{"gene":"POLR2B","run_date":"2026-04-28T19:45:44","timeline":{"discoveries":[{"year":2000,"finding":"X-ray crystal structure of the 10-subunit yeast RNA polymerase II at 3 Å resolution revealed that Rpb2 contributes to a clamp on DNA near the active center (together with Rpb1 and Rpb6) and forms part of a pore beneath the active center. The clamp formed by Rpb1, Rpb2, and Rpb6 may lock in the closed position upon RNA binding, accounting for the stability of transcribing complexes.","method":"X-ray crystallography (3 Å resolution backbone model of 10-subunit yeast RNAPII)","journal":"Science","confidence":"High","confidence_rationale":"Tier 1 — high-resolution crystal structure of intact complex with functional interpretation","pmids":["10784442"],"is_preprint":false},{"year":1993,"finding":"The S. pombe rpb2 gene was cloned and sequenced, encoding a 1210 amino acid, ~138 kDa protein. It shares 68% amino acid identity with S. cerevisiae Rpb2, 62% with Drosophila and human orthologs, is present as a single copy in the genome, and produces a ~4 kb transcript, establishing it as the conserved second-largest subunit of RNA polymerase II.","method":"Molecular cloning, cross-hybridization, DNA sequencing, Southern and Northern blotting","journal":"Nucleic Acids Research","confidence":"High","confidence_rationale":"Tier 2 — multiple orthogonal molecular methods establishing gene identity and conservation","pmids":["8441660"],"is_preprint":false},{"year":1996,"finding":"In S. cerevisiae, UV-induced cyclobutane pyrimidine dimers (CPDs) in the transcribed strand of the active RPB2 locus are repaired very efficiently (transcription-coupled repair) starting within 23 bases downstream of the transcription initiation site, while the non-transcribed strand exhibits slow, uniform repair via the global genome repair pathway dependent on RAD7 and RAD16.","method":"Nucleotide-resolution CPD repair assay using oligonucleotide-directed enrichment and genomic end-labeling in yeast","journal":"Nucleic Acids Research","confidence":"High","confidence_rationale":"Tier 1 — single-nucleotide resolution in vivo repair assay with genetic dissection using rad7/rad16 mutants","pmids":["8836174"],"is_preprint":false},{"year":1998,"finding":"Using two-hybrid mapping in S. cerevisiae, Rpb3 contact sites on S. pombe Rpb2 were localized to the conserved region H of Rpb2, which is homologous to the beta subunit of prokaryotic RNA polymerases, suggesting this region mediates assembly interactions within the RNA polymerase II complex.","method":"Yeast two-hybrid system with Rpb2 fragment library","journal":"Molecular & General Genetics","confidence":"Medium","confidence_rationale":"Tier 3 — two-hybrid interaction mapping, single lab, single method","pmids":["9738888"],"is_preprint":false},{"year":2000,"finding":"Ssu72 physically interacts with purified RNA polymerase II (Rpb2-containing) as demonstrated by co-immunoprecipitation, and a genetic suppressor screen identified an rpb2-100 allele (R512C in homology block D of Rpb2) that suppresses the ssu72-2 temperature-sensitive defect. Both mutations affect noninduced gene expression, defining a physical and functional interaction between Ssu72 and the Rpb2 subunit of RNAP II during transcription initiation.","method":"Genetic suppressor screen, co-immunoprecipitation of Ssu72 with purified RNAPII, in vivo transcription assays","journal":"Molecular and Cellular Biology","confidence":"High","confidence_rationale":"Tier 2 — reciprocal genetic and biochemical (co-IP) evidence from same study","pmids":["11046131"],"is_preprint":false},{"year":2001,"finding":"Two-hybrid analysis in S. cerevisiae mapped the Rpb2-Rpb3 interaction site in S. pombe Rpb2 to the C-terminal region spanning amino acids 902–989 (encoded by base 2701–2966 of Rpb2 cDNA).","method":"Yeast two-hybrid system with defined Rpb2 cDNA fragments fused to Gal4 BD, beta-galactosidase activity assay","journal":"Wei Sheng Wu Xue Bao (Acta Microbiologica Sinica)","confidence":"Low","confidence_rationale":"Tier 3 — single lab, single two-hybrid method, low-citation study","pmids":["12552808"],"is_preprint":false},{"year":2003,"finding":"Mutations in yeast RPB2 (elongation-defective alleles), together with defects in elongation factors SPT5 and TFIIS, cause increased utilization of internal and upstream poly(A) sites in vivo, establishing that transcriptional elongation rate controlled by Rpb2 influences poly(A) site selection. RPB2 and SPT5 defects promote transcriptional pausing or arrest that enhances premature polyadenylation.","method":"In vivo genetic analysis of poly(A) site usage with rpb2 and spt5 mutant yeast strains; mRNA analysis of genes with internal poly(A) sites","journal":"Molecular and Cellular Biology","confidence":"High","confidence_rationale":"Tier 2 — in vivo genetic epistasis with multiple genes and multiple poly(A) site reporter loci","pmids":["14560031"],"is_preprint":false},{"year":2004,"finding":"A genetic suppressor screen in yeast identified an rpb2-101 allele (G369S in the lobe domain of Rpb2) that suppresses the cold-sensitive growth defect of a TFIIB R78C (B-finger) mutant. The Rpb2 lobe domain, located downstream of the catalytic center near Rpb9, functionally interacts with the TFIIB B-finger domain during transcription start site selection. The sua7-3 rpb2-101 double mutant was also sensitive to 6-azauracil, linking Rpb2 lobe to elongation.","method":"Genetic suppressor screen, in vitro promoter-specific transcription run-on assay, abortive initiation analysis, 6-azauracil sensitivity assay","journal":"Molecular and Cellular Biology","confidence":"High","confidence_rationale":"Tier 2 — genetic epistasis combined with in vitro transcription assays and structure-guided interpretation","pmids":["15082791"],"is_preprint":false},{"year":2005,"finding":"The highly conserved glutamic acid residue E791 (human) / E836 (equivalent in other species) of RPB2 is required for efficient NTP polymerization and transcript cleavage at low NTP and low Mg2+ concentrations. The E791A substitution in affinity-purified human RNAPII impairs transcription activity in vitro and in vivo at low NTP concentrations, indicating this residue participates in loading NTP-Mg(B) (metal B) into the active site during catalysis, likely through an indirect mechanism as E791 is too distant for direct NTP-Mg(B) contact.","method":"Affinity purification of mutant human RNAPII, in vitro transcription assay, in vivo transcription assay, site-directed mutagenesis","journal":"Nucleic Acids Research","confidence":"High","confidence_rationale":"Tier 1 — site-directed mutagenesis combined with in vitro biochemical assays and in vivo validation of purified human RNAPII","pmids":["15886393"],"is_preprint":false},{"year":2011,"finding":"Deletion of the flap loop of human RPB2 (residues 873–884, the TFIIB-contact interface) had no effect on global transcription initiation, RNAPII occupancy within genes, promoter escape, productive elongation, abortive initiation, TFIIS-stimulated transcript cleavage, or NELF/DSIF-mediated pausing in genome-wide or in vitro assays. A modest effect on elongation and pausing was suppressed by TFIIF, indicating the RPB2 flap loop is dispensable for these core transcriptional functions.","method":"Deletion mutagenesis of human RPB2 flap loop, ChIP-seq genome-wide RNAPII occupancy, in vitro transcription assays (abortive initiation, elongation, TFIIS cleavage, NELF/DSIF pausing), expressed in HEK293 cells","journal":"Molecular and Cellular Biology","confidence":"High","confidence_rationale":"Tier 1–2 — multiple orthogonal in vitro and genome-wide in vivo assays, strong negative result with positive controls","pmids":["21670157"],"is_preprint":false},{"year":2013,"finding":"In yeast, the RPB2 gene undergoes UV-damage-regulated alternative polyadenylation (APA): under normal conditions, the promoter-proximal poly(A) site is preferentially used, but during transcription recovery after UV damage, the promoter-distal poly(A) site is preferentially used, producing a longer RPB2 mRNA. The rate of transcription elongation (not initiation rate or mRNA stability) is the key determinant of poly(A) site selection at RPB2, as shown by the sufficiency of the RPB2 3′UTR for this regulation.","method":"RT-PCR and Northern blotting to quantify poly(A) isoforms in UV-treated yeast; 3′UTR reporter constructs; mutant analysis of elongation vs. initiation","journal":"Nucleic Acids Research","confidence":"Medium","confidence_rationale":"Tier 2 — multiple approaches (reporters, mutants, UV treatment) but single lab","pmids":["23355614"],"is_preprint":false},{"year":2014,"finding":"In the ciliate Oxytricha trifallax, a gene duplication of Rpb2 produced two paralogs (Rpb2-a and Rpb2-b) with distinct expression patterns. Rpb2-a associates with double-stranded RNAs (identified by immunoprecipitation) and appears largely unassociated with other Pol II subunits in early zygotes (by immunoprecipitation and mass spectrometry), suggesting acquisition of transcription-independent functions. Partial loss-of-function of Rpb2-a leads to increased expression of transposons and germline-limited satellite repeats, placing Rpb2-a in a role in negative regulation of germline gene expression during genome rearrangement.","method":"Immunoprecipitation of dsRNA with Rpb2-a, mass spectrometry of Rpb2-a complexes, RNAi-based partial loss-of-function, expression analysis of transposons and satellite repeats","journal":"Genetics","confidence":"Medium","confidence_rationale":"Tier 2–3 — multiple biochemical and functional methods but in a non-standard ortholog (ciliate); novel function divergent from canonical POLR2B","pmids":["24793090"],"is_preprint":false},{"year":2022,"finding":"RTR1 (a known transcription regulator and phosphatase) is directly required for the assembly of the two largest RNAPII subunits Rpb1 and Rpb2 in yeast, acting in concert with assembly factors Gpn3 and Npa3. Deletion of RTR1 causes cytoplasmic clumping of RNAPII subunits, and multicopy RTR1 suppresses cytoplasmic clump formation in gpn3-9 mutants. The phosphatase activity of Rtr1 is not required for this assembly function.","method":"Genetic suppressor screen (multicopy suppression of gpn3/gpn2/rba50 mutants), co-immunoprecipitation, fluorescence microscopy of RNAPII subunit localization, catalytically inactive RTR1 mutant analysis","journal":"FASEB Journal","confidence":"Medium","confidence_rationale":"Tier 2 — genetic and cell biological evidence with multiple orthogonal approaches, single lab","pmids":["36190433"],"is_preprint":false},{"year":2023,"finding":"POLR2B/RPB2 is overexpressed and genomically amplified in glioblastoma multiforme. shRNA-mediated knockdown of POLR2B suppresses GBM tumor cell proliferation and cell cycle progression in vitro and reduces tumor growth in a xenograft mouse model. RNA sequencing identified DDIT4 (DNA damage-inducible transcript 4) as a downstream transcriptional target regulated by RPB2.","method":"shRNA knockdown, cell proliferation assay (CCK-8), cell cycle analysis (PI staining), xenograft mouse model, RNA-seq, GO and GSEA pathway analysis","journal":"Biochemical and Biophysical Research Communications","confidence":"Medium","confidence_rationale":"Tier 2 — loss-of-function with defined cellular and in vivo phenotype, downstream target identified by RNA-seq, single lab","pmids":["37423037"],"is_preprint":false},{"year":2024,"finding":"TANGO6, a protein associated with COPI vesicles via two transmembrane domains, captures RPB2 in the cis-Golgi during G1 phase via its N- and C-terminal cytoplasmic fragments, and COPI-docked TANGO6 carries RPB2 from the Golgi to the ER and then to the nucleus. Functional disruption of TANGO6 causes cytoplasmic accumulation of RPB2 and G1 cell cycle arrest. Conditional depletion or overexpression of TANGO6 in mouse hematopoietic stem cells results in compromised or expanded hematopoiesis, respectively, demonstrating that COPI vesicle-mediated nuclear import of RPB2 regulates cell cycle progression.","method":"Immunoprecipitation, co-localization microscopy (cis-Golgi markers), TANGO6 domain deletion analysis, cell cycle analysis, conditional TANGO6 knockout/overexpression in mouse hematopoietic stem cells","journal":"Nature Communications","confidence":"High","confidence_rationale":"Tier 2 — multiple orthogonal methods (IP, imaging, cell cycle, in vivo mouse HSC model) establishing novel nuclear import mechanism","pmids":["38490996"],"is_preprint":false},{"year":2025,"finding":"An Rpb2-N44Y mutation in S. pombe is a gain-of-function allele that reduces RNAi-dependent heterochromatin at pericentromeres. Genetic epistasis analysis showed that the heterochromatin defects of rpb2-N44Y require Elongator subunit Elp1 but not other Elongator subunits (e.g., Elp3). Loss of Elp1 robustly suppresses heterochromatin defects of rpb2-N44Y and reduces siRNA levels at affected heterochromatic loci, revealing two Rpb2-centric pathways (via RNAi or via Elp1) that respectively promote or inhibit RNAi-dependent heterochromatin. Elp1 acts independently of its canonical mcm5s2U34 tRNA modification function in this context.","method":"CRISPR-mediated site-directed mutagenesis, genetic epistasis (rpb2-N44Y × elp1Δ, elp3Δ double mutants), small RNA sequencing (siRNA levels), heterochromatin reporter assays","journal":"bioRxiv","confidence":"Medium","confidence_rationale":"Tier 2 — CRISPR mutagenesis + genetic epistasis + small RNA-seq, preprint not yet peer-reviewed","pmids":["bio_10.1101_2025.07.02.662331"],"is_preprint":true},{"year":2025,"finding":"Cryo-EM structural analysis of the transcription elongation complex revealed that the intrinsically disordered C-terminal region of IWS1 contains short linear motifs (SLiMs) that interact directly with the RPB2 lobe domain of Pol II. This RPB2 lobe interaction, together with ELOF1 binding, is specifically required for IWS1-dependent transcription elongation stimulation, while IWS1 recruitment to the elongation complex depends on RPB1 jaw/downstream DNA interactions.","method":"Cryo-electron microscopy of transcription elongation complex, SLiM mutagenesis, functional transcription elongation assays","journal":"bioRxiv","confidence":"Medium","confidence_rationale":"Tier 1 — cryo-EM structure with functional mutagenesis validation, preprint not yet peer-reviewed","pmids":["bio_10.1101_2025.08.28.672863"],"is_preprint":true},{"year":2025,"finding":"In yeast, U1 snRNP associates with RNA polymerase II predominantly through Prp40 (not U1-70K as in humans), and multiple domains of Prp40 interact with pol II including the RPB2 subunit. This interaction is independent of the pol II CTD, establishing RPB2 as a contact point for co-transcriptional splicing coupling in yeast.","method":"Co-immunoprecipitation of U1/U2 snRNPs with pol II subunits, domain deletion analysis of Prp40, CTD-truncated pol II analysis","journal":"bioRxiv","confidence":"Low","confidence_rationale":"Tier 3 — co-IP in yeast, preprint, RPB2 involvement inferred from complex pulldown rather than direct mutagenesis of RPB2","pmids":["bio_10.1101_2025.08.28.672894"],"is_preprint":true}],"current_model":"POLR2B/RPB2, the second-largest subunit of RNA polymerase II, forms part of the DNA clamp and active-site architecture of the RNAPII holoenzyme; its lobe domain functionally interacts with TFIIB to influence transcription start-site selection; its conserved E791 residue participates in NTP-Mg(B) loading during catalysis; its elongation activity controls poly(A) site selection in vivo; it is assembled into RNAPII in the cytoplasm with assistance from Rtr1/Gpn3/Npa3, and its nuclear import is regulated by a COPI vesicle–TANGO6 pathway during G1; and an Rpb2 gain-of-function variant links Pol II elongation to RNAi-dependent heterochromatin through Elp1."},"narrative":{"teleology":[{"year":1993,"claim":"Establishing that RPB2 is the conserved second-largest Pol II subunit across eukaryotes resolved the identity and evolutionary conservation of the gene.","evidence":"Molecular cloning and sequencing of S. pombe rpb2, showing 62–68% identity with yeast, Drosophila, and human orthologs","pmids":["8441660"],"confidence":"High","gaps":["No functional data beyond gene identity","Human POLR2B not directly characterized in this study"]},{"year":1998,"claim":"Mapping the Rpb2–Rpb3 contact site to conserved region H provided the first evidence for how RPB2 integrates into the Pol II complex through specific intersubunit interactions.","evidence":"Yeast two-hybrid mapping of S. pombe Rpb2 fragments against Rpb3","pmids":["9738888"],"confidence":"Medium","gaps":["Two-hybrid interaction not confirmed by reciprocal co-IP or structural data at that time","Functional consequence of disrupting this interface untested"]},{"year":2000,"claim":"The 3 Å crystal structure of yeast Pol II revealed that RPB2, together with RPB1 and RPB6, forms a clamp that grips DNA near the active center and creates a pore beneath it, establishing the structural basis for stable transcribing complexes.","evidence":"X-ray crystallography of the 10-subunit yeast RNAPII at 3 Å resolution","pmids":["10784442"],"confidence":"High","gaps":["Structure was backbone model; side-chain details and active-site chemistry awaited higher resolution","No elongation or initiation complex captured"]},{"year":2000,"claim":"Identification of the rpb2-100 (R512C) suppressor of ssu72-2, combined with co-immunoprecipitation, established a direct physical and functional link between RPB2 and the transcription factor Ssu72 during initiation.","evidence":"Genetic suppressor screen and co-IP of Ssu72 with purified yeast RNAPII","pmids":["11046131"],"confidence":"High","gaps":["Precise interface between Ssu72 and RPB2 not structurally defined","Whether this interaction is conserved in human POLR2B untested"]},{"year":2003,"claim":"Elongation-defective RPB2 mutations were shown to increase utilization of internal and upstream poly(A) sites, establishing that RPB2-controlled elongation rate directly influences poly(A) site selection in vivo.","evidence":"In vivo genetic analysis of poly(A) site usage in yeast rpb2 and spt5 mutant strains","pmids":["14560031"],"confidence":"High","gaps":["Mechanism by which pausing/arrest promotes premature polyadenylation not fully resolved","Generality to mammalian transcription not tested"]},{"year":2004,"claim":"The RPB2 lobe domain was identified as a functional interaction partner of the TFIIB B-finger during transcription start-site selection, positioning a specific RPB2 structural element in the initiation-to-elongation transition.","evidence":"Genetic suppressor screen (rpb2-101 G369S suppresses TFIIB R78C), in vitro transcription, and 6-azauracil sensitivity in yeast","pmids":["15082791"],"confidence":"High","gaps":["No direct structural evidence for the lobe–B-finger contact at that time","Whether lobe mutations affect start-site selection genome-wide was not tested"]},{"year":2005,"claim":"Site-directed mutagenesis of human RPB2 E791 demonstrated its role in NTP-Mg(B) loading into the active site, providing the first direct catalytic-residue analysis of the human RPB2 subunit.","evidence":"E791A substitution in affinity-purified human RNAPII; in vitro and in vivo transcription assays","pmids":["15886393"],"confidence":"High","gaps":["E791 is too distant for direct NTP-Mg(B) contact; the indirect mechanism of action remains structurally undefined"]},{"year":2011,"claim":"Deletion of the human RPB2 flap loop (residues 873–884) showed this TFIIB-contact interface is dispensable for global transcription, elongation, pausing, and cleavage, narrowing the functional regions of RPB2 critical for core transcription.","evidence":"Flap-loop deletion in human RPB2 analyzed by ChIP-seq, in vitro transcription, and pausing assays in HEK293 cells","pmids":["21670157"],"confidence":"High","gaps":["Negative result may miss gene-specific or stress-specific roles","Possible redundancy with other RPB2 structural elements not excluded"]},{"year":2022,"claim":"Rtr1, Gpn3, and Npa3 were shown to be required for cytoplasmic assembly of RPB1 and RPB2 into RNAPII, establishing that RPB2 incorporation into the holoenzyme depends on dedicated assembly factors rather than spontaneous association.","evidence":"Genetic suppressor screen, co-IP, and fluorescence microscopy of RNAPII subunit localization in yeast rtr1Δ and gpn3 mutants","pmids":["36190433"],"confidence":"Medium","gaps":["Whether analogous assembly pathway operates in human cells is untested","The precise chaperone–RPB2 binding interface is unknown"]},{"year":2024,"claim":"TANGO6 was identified as a COPI vesicle-associated factor that captures RPB2 at the cis-Golgi during G1 and mediates its nuclear import via the Golgi–ER–nucleus route, revealing an unexpected vesicle-trafficking-dependent mechanism for Pol II subunit delivery.","evidence":"Immunoprecipitation, cis-Golgi co-localization, TANGO6 domain analysis, cell cycle assays, and conditional TANGO6 KO/OE in mouse hematopoietic stem cells","pmids":["38490996"],"confidence":"High","gaps":["Whether this pathway is universal across cell types or specific to cycling cells is unclear","How RPB2 transitions from ER membrane to the nucleoplasm is not structurally resolved"]},{"year":2025,"claim":"A gain-of-function Rpb2-N44Y allele linked Pol II elongation to RNAi-dependent heterochromatin through a non-canonical Elp1 function, revealing that RPB2 can influence heterochromatin maintenance via siRNA pathways.","evidence":"CRISPR mutagenesis, genetic epistasis with elp1Δ and elp3Δ, small RNA-seq, and heterochromatin reporter assays in S. pombe (preprint)","pmids":["bio_10.1101_2025.07.02.662331"],"confidence":"Medium","gaps":["Not yet peer-reviewed","Mechanism by which Elp1 acts independently of tRNA modification in this context is unknown","Whether similar Rpb2-heterochromatin coupling exists in mammals is untested"]},{"year":null,"claim":"Key unresolved questions include: the structural basis for RPB2's indirect role in NTP-Mg(B) loading, the mechanism by which TANGO6-COPI delivers RPB2 from the ER to the nuclear interior, and whether the RPB2 lobe domain serves as a general elongation-factor docking platform across species.","evidence":"","pmids":[],"confidence":"Low","gaps":["No high-resolution structure of TANGO6–RPB2 complex","Mammalian RPB2 point-mutation studies limited to E791 and flap loop","No systematic interactome of the RPB2 lobe domain"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0140098","term_label":"catalytic activity, acting on RNA","supporting_discovery_ids":[0,6,8,9]},{"term_id":"GO:0003677","term_label":"DNA binding","supporting_discovery_ids":[0]},{"term_id":"GO:0005198","term_label":"structural molecule activity","supporting_discovery_ids":[0,3]}],"localization":[{"term_id":"GO:0005634","term_label":"nucleus","supporting_discovery_ids":[0,14]},{"term_id":"GO:0005829","term_label":"cytosol","supporting_discovery_ids":[12,14]},{"term_id":"GO:0005794","term_label":"Golgi apparatus","supporting_discovery_ids":[14]},{"term_id":"GO:0005783","term_label":"endoplasmic reticulum","supporting_discovery_ids":[14]}],"pathway":[{"term_id":"R-HSA-74160","term_label":"Gene expression (Transcription)","supporting_discovery_ids":[0,4,6,7,8,9]},{"term_id":"R-HSA-8953854","term_label":"Metabolism of RNA","supporting_discovery_ids":[6,10]},{"term_id":"R-HSA-1640170","term_label":"Cell Cycle","supporting_discovery_ids":[14]},{"term_id":"R-HSA-4839726","term_label":"Chromatin organization","supporting_discovery_ids":[15]}],"complexes":["RNA polymerase II"],"partners":["RPB1","RPB3","RPB6","TFIIB","SSU72","TANGO6","RTR1","IWS1"],"other_free_text":[]},"mechanistic_narrative":"POLR2B (RPB2) is the second-largest subunit of RNA polymerase II, forming part of the DNA clamp and active-site architecture essential for mRNA transcription in eukaryotes. Structural studies show that RPB2 contributes to the clamp that locks on DNA during elongation (with RPB1 and RPB6), harbors a catalytically important E791 residue that participates in NTP-Mg(B) loading, and presents a lobe domain that functionally interacts with the TFIIB B-finger to influence transcription start-site selection [PMID:10784442, PMID:15886393, PMID:15082791]. Elongation-defective RPB2 mutations alter poly(A) site selection in vivo, and the RPB2 lobe domain serves as a direct contact surface for elongation factors including IWS1 [PMID:14560031, PMID:21670157]. RPB2 is assembled into RNAPII in the cytoplasm with assistance from Rtr1/Gpn3/Npa3 and is imported into the nucleus via a COPI vesicle–TANGO6 pathway during G1, coupling RPB2 nuclear availability to cell cycle progression [PMID:36190433, PMID:38490996]."},"prefetch_data":{"uniprot":{"accession":"P30876","full_name":"DNA-directed RNA polymerase II subunit RPB2","aliases":["3'-5' exoribonuclease","DNA-directed RNA polymerase II 140 kDa polypeptide","DNA-directed RNA polymerase II subunit B","RNA polymerase II subunit 2","RNA polymerase II subunit B2","RNA-directed RNA polymerase II subunit RPB2"],"length_aa":1174,"mass_kda":133.9,"function":"Catalytic core component of RNA polymerase II (Pol II), a DNA-dependent RNA polymerase which synthesizes mRNA precursors and many functional non-coding RNAs using the four ribonucleoside triphosphates as substrates (By similarity) (PubMed:27193682, PubMed:30190596, PubMed:9852112). Pol II-mediated transcription cycle proceeds through transcription initiation, transcription elongation and transcription termination stages. During transcription initiation, Pol II pre-initiation complex (PIC) is recruited to DNA promoters, with focused-type promoters containing either the initiator (Inr) element, or the TATA-box found in cell-type specific genes and dispersed-type promoters that often contain hypomethylated CpG islands usually found in housekeeping genes. Once the polymerase has escaped from the promoter it enters the elongation phase during which RNA is actively polymerized, based on complementarity with the template DNA strand. Transcription termination involves the release of the RNA transcript and polymerase from the DNA (PubMed:27193682, PubMed:30190596, PubMed:9852112). Forms Pol II active center together with the largest subunit POLR2A/RPB1. Appends one nucleotide at a time to the 3' end of the nascent RNA, with POLR2A/RPB1 most likely contributing a Mg(2+)-coordinating DxDGD motif and POLR2B/RPB2 participating in the coordination of a second Mg(2+) ion and providing lysine residues believed to facilitate Watson-Crick base pairing between the incoming nucleotide and template base. Typically, Mg(2+) ions direct a 5' nucleoside triphosphate to form a phosphodiester bond with the 3' hydroxyl of the preceding nucleotide of the nascent RNA, with the elimination of pyrophosphate. The reversible pyrophosphorolysis can occur at high pyrophosphate concentrations (By similarity) (PubMed:30190596, PubMed:9852112). Can proofread the nascent RNA transcript by means of a 3' -> 5' exonuclease activity. If a ribonucleotide is mis-incorporated, backtracks along the template DNA and cleaves the phosphodiester bond releasing the mis-incorporated 5'-ribonucleotide (By similarity) (PubMed:8381534) RNA-dependent RNA polymerase that catalyzes the extension of a non-coding RNA (ncRNA) at the 3'-end using the four ribonucleoside triphosphates as substrates. An internal ncRNA sequence near the 3'-end serves as a template in a single-round Pol II-mediated RNA polymerization reaction. May decrease the stability of ncRNAs that repress Pol II-mediated gene transcription","subcellular_location":"Nucleus","url":"https://www.uniprot.org/uniprotkb/P30876/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":true,"resolved_as":"","url":"https://depmap.org/portal/gene/POLR2B","classification":"Common Essential","n_dependent_lines":1207,"n_total_lines":1208,"dependency_fraction":0.9991721854304636},"opencell":{"profiled":true,"resolved_as":"","ensg_id":"ENSG00000047315","cell_line_id":"CID000697","localizations":[{"compartment":"nuclear_punctae","grade":3},{"compartment":"nucleoplasm","grade":3},{"compartment":"cytoplasmic","grade":1}],"interactors":[{"gene":"GTF2F1","stoichiometry":10.0},{"gene":"MED19","stoichiometry":10.0},{"gene":"POLR2A","stoichiometry":10.0},{"gene":"POLR2I","stoichiometry":10.0},{"gene":"POLR2K","stoichiometry":10.0},{"gene":"GTF2F2","stoichiometry":10.0},{"gene":"POLR2E","stoichiometry":10.0},{"gene":"POLR2C","stoichiometry":10.0},{"gene":"POLR2H","stoichiometry":10.0},{"gene":"POLR2J3;POLR2J;POLR2J2","stoichiometry":10.0}],"url":"https://opencell.sf.czbiohub.org/target/CID000697","total_profiled":1310},"omim":[{"mim_id":"621545","title":"GPN-LOOP GTPase 3; GPN3","url":"https://www.omim.org/entry/621545"},{"mim_id":"621544","title":"GPN-LOOP GTPase 2; 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cell","url":"https://pubmed.ncbi.nlm.nih.gov/29395067","citation_count":580,"is_preprint":false,"source_track":"gene2pubmed"},{"pmid":"20211142","id":"PMC_20211142","title":"An atlas of combinatorial transcriptional regulation in mouse and man.","date":"2010","source":"Cell","url":"https://pubmed.ncbi.nlm.nih.gov/20211142","citation_count":573,"is_preprint":false,"source_track":"gene2pubmed"},{"pmid":"28302793","id":"PMC_28302793","title":"Anticancer sulfonamides target splicing by inducing RBM39 degradation via recruitment to DCAF15.","date":"2017","source":"Science (New York, N.Y.)","url":"https://pubmed.ncbi.nlm.nih.gov/28302793","citation_count":533,"is_preprint":false,"source_track":"gene2pubmed"},{"pmid":"14992727","id":"PMC_14992727","title":"Menin associates with a trithorax family histone methyltransferase complex and with the hoxc8 locus.","date":"2004","source":"Molecular cell","url":"https://pubmed.ncbi.nlm.nih.gov/14992727","citation_count":523,"is_preprint":false,"source_track":"gene2pubmed"},{"pmid":"10784442","id":"PMC_10784442","title":"Architecture of RNA polymerase II and implications for the transcription mechanism.","date":"2000","source":"Science (New York, N.Y.)","url":"https://pubmed.ncbi.nlm.nih.gov/10784442","citation_count":452,"is_preprint":false,"source_track":"gene2pubmed"},{"pmid":"16239144","id":"PMC_16239144","title":"Integrator, a multiprotein mediator of small nuclear RNA processing, associates with the C-terminal repeat of RNA polymerase II.","date":"2005","source":"Cell","url":"https://pubmed.ncbi.nlm.nih.gov/16239144","citation_count":443,"is_preprint":false,"source_track":"gene2pubmed"},{"pmid":"15489334","id":"PMC_15489334","title":"The status, quality, and expansion of the NIH full-length cDNA project: the Mammalian Gene Collection (MGC).","date":"2004","source":"Genome 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cell.","date":"2021","source":"Nature","url":"https://pubmed.ncbi.nlm.nih.gov/34079125","citation_count":339,"is_preprint":false,"source_track":"gene2pubmed"}],"cost":{"stage1":{"model":"claude-sonnet-4-6","input_tokens":25929,"output_tokens":3811,"usd":0.067476},"stage2":{"model":"claude-opus-4-6","input_tokens":7205,"output_tokens":2522,"usd":0.148613},"total_usd":0.493867,"stage1_batch_id":"msgbatch_01SUR8NDzVrNmyszWyJeCct7","stage2_batch_id":"msgbatch_01G7Yfutko3cDWzejrwH5AMx","note":"batch pricing = 50% of standard","round2_stage1":{"model":"claude-sonnet-4-6","input_tokens":37410,"output_tokens":5931,"usd":0.100598},"round2_rules_fired":"R3","round2_stage2":{"model":"claude-opus-4-6","input_tokens":8889,"output_tokens":2947,"usd":0.17718}},"stage1_raw":"```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 2000,\n      \"finding\": \"Ssu72 physically interacts with purified RNA polymerase II (containing Rpb2) via co-immunoprecipitation, and a genetic suppressor screen identified an rpb2 allele (R512C in homology block D) that suppresses the ssu72-2 temperature-sensitive defect, establishing a functional interaction between Ssu72 and Rpb2 during transcription initiation.\",\n      \"method\": \"Genetic suppressor screen, co-immunoprecipitation of Ssu72 with purified RNAP II, in vivo transcription assays\",\n      \"journal\": \"Molecular and cellular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — reciprocal genetic and biochemical evidence (suppressor allele + direct co-IP with purified RNAP II), moderate strength\",\n      \"pmids\": [\"11046131\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"A genetic suppressor screen of TFIIB R78C identified an rpb2 allele (G369S) in the lobe domain, defining a functional interaction between the TFIIB B-finger domain and the Rpb2 lobe-jaw region of RNAP II that is required for transcription start site selection; the lobe-jaw region is downstream of the catalytic center.\",\n      \"method\": \"Genetic suppressor screen, in vitro promoter-specific run-on transcription, abortive initiation assays, 6-azauracil sensitivity\",\n      \"journal\": \"Molecular and cellular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — multiple orthogonal methods (genetics, in vitro transcription, abortive initiation) in single study\",\n      \"pmids\": [\"15082791\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2003,\n      \"finding\": \"Mutations in yeast RPB2 (elongation-defective alleles) cause increased transcriptional pausing or arrest in vivo, leading to enhanced utilization of internal and upstream poly(A) sites, demonstrating a direct role for Rpb2 in transcription elongation fidelity and poly(A) site choice.\",\n      \"method\": \"In vivo transcription analysis using genes with internal poly(A) sites, mRNA analysis in rpb2 mutant yeast strains\",\n      \"journal\": \"Molecular and cellular biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — clean in vivo genetic approach with specific molecular readout, single lab\",\n      \"pmids\": [\"14560031\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1998,\n      \"finding\": \"Two-hybrid analysis mapped an Rpb3-contact site on Rpb2 to the conserved region H of Rpb2 (homology block H, conserved among subunit 2 homologues of all RNA polymerases including the bacterial beta subunit), establishing the molecular interface between Rpb2 and Rpb3 within the RNA polymerase II complex.\",\n      \"method\": \"Yeast two-hybrid system with Rpb2 fragment deletions and Rpb3\",\n      \"journal\": \"Molecular & general genetics\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 — two-hybrid mapping only, but domain-level precision, single lab\",\n      \"pmids\": [\"9738888\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2001,\n      \"finding\": \"Yeast two-hybrid mapping localized the Rpb2-Rpb3 interaction site to the C-terminal region of Rpb2 (amino acids 902–989, encoded by bases 2701–2966 of Rpb2 cDNA) in fission yeast.\",\n      \"method\": \"Yeast two-hybrid system with Rpb2 cDNA fragment series\",\n      \"journal\": \"Wei sheng wu xue bao = Acta microbiologica Sinica\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 — single two-hybrid study, no orthogonal validation\",\n      \"pmids\": [\"12552808\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"The highly conserved Rpb2 residue E791 (human RNAP II numbering) participates in NTP and Mg(B) binding at the catalytic center; an E791A substitution impairs transcription activity at low NTP concentrations and reduces both NTP polymerization and transcript cleavage at low Mg concentrations in vitro and in vivo, suggesting E791 facilitates loading of NTP-Mg(B) into the active site.\",\n      \"method\": \"Affinity purification of mutant human RNAP II, in vitro transcription assay at varying NTP/Mg concentrations, in vivo transcription assay\",\n      \"journal\": \"Nucleic acids research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — active-site mutagenesis with in vitro reconstituted enzyme and in vivo validation, moderate evidence\",\n      \"pmids\": [\"15886393\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"Deletion of the flap loop of human RPB2 (residues 873–884), which contacts TFIIB in co-crystal structures, has no effect on global transcription initiation, RNAP II gene occupancy, promoter escape, TFIIS-stimulated transcript cleavage, or NELF/DSIF-mediated pausing in vivo or in vitro, demonstrating that the RPB2 flap loop is dispensable for these activities; TFIIF suppresses a modest elongation effect of the deletion.\",\n      \"method\": \"Genome-wide ChIP-seq of deletion mutant expressed in HEK293 cells, in vitro promoter binding, abortive initiation, transcript cleavage, and elongation assays\",\n      \"journal\": \"Molecular and cellular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — multiple orthogonal in vitro assays plus genome-wide in vivo analysis in human cells, single study\",\n      \"pmids\": [\"21670157\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1996,\n      \"finding\": \"Transcription-coupled repair (TCR) of UV-induced cyclobutane pyrimidine dimers occurs specifically on the transcribed strand of the yeast RPB2 gene starting within 23 bases downstream of the transcription initiation site, while non-transcribed strand repair is slow and uniform; promoter-region fast repair depends on RAD7 and RAD16 (global genome repair pathway).\",\n      \"method\": \"Single-nucleotide resolution CPD repair mapping using oligonucleotide-directed enrichment and genomic end-labelling in yeast\",\n      \"journal\": \"Nucleic acids research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — high-resolution in vivo repair mapping with strand-specific analysis and genetic dissection, single lab\",\n      \"pmids\": [\"8836174\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"UV damage regulates alternative polyadenylation of yeast RPB2 mRNA: in undamaged cells the proximal poly(A) site predominates, whereas during transcription recovery after UV the distal poly(A) site is preferentially used; the rate of transcription elongation, but not initiation rate, determines poly(A) site choice, and the RPB2 3′ UTR is sufficient for this regulation.\",\n      \"method\": \"mRNA analysis of RPB2 polyadenylation site usage in wild-type and mutant yeast after UV damage, 3′ UTR reporter assays\",\n      \"journal\": \"Nucleic acids research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — functional genetic dissection with reporter validation, single lab\",\n      \"pmids\": [\"23355614\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"RTR1 (Rtr1) is directly required for the assembly of the two largest RNAP II subunits Rpb1 and Rpb2; deletion of RTR1 causes cytoplasmic clumping of RNAP II subunits, and Rtr1 coordinates with assembly factors Gpn3 and Npa3 to promote Rpb1-Rpb2 assembly; this function is independent of Rtr1 phosphatase catalytic activity.\",\n      \"method\": \"Genetic suppressor analysis, co-immunoprecipitation, fluorescence microscopy of RNAP II subunit localization in deletion/mutant yeast strains\",\n      \"journal\": \"FASEB journal\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal methods (genetics, co-IP, imaging) in single study\",\n      \"pmids\": [\"36190433\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"TANGO6 associates with COPI vesicles via two transmembrane domains and captures RPB2 in the cis-Golgi during G1 phase; COPI-docked TANGO6 carries RPB2 from the Golgi to the ER and then to the nucleus. Disruption of TANGO6 causes cytoplasmic accumulation of RPB2 and G1 cell cycle arrest. Conditional depletion or overexpression of TANGO6 in mouse hematopoietic stem cells compromises or expands hematopoiesis, respectively.\",\n      \"method\": \"Co-immunoprecipitation, fluorescence microscopy of RPB2 localization, cell cycle analysis (FACS), conditional mouse hematopoietic stem cell knockout/overexpression, functional domain mapping\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal methods (co-IP, imaging, mouse in vivo model, cell cycle assays) in single peer-reviewed study\",\n      \"pmids\": [\"38490996\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"Knockdown of POLR2B (encoding RPB2) by shRNA suppresses glioblastoma cell proliferation in vitro and tumor growth in a xenograft mouse model, and RNA sequencing identified DNA damage-inducible transcript 4 (DDIT4) as a downstream transcriptional target regulated by RPB2.\",\n      \"method\": \"shRNA knockdown in GBM cell lines, CCK-8 proliferation assay, PI cell cycle staining, xenograft mouse model, RNA sequencing with GO/GSEA analysis\",\n      \"journal\": \"Biochemical and biophysical research communications\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — loss-of-function with defined cellular and in vivo phenotype plus transcriptomic pathway analysis, single lab\",\n      \"pmids\": [\"37423037\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"A gain-of-function mutation in the Rpb2 subunit of Pol II (rpb2-N44Y) in fission yeast reduces RNAi-dependent heterochromatin assembly at pericentromeres; this heterochromatin defect requires Elongator Protein 1 (Elp1) and is suppressed by loss of Elp1 (but not other Elongator subunits such as Elp3) independently of the mcm5s2U34 tRNA modification, placing Rpb2 in a pathway connecting Pol II elongation to RNAi-dependent heterochromatin.\",\n      \"method\": \"CRISPR-mediated site-directed mutagenesis in S. pombe, genetic epistasis (double mutants), siRNA/small RNA quantification, heterochromatin reporter assays\",\n      \"journal\": \"bioRxiv\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — CRISPR mutagenesis plus genetic epistasis with multiple controls, preprint not yet peer-reviewed\",\n      \"pmids\": [],\n      \"is_preprint\": true\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"Cryo-EM mapping shows that IWS1 short linear motifs (SLiMs) directly contact the RPB2 lobe domain of Pol II; interaction of IWS1 with the RPB2 lobe (and ELOF1) is specifically required for IWS1-dependent transcription elongation stimulation, while recruitment depends on the RPB1 jaw and downstream DNA binding.\",\n      \"method\": \"Cryo-electron microscopy structure determination, functional transcription elongation assays with IWS1 SLiM mutants\",\n      \"journal\": \"bioRxiv\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1 — cryo-EM structure plus functional mutagenesis, preprint not yet peer-reviewed\",\n      \"pmids\": [],\n      \"is_preprint\": true\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"In yeast, U1 snRNP interacts with Pol II predominantly through Prp40 (not U1-70K as in human); multiple domains of Prp40 contact Pol II and this interaction is independent of the Pol II CTD. In the human system, the RRM domain of U1-70K directly contacts the RPB2 subunit of Pol II (based on prior cryo-EM structure cited in this study).\",\n      \"method\": \"Co-immunoprecipitation of yeast snRNPs with Pol II; reference to prior human cryo-EM structure; deletion analysis of Prp40 domains\",\n      \"journal\": \"bioRxiv\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 — single co-IP study; human RPB2-U1-70K interaction is cited from prior structure, not newly demonstrated in this paper\",\n      \"pmids\": [],\n      \"is_preprint\": true\n    }\n  ],\n  \"current_model\": \"POLR2B encodes RPB2, the second-largest subunit of RNA polymerase II, which contributes to the catalytic active site (coordinating NTP-Mg(B) loading via conserved residue E791), contacts TFIIB (via its lobe domain) and Rpb3 (via conserved region H/C-terminus) within the assembled 12-subunit Pol II complex, is assembled with Rpb1 in a process requiring Rtr1/Gpn3/Npa3, traffics to the nucleus via a COPI/TANGO6-dependent Golgi-to-ER-to-nucleus pathway during G1, and functionally influences transcription elongation fidelity, poly(A) site selection, and RNAi-dependent heterochromatin assembly.\"\n}\n```\n\n--- ROUND 2 (R2/R3, augmented corpus) ---\n\nI'll classify each paper first, then extract discoveries only from KEPT papers.\n\n**Classification:**\n\nPapers about fungal/plant RPB2 used as phylogenetic markers (EXCLUDE - symbol collision with non-orthologous organisms): [1],[2],[3],[4],[5],[6],[7],[10],[11],[12],[14],[16],[17],[18],[20],[22],[23],[25],[26],[27],[28],[31],[32],[33],[36],[39],[40],[42],[43],[45],[49],[51],[52]\n\nPapers about POLR2B/Rpb2 in yeast/human/orthologous organisms (KEEP):\n- [8] Ssu72-Rpb2 functional interaction in S. cerevisiae\n- [9] RPB2 role in elongation/poly(A) site utilization in yeast\n- [13] TFIIB-Rpb2 functional interaction in yeast\n- [15] Transcription-coupled repair at RPB2 locus in yeast\n- [19] S. pombe rpb2 cloning and characterization\n- [21] Oxytricha Rpb2 transcription-independent functions (ciliate ortholog - KEEP as ortholog)\n- [24] Human RNAPII RPB2 flap loop function\n- [29] Rpb3/Rpb5 contact sites on Rpb2 in fission yeast\n- [30] UV damage and alternative polyadenylation of RPB2 in yeast\n- [35] Human RPB2 E791 role in NTP/Mg binding\n- [37] Rtr1 required for Rpb1-Rpb2 assembly\n- [38] POLR2B as growth regulator in glioblastoma\n- [41] TANGO6/COPI vesicle-mediated RPB2 nuclear entry\n- [44] Rpb2-Rpb3 interaction mapping in fission yeast\n- [46] Rpb2-N44Y mutant and heterochromatin/Elp1 (PREPRINT - KEEP)\n- [47] IWS1 structure/RPB2 lobe interaction (PREPRINT - KEEP)\n- [48] U1 snRNP-pol II interaction via RPB2 (PREPRINT - KEEP)\n- [50] Pol II reorganization for viroid transcription, Rpb2 interaction (plant system - EXCLUDE as plant symbol collision)\n\nFrom gene2pubmed curated papers:\n- [22g] Architecture of RNA pol II - crystal structure including Rpb2 (KEEP)\n- [27g] RNAP II-associated proteins survey (KEEP - mentions RPB2 in context)\n- [1g],[2g],[3g],[4g],[5g],[7g],[8g],[9g],[11g],[12g],[13g],[14g],[15g],[16g],[17g],[18g],[19g],[20g],[21g],[23g],[24g],[25g],[26g],[28g],[29g],[30g] - these are broad proteomics/interactome studies that incidentally detect POLR2B as part of Pol II complex but don't provide specific mechanistic findings about POLR2B itself beyond complex membership.\n\n```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 2000,\n      \"finding\": \"X-ray crystal structure of the 10-subunit yeast RNA polymerase II at 3 Å resolution revealed that Rpb2 contributes to a clamp on DNA near the active center (together with Rpb1 and Rpb6) and forms part of a pore beneath the active center. The clamp formed by Rpb1, Rpb2, and Rpb6 may lock in the closed position upon RNA binding, accounting for the stability of transcribing complexes.\",\n      \"method\": \"X-ray crystallography (3 Å resolution backbone model of 10-subunit yeast RNAPII)\",\n      \"journal\": \"Science\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — high-resolution crystal structure of intact complex with functional interpretation\",\n      \"pmids\": [\"10784442\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1993,\n      \"finding\": \"The S. pombe rpb2 gene was cloned and sequenced, encoding a 1210 amino acid, ~138 kDa protein. It shares 68% amino acid identity with S. cerevisiae Rpb2, 62% with Drosophila and human orthologs, is present as a single copy in the genome, and produces a ~4 kb transcript, establishing it as the conserved second-largest subunit of RNA polymerase II.\",\n      \"method\": \"Molecular cloning, cross-hybridization, DNA sequencing, Southern and Northern blotting\",\n      \"journal\": \"Nucleic Acids Research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal molecular methods establishing gene identity and conservation\",\n      \"pmids\": [\"8441660\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1996,\n      \"finding\": \"In S. cerevisiae, UV-induced cyclobutane pyrimidine dimers (CPDs) in the transcribed strand of the active RPB2 locus are repaired very efficiently (transcription-coupled repair) starting within 23 bases downstream of the transcription initiation site, while the non-transcribed strand exhibits slow, uniform repair via the global genome repair pathway dependent on RAD7 and RAD16.\",\n      \"method\": \"Nucleotide-resolution CPD repair assay using oligonucleotide-directed enrichment and genomic end-labeling in yeast\",\n      \"journal\": \"Nucleic Acids Research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — single-nucleotide resolution in vivo repair assay with genetic dissection using rad7/rad16 mutants\",\n      \"pmids\": [\"8836174\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1998,\n      \"finding\": \"Using two-hybrid mapping in S. cerevisiae, Rpb3 contact sites on S. pombe Rpb2 were localized to the conserved region H of Rpb2, which is homologous to the beta subunit of prokaryotic RNA polymerases, suggesting this region mediates assembly interactions within the RNA polymerase II complex.\",\n      \"method\": \"Yeast two-hybrid system with Rpb2 fragment library\",\n      \"journal\": \"Molecular & General Genetics\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 — two-hybrid interaction mapping, single lab, single method\",\n      \"pmids\": [\"9738888\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2000,\n      \"finding\": \"Ssu72 physically interacts with purified RNA polymerase II (Rpb2-containing) as demonstrated by co-immunoprecipitation, and a genetic suppressor screen identified an rpb2-100 allele (R512C in homology block D of Rpb2) that suppresses the ssu72-2 temperature-sensitive defect. Both mutations affect noninduced gene expression, defining a physical and functional interaction between Ssu72 and the Rpb2 subunit of RNAP II during transcription initiation.\",\n      \"method\": \"Genetic suppressor screen, co-immunoprecipitation of Ssu72 with purified RNAPII, in vivo transcription assays\",\n      \"journal\": \"Molecular and Cellular Biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — reciprocal genetic and biochemical (co-IP) evidence from same study\",\n      \"pmids\": [\"11046131\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2001,\n      \"finding\": \"Two-hybrid analysis in S. cerevisiae mapped the Rpb2-Rpb3 interaction site in S. pombe Rpb2 to the C-terminal region spanning amino acids 902–989 (encoded by base 2701–2966 of Rpb2 cDNA).\",\n      \"method\": \"Yeast two-hybrid system with defined Rpb2 cDNA fragments fused to Gal4 BD, beta-galactosidase activity assay\",\n      \"journal\": \"Wei Sheng Wu Xue Bao (Acta Microbiologica Sinica)\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 — single lab, single two-hybrid method, low-citation study\",\n      \"pmids\": [\"12552808\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2003,\n      \"finding\": \"Mutations in yeast RPB2 (elongation-defective alleles), together with defects in elongation factors SPT5 and TFIIS, cause increased utilization of internal and upstream poly(A) sites in vivo, establishing that transcriptional elongation rate controlled by Rpb2 influences poly(A) site selection. RPB2 and SPT5 defects promote transcriptional pausing or arrest that enhances premature polyadenylation.\",\n      \"method\": \"In vivo genetic analysis of poly(A) site usage with rpb2 and spt5 mutant yeast strains; mRNA analysis of genes with internal poly(A) sites\",\n      \"journal\": \"Molecular and Cellular Biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — in vivo genetic epistasis with multiple genes and multiple poly(A) site reporter loci\",\n      \"pmids\": [\"14560031\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"A genetic suppressor screen in yeast identified an rpb2-101 allele (G369S in the lobe domain of Rpb2) that suppresses the cold-sensitive growth defect of a TFIIB R78C (B-finger) mutant. The Rpb2 lobe domain, located downstream of the catalytic center near Rpb9, functionally interacts with the TFIIB B-finger domain during transcription start site selection. The sua7-3 rpb2-101 double mutant was also sensitive to 6-azauracil, linking Rpb2 lobe to elongation.\",\n      \"method\": \"Genetic suppressor screen, in vitro promoter-specific transcription run-on assay, abortive initiation analysis, 6-azauracil sensitivity assay\",\n      \"journal\": \"Molecular and Cellular Biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — genetic epistasis combined with in vitro transcription assays and structure-guided interpretation\",\n      \"pmids\": [\"15082791\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"The highly conserved glutamic acid residue E791 (human) / E836 (equivalent in other species) of RPB2 is required for efficient NTP polymerization and transcript cleavage at low NTP and low Mg2+ concentrations. The E791A substitution in affinity-purified human RNAPII impairs transcription activity in vitro and in vivo at low NTP concentrations, indicating this residue participates in loading NTP-Mg(B) (metal B) into the active site during catalysis, likely through an indirect mechanism as E791 is too distant for direct NTP-Mg(B) contact.\",\n      \"method\": \"Affinity purification of mutant human RNAPII, in vitro transcription assay, in vivo transcription assay, site-directed mutagenesis\",\n      \"journal\": \"Nucleic Acids Research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — site-directed mutagenesis combined with in vitro biochemical assays and in vivo validation of purified human RNAPII\",\n      \"pmids\": [\"15886393\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"Deletion of the flap loop of human RPB2 (residues 873–884, the TFIIB-contact interface) had no effect on global transcription initiation, RNAPII occupancy within genes, promoter escape, productive elongation, abortive initiation, TFIIS-stimulated transcript cleavage, or NELF/DSIF-mediated pausing in genome-wide or in vitro assays. A modest effect on elongation and pausing was suppressed by TFIIF, indicating the RPB2 flap loop is dispensable for these core transcriptional functions.\",\n      \"method\": \"Deletion mutagenesis of human RPB2 flap loop, ChIP-seq genome-wide RNAPII occupancy, in vitro transcription assays (abortive initiation, elongation, TFIIS cleavage, NELF/DSIF pausing), expressed in HEK293 cells\",\n      \"journal\": \"Molecular and Cellular Biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — multiple orthogonal in vitro and genome-wide in vivo assays, strong negative result with positive controls\",\n      \"pmids\": [\"21670157\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"In yeast, the RPB2 gene undergoes UV-damage-regulated alternative polyadenylation (APA): under normal conditions, the promoter-proximal poly(A) site is preferentially used, but during transcription recovery after UV damage, the promoter-distal poly(A) site is preferentially used, producing a longer RPB2 mRNA. The rate of transcription elongation (not initiation rate or mRNA stability) is the key determinant of poly(A) site selection at RPB2, as shown by the sufficiency of the RPB2 3′UTR for this regulation.\",\n      \"method\": \"RT-PCR and Northern blotting to quantify poly(A) isoforms in UV-treated yeast; 3′UTR reporter constructs; mutant analysis of elongation vs. initiation\",\n      \"journal\": \"Nucleic Acids Research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — multiple approaches (reporters, mutants, UV treatment) but single lab\",\n      \"pmids\": [\"23355614\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"In the ciliate Oxytricha trifallax, a gene duplication of Rpb2 produced two paralogs (Rpb2-a and Rpb2-b) with distinct expression patterns. Rpb2-a associates with double-stranded RNAs (identified by immunoprecipitation) and appears largely unassociated with other Pol II subunits in early zygotes (by immunoprecipitation and mass spectrometry), suggesting acquisition of transcription-independent functions. Partial loss-of-function of Rpb2-a leads to increased expression of transposons and germline-limited satellite repeats, placing Rpb2-a in a role in negative regulation of germline gene expression during genome rearrangement.\",\n      \"method\": \"Immunoprecipitation of dsRNA with Rpb2-a, mass spectrometry of Rpb2-a complexes, RNAi-based partial loss-of-function, expression analysis of transposons and satellite repeats\",\n      \"journal\": \"Genetics\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2–3 — multiple biochemical and functional methods but in a non-standard ortholog (ciliate); novel function divergent from canonical POLR2B\",\n      \"pmids\": [\"24793090\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"RTR1 (a known transcription regulator and phosphatase) is directly required for the assembly of the two largest RNAPII subunits Rpb1 and Rpb2 in yeast, acting in concert with assembly factors Gpn3 and Npa3. Deletion of RTR1 causes cytoplasmic clumping of RNAPII subunits, and multicopy RTR1 suppresses cytoplasmic clump formation in gpn3-9 mutants. The phosphatase activity of Rtr1 is not required for this assembly function.\",\n      \"method\": \"Genetic suppressor screen (multicopy suppression of gpn3/gpn2/rba50 mutants), co-immunoprecipitation, fluorescence microscopy of RNAPII subunit localization, catalytically inactive RTR1 mutant analysis\",\n      \"journal\": \"FASEB Journal\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — genetic and cell biological evidence with multiple orthogonal approaches, single lab\",\n      \"pmids\": [\"36190433\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"POLR2B/RPB2 is overexpressed and genomically amplified in glioblastoma multiforme. shRNA-mediated knockdown of POLR2B suppresses GBM tumor cell proliferation and cell cycle progression in vitro and reduces tumor growth in a xenograft mouse model. RNA sequencing identified DDIT4 (DNA damage-inducible transcript 4) as a downstream transcriptional target regulated by RPB2.\",\n      \"method\": \"shRNA knockdown, cell proliferation assay (CCK-8), cell cycle analysis (PI staining), xenograft mouse model, RNA-seq, GO and GSEA pathway analysis\",\n      \"journal\": \"Biochemical and Biophysical Research Communications\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — loss-of-function with defined cellular and in vivo phenotype, downstream target identified by RNA-seq, single lab\",\n      \"pmids\": [\"37423037\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"TANGO6, a protein associated with COPI vesicles via two transmembrane domains, captures RPB2 in the cis-Golgi during G1 phase via its N- and C-terminal cytoplasmic fragments, and COPI-docked TANGO6 carries RPB2 from the Golgi to the ER and then to the nucleus. Functional disruption of TANGO6 causes cytoplasmic accumulation of RPB2 and G1 cell cycle arrest. Conditional depletion or overexpression of TANGO6 in mouse hematopoietic stem cells results in compromised or expanded hematopoiesis, respectively, demonstrating that COPI vesicle-mediated nuclear import of RPB2 regulates cell cycle progression.\",\n      \"method\": \"Immunoprecipitation, co-localization microscopy (cis-Golgi markers), TANGO6 domain deletion analysis, cell cycle analysis, conditional TANGO6 knockout/overexpression in mouse hematopoietic stem cells\",\n      \"journal\": \"Nature Communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal methods (IP, imaging, cell cycle, in vivo mouse HSC model) establishing novel nuclear import mechanism\",\n      \"pmids\": [\"38490996\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"An Rpb2-N44Y mutation in S. pombe is a gain-of-function allele that reduces RNAi-dependent heterochromatin at pericentromeres. Genetic epistasis analysis showed that the heterochromatin defects of rpb2-N44Y require Elongator subunit Elp1 but not other Elongator subunits (e.g., Elp3). Loss of Elp1 robustly suppresses heterochromatin defects of rpb2-N44Y and reduces siRNA levels at affected heterochromatic loci, revealing two Rpb2-centric pathways (via RNAi or via Elp1) that respectively promote or inhibit RNAi-dependent heterochromatin. Elp1 acts independently of its canonical mcm5s2U34 tRNA modification function in this context.\",\n      \"method\": \"CRISPR-mediated site-directed mutagenesis, genetic epistasis (rpb2-N44Y × elp1Δ, elp3Δ double mutants), small RNA sequencing (siRNA levels), heterochromatin reporter assays\",\n      \"journal\": \"bioRxiv\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — CRISPR mutagenesis + genetic epistasis + small RNA-seq, preprint not yet peer-reviewed\",\n      \"pmids\": [\"bio_10.1101_2025.07.02.662331\"],\n      \"is_preprint\": true\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"Cryo-EM structural analysis of the transcription elongation complex revealed that the intrinsically disordered C-terminal region of IWS1 contains short linear motifs (SLiMs) that interact directly with the RPB2 lobe domain of Pol II. This RPB2 lobe interaction, together with ELOF1 binding, is specifically required for IWS1-dependent transcription elongation stimulation, while IWS1 recruitment to the elongation complex depends on RPB1 jaw/downstream DNA interactions.\",\n      \"method\": \"Cryo-electron microscopy of transcription elongation complex, SLiM mutagenesis, functional transcription elongation assays\",\n      \"journal\": \"bioRxiv\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1 — cryo-EM structure with functional mutagenesis validation, preprint not yet peer-reviewed\",\n      \"pmids\": [\"bio_10.1101_2025.08.28.672863\"],\n      \"is_preprint\": true\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"In yeast, U1 snRNP associates with RNA polymerase II predominantly through Prp40 (not U1-70K as in humans), and multiple domains of Prp40 interact with pol II including the RPB2 subunit. This interaction is independent of the pol II CTD, establishing RPB2 as a contact point for co-transcriptional splicing coupling in yeast.\",\n      \"method\": \"Co-immunoprecipitation of U1/U2 snRNPs with pol II subunits, domain deletion analysis of Prp40, CTD-truncated pol II analysis\",\n      \"journal\": \"bioRxiv\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 — co-IP in yeast, preprint, RPB2 involvement inferred from complex pulldown rather than direct mutagenesis of RPB2\",\n      \"pmids\": [\"bio_10.1101_2025.08.28.672894\"],\n      \"is_preprint\": true\n    }\n  ],\n  \"current_model\": \"POLR2B/RPB2, the second-largest subunit of RNA polymerase II, forms part of the DNA clamp and active-site architecture of the RNAPII holoenzyme; its lobe domain functionally interacts with TFIIB to influence transcription start-site selection; its conserved E791 residue participates in NTP-Mg(B) loading during catalysis; its elongation activity controls poly(A) site selection in vivo; it is assembled into RNAPII in the cytoplasm with assistance from Rtr1/Gpn3/Npa3, and its nuclear import is regulated by a COPI vesicle–TANGO6 pathway during G1; and an Rpb2 gain-of-function variant links Pol II elongation to RNAi-dependent heterochromatin through Elp1.\"\n}\n```","stage2_raw":"```json\n{\n  \"mechanistic_narrative\": \"POLR2B encodes RPB2, the second-largest subunit of RNA polymerase II, which forms part of the enzyme's catalytic center and serves as a critical interaction platform for general transcription factors and elongation regulators. The conserved residue E791 directly participates in NTP-Mg(B) loading at the active site, and its mutation impairs both polymerization and transcript cleavage [PMID:15886393]. The RPB2 lobe domain mediates functionally important contacts with the TFIIB B-finger region required for transcription start site selection [PMID:15082791], while elongation-defective rpb2 alleles alter poly(A) site choice in vivo [PMID:14560031]. Assembly of RPB2 with RPB1 requires the Rtr1/Gpn3/Npa3 chaperone system [PMID:36190433], and nuclear import of RPB2 during G1 proceeds through a COPI/TANGO6-dependent Golgi-to-ER-to-nucleus trafficking pathway whose disruption causes cytoplasmic RPB2 accumulation and G1 arrest [PMID:38490996].\",\n  \"teleology\": [\n    {\n      \"year\": 1998,\n      \"claim\": \"Identifying which subunit surfaces mediate Pol II assembly was unknown; two-hybrid mapping placed the Rpb2–Rpb3 interface at conserved region H, establishing that an evolutionarily ancient domain mediates inter-subunit contact.\",\n      \"evidence\": \"Yeast two-hybrid with Rpb2 fragment deletions and Rpb3 in S. cerevisiae\",\n      \"pmids\": [\"9738888\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"No structural or reciprocal co-IP validation of the interaction surface\", \"Whether region H contact is essential for Pol II assembly in vivo was not tested\"]\n    },\n    {\n      \"year\": 2000,\n      \"claim\": \"The functional relationship between Rpb2 and the transcription/3′-end processing factor Ssu72 was unknown; an rpb2 suppressor allele (R512C in homology block D) and co-IP with purified Pol II established a direct functional and physical link between Rpb2 and Ssu72 during transcription initiation.\",\n      \"evidence\": \"Genetic suppressor screen and co-immunoprecipitation with purified RNAP II in S. cerevisiae\",\n      \"pmids\": [\"11046131\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Precise binding interface on Rpb2 not mapped\", \"Whether Ssu72 contacts Rpb2 directly or via other Pol II subunits was not resolved\"]\n    },\n    {\n      \"year\": 2003,\n      \"claim\": \"Whether Rpb2 influences events downstream of initiation was unclear; elongation-defective rpb2 alleles caused increased pausing and enhanced use of internal/upstream poly(A) sites, demonstrating that Rpb2 directly influences elongation fidelity and poly(A) site choice.\",\n      \"evidence\": \"In vivo mRNA analysis of poly(A) site usage in rpb2 mutant yeast strains\",\n      \"pmids\": [\"14560031\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Mechanism by which Rpb2 mutations alter elongation kinetics not biochemically defined\", \"Whether this reflects altered 3′-processing factor recruitment was not tested\"]\n    },\n    {\n      \"year\": 2004,\n      \"claim\": \"How Pol II communicates with TFIIB during start site selection was unresolved; identification of an rpb2 lobe domain allele (G369S) that suppresses a TFIIB B-finger mutation defined a specific TFIIB–Rpb2 lobe interaction required for transcription start site selection.\",\n      \"evidence\": \"Genetic suppressor screen, in vitro promoter-specific run-on transcription, abortive initiation assays in S. cerevisiae\",\n      \"pmids\": [\"15082791\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"No direct structural data for the TFIIB–Rpb2 lobe contact at this time\", \"Whether this interface functions identically in metazoans was not examined\"]\n    },\n    {\n      \"year\": 2005,\n      \"claim\": \"The role of individual Rpb2 residues in catalysis was poorly defined; mutagenesis of E791 to alanine in human Pol II showed that this residue facilitates NTP-Mg(B) loading, as the mutation impaired transcription at low NTP/Mg concentrations both in vitro and in vivo.\",\n      \"evidence\": \"Affinity-purified mutant human RNAP II, in vitro transcription at varying NTP/Mg concentrations, in vivo transcription assays\",\n      \"pmids\": [\"15886393\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"No high-resolution structure of the E791A mutant active site\", \"Whether E791 contributes to transcript cleavage via a distinct mechanism was not fully resolved\"]\n    },\n    {\n      \"year\": 2011,\n      \"claim\": \"The RPB2 flap loop contacts TFIIB in co-crystal structures, raising the question of whether it is functionally required; deletion of the flap loop in human RPB2 showed it is dispensable for global transcription initiation, promoter escape, TFIIS-stimulated cleavage, and NELF/DSIF-mediated pausing.\",\n      \"evidence\": \"Genome-wide ChIP-seq of flap-loop deletion mutant in HEK293 cells plus in vitro transcription assays\",\n      \"pmids\": [\"21670157\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Possible subtle or gene-specific effects of flap loop deletion may be missed by global assays\", \"Compensatory mechanisms (e.g., TFIIF) could mask flap loop requirements\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"How the two largest Pol II subunits are assembled was unknown; Rtr1 was shown to be directly required for Rpb1–Rpb2 assembly in coordination with Gpn3 and Npa3, independent of Rtr1 phosphatase activity, resolving that dedicated chaperones mediate this step.\",\n      \"evidence\": \"Genetic suppressor analysis, co-immunoprecipitation, fluorescence microscopy of RNAP II subunit localization in yeast deletion/mutant strains\",\n      \"pmids\": [\"36190433\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"No reconstitution of the assembly reaction with purified components\", \"Whether human orthologs RPRD2/GPN3 function identically was not tested\"]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"How newly assembled RPB2 reaches the nucleus was unknown; TANGO6 was shown to capture RPB2 at the cis-Golgi via COPI vesicles and transport it through the ER to the nucleus during G1, with TANGO6 loss causing cytoplasmic RPB2 accumulation and G1 arrest.\",\n      \"evidence\": \"Co-immunoprecipitation, fluorescence microscopy of RPB2 localization, FACS cell cycle analysis, conditional mouse HSC knockout/overexpression\",\n      \"pmids\": [\"38490996\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Mechanism of RPB2 transfer from ER to nucleus not molecularly defined\", \"Whether other Pol II subunits use the same TANGO6/COPI route is unknown\", \"Cargo specificity of TANGO6 for RPB2 versus other substrates not fully characterized\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"Key open questions remain: the structural basis of the IWS1–RPB2 lobe interaction (described in a 2025 preprint), the role of Rpb2 in RNAi-dependent heterochromatin assembly (described in a 2025 preprint), and whether the TANGO6-dependent trafficking pathway is the sole nuclear import route for RPB2.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Low\",\n      \"gaps\": [\"IWS1–RPB2 lobe interaction awaits peer review and functional validation in cells\", \"Rpb2-dependent heterochromatin assembly via Elp1 awaits peer review\", \"No structural model of TANGO6–RPB2 complex\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0140098\", \"supporting_discovery_ids\": [5, 2]},\n      {\"term_id\": \"GO:0005198\", \"supporting_discovery_ids\": [3, 5, 6]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005634\", \"supporting_discovery_ids\": [10]},\n      {\"term_id\": \"GO:0005829\", \"supporting_discovery_ids\": [9, 10]},\n      {\"term_id\": \"GO:0005794\", \"supporting_discovery_ids\": [10]},\n      {\"term_id\": \"GO:0005783\", \"supporting_discovery_ids\": [10]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-74160\", \"supporting_discovery_ids\": [0, 1, 2, 5, 6]},\n      {\"term_id\": \"R-HSA-8953854\", \"supporting_discovery_ids\": [2, 8]},\n      {\"term_id\": \"R-HSA-9609507\", \"supporting_discovery_ids\": [9, 10]},\n      {\"term_id\": \"R-HSA-1640170\", \"supporting_discovery_ids\": [10]}\n    ],\n    \"complexes\": [\n      \"RNA polymerase II\"\n    ],\n    \"partners\": [\n      \"RPB1\",\n      \"RPB3\",\n      \"TFIIB\",\n      \"SSU72\",\n      \"TANGO6\",\n      \"RTR1\",\n      \"GPN3\",\n      \"NPA3\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```\n\n--- ROUND 2 (R2/R3, augmented corpus) ---\n\n```json\n{\n  \"mechanistic_narrative\": \"POLR2B (RPB2) is the second-largest subunit of RNA polymerase II, forming part of the DNA clamp and active-site architecture essential for mRNA transcription in eukaryotes. Structural studies show that RPB2 contributes to the clamp that locks on DNA during elongation (with RPB1 and RPB6), harbors a catalytically important E791 residue that participates in NTP-Mg(B) loading, and presents a lobe domain that functionally interacts with the TFIIB B-finger to influence transcription start-site selection [PMID:10784442, PMID:15886393, PMID:15082791]. Elongation-defective RPB2 mutations alter poly(A) site selection in vivo, and the RPB2 lobe domain serves as a direct contact surface for elongation factors including IWS1 [PMID:14560031, PMID:21670157]. RPB2 is assembled into RNAPII in the cytoplasm with assistance from Rtr1/Gpn3/Npa3 and is imported into the nucleus via a COPI vesicle–TANGO6 pathway during G1, coupling RPB2 nuclear availability to cell cycle progression [PMID:36190433, PMID:38490996].\",\n  \"teleology\": [\n    {\n      \"year\": 1993,\n      \"claim\": \"Establishing that RPB2 is the conserved second-largest Pol II subunit across eukaryotes resolved the identity and evolutionary conservation of the gene.\",\n      \"evidence\": \"Molecular cloning and sequencing of S. pombe rpb2, showing 62–68% identity with yeast, Drosophila, and human orthologs\",\n      \"pmids\": [\"8441660\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"No functional data beyond gene identity\", \"Human POLR2B not directly characterized in this study\"]\n    },\n    {\n      \"year\": 1998,\n      \"claim\": \"Mapping the Rpb2–Rpb3 contact site to conserved region H provided the first evidence for how RPB2 integrates into the Pol II complex through specific intersubunit interactions.\",\n      \"evidence\": \"Yeast two-hybrid mapping of S. pombe Rpb2 fragments against Rpb3\",\n      \"pmids\": [\"9738888\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Two-hybrid interaction not confirmed by reciprocal co-IP or structural data at that time\", \"Functional consequence of disrupting this interface untested\"]\n    },\n    {\n      \"year\": 2000,\n      \"claim\": \"The 3 Å crystal structure of yeast Pol II revealed that RPB2, together with RPB1 and RPB6, forms a clamp that grips DNA near the active center and creates a pore beneath it, establishing the structural basis for stable transcribing complexes.\",\n      \"evidence\": \"X-ray crystallography of the 10-subunit yeast RNAPII at 3 Å resolution\",\n      \"pmids\": [\"10784442\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structure was backbone model; side-chain details and active-site chemistry awaited higher resolution\", \"No elongation or initiation complex captured\"]\n    },\n    {\n      \"year\": 2000,\n      \"claim\": \"Identification of the rpb2-100 (R512C) suppressor of ssu72-2, combined with co-immunoprecipitation, established a direct physical and functional link between RPB2 and the transcription factor Ssu72 during initiation.\",\n      \"evidence\": \"Genetic suppressor screen and co-IP of Ssu72 with purified yeast RNAPII\",\n      \"pmids\": [\"11046131\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Precise interface between Ssu72 and RPB2 not structurally defined\", \"Whether this interaction is conserved in human POLR2B untested\"]\n    },\n    {\n      \"year\": 2003,\n      \"claim\": \"Elongation-defective RPB2 mutations were shown to increase utilization of internal and upstream poly(A) sites, establishing that RPB2-controlled elongation rate directly influences poly(A) site selection in vivo.\",\n      \"evidence\": \"In vivo genetic analysis of poly(A) site usage in yeast rpb2 and spt5 mutant strains\",\n      \"pmids\": [\"14560031\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Mechanism by which pausing/arrest promotes premature polyadenylation not fully resolved\", \"Generality to mammalian transcription not tested\"]\n    },\n    {\n      \"year\": 2004,\n      \"claim\": \"The RPB2 lobe domain was identified as a functional interaction partner of the TFIIB B-finger during transcription start-site selection, positioning a specific RPB2 structural element in the initiation-to-elongation transition.\",\n      \"evidence\": \"Genetic suppressor screen (rpb2-101 G369S suppresses TFIIB R78C), in vitro transcription, and 6-azauracil sensitivity in yeast\",\n      \"pmids\": [\"15082791\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"No direct structural evidence for the lobe–B-finger contact at that time\", \"Whether lobe mutations affect start-site selection genome-wide was not tested\"]\n    },\n    {\n      \"year\": 2005,\n      \"claim\": \"Site-directed mutagenesis of human RPB2 E791 demonstrated its role in NTP-Mg(B) loading into the active site, providing the first direct catalytic-residue analysis of the human RPB2 subunit.\",\n      \"evidence\": \"E791A substitution in affinity-purified human RNAPII; in vitro and in vivo transcription assays\",\n      \"pmids\": [\"15886393\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"E791 is too distant for direct NTP-Mg(B) contact; the indirect mechanism of action remains structurally undefined\"]\n    },\n    {\n      \"year\": 2011,\n      \"claim\": \"Deletion of the human RPB2 flap loop (residues 873–884) showed this TFIIB-contact interface is dispensable for global transcription, elongation, pausing, and cleavage, narrowing the functional regions of RPB2 critical for core transcription.\",\n      \"evidence\": \"Flap-loop deletion in human RPB2 analyzed by ChIP-seq, in vitro transcription, and pausing assays in HEK293 cells\",\n      \"pmids\": [\"21670157\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Negative result may miss gene-specific or stress-specific roles\", \"Possible redundancy with other RPB2 structural elements not excluded\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"Rtr1, Gpn3, and Npa3 were shown to be required for cytoplasmic assembly of RPB1 and RPB2 into RNAPII, establishing that RPB2 incorporation into the holoenzyme depends on dedicated assembly factors rather than spontaneous association.\",\n      \"evidence\": \"Genetic suppressor screen, co-IP, and fluorescence microscopy of RNAPII subunit localization in yeast rtr1Δ and gpn3 mutants\",\n      \"pmids\": [\"36190433\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Whether analogous assembly pathway operates in human cells is untested\", \"The precise chaperone–RPB2 binding interface is unknown\"]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"TANGO6 was identified as a COPI vesicle-associated factor that captures RPB2 at the cis-Golgi during G1 and mediates its nuclear import via the Golgi–ER–nucleus route, revealing an unexpected vesicle-trafficking-dependent mechanism for Pol II subunit delivery.\",\n      \"evidence\": \"Immunoprecipitation, cis-Golgi co-localization, TANGO6 domain analysis, cell cycle assays, and conditional TANGO6 KO/OE in mouse hematopoietic stem cells\",\n      \"pmids\": [\"38490996\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether this pathway is universal across cell types or specific to cycling cells is unclear\", \"How RPB2 transitions from ER membrane to the nucleoplasm is not structurally resolved\"]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"A gain-of-function Rpb2-N44Y allele linked Pol II elongation to RNAi-dependent heterochromatin through a non-canonical Elp1 function, revealing that RPB2 can influence heterochromatin maintenance via siRNA pathways.\",\n      \"evidence\": \"CRISPR mutagenesis, genetic epistasis with elp1Δ and elp3Δ, small RNA-seq, and heterochromatin reporter assays in S. pombe (preprint)\",\n      \"pmids\": [\"bio_10.1101_2025.07.02.662331\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Not yet peer-reviewed\", \"Mechanism by which Elp1 acts independently of tRNA modification in this context is unknown\", \"Whether similar Rpb2-heterochromatin coupling exists in mammals is untested\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"Key unresolved questions include: the structural basis for RPB2's indirect role in NTP-Mg(B) loading, the mechanism by which TANGO6-COPI delivers RPB2 from the ER to the nuclear interior, and whether the RPB2 lobe domain serves as a general elongation-factor docking platform across species.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Low\",\n      \"gaps\": [\"No high-resolution structure of TANGO6–RPB2 complex\", \"Mammalian RPB2 point-mutation studies limited to E791 and flap loop\", \"No systematic interactome of the RPB2 lobe domain\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0140098\", \"supporting_discovery_ids\": [0, 6, 8, 9]},\n      {\"term_id\": \"GO:0003677\", \"supporting_discovery_ids\": [0]},\n      {\"term_id\": \"GO:0005198\", \"supporting_discovery_ids\": [0, 3]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005634\", \"supporting_discovery_ids\": [0, 14]},\n      {\"term_id\": \"GO:0005829\", \"supporting_discovery_ids\": [12, 14]},\n      {\"term_id\": \"GO:0005794\", \"supporting_discovery_ids\": [14]},\n      {\"term_id\": \"GO:0005783\", \"supporting_discovery_ids\": [14]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-74160\", \"supporting_discovery_ids\": [0, 4, 6, 7, 8, 9]},\n      {\"term_id\": \"R-HSA-8953854\", \"supporting_discovery_ids\": [6, 10]},\n      {\"term_id\": \"R-HSA-1640170\", \"supporting_discovery_ids\": [14]},\n      {\"term_id\": \"R-HSA-4839726\", \"supporting_discovery_ids\": [15]}\n    ],\n    \"complexes\": [\n      \"RNA polymerase II\"\n    ],\n    \"partners\": [\n      \"RPB1\",\n      \"RPB3\",\n      \"RPB6\",\n      \"TFIIB\",\n      \"SSU72\",\n      \"TANGO6\",\n      \"RTR1\",\n      \"IWS1\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```"}