{"gene":"MED20","run_date":"2026-04-28T18:30:28","timeline":{"discoveries":[{"year":2006,"finding":"Med8, Med18, and Med20 form a head subcomplex (Med8/18/20) with two submodules: the N-terminal domain of Med8 binds TBP in vitro and is essential in vivo, while the C-terminal region of Med8 (Med8C), Med18, and Med20 form a second submodule. X-ray crystallography revealed that Med18 and Med20 adopt related beta-barrel folds, and a conserved putative protein-interaction face on the Med8C/18/20 submodule includes sites altered by srb mutations that counteract defects from RNA Pol II truncation.","method":"X-ray crystallography, in vitro TBP binding assay, genetic analysis of srb mutations","journal":"Nature structural & molecular biology","confidence":"High","confidence_rationale":"Tier 1 — crystal structure combined with in vitro binding and in vivo genetic validation in single rigorous study","pmids":["16964259"],"is_preprint":false},{"year":1996,"finding":"Yeast SRB2 (Med20 ortholog) is required for normal DNA repair of MMS-induced damage and modulates transcription-associated recombination; loss of SRB2 suppresses hyperrecombination between direct repeats in hpr1Δ cells, linking this basal transcription factor to both recombination and DNA repair pathways.","method":"Genetic suppressor screen, null allele construction, MMS sensitivity assays, recombination frequency measurement","journal":"Genetics","confidence":"Medium","confidence_rationale":"Tier 2 — genetic epistasis with defined phenotypic readouts, single lab","pmids":["8844143"],"is_preprint":false},{"year":2008,"finding":"In Saccharomyces cerevisiae, Med20 (head module subunit) plays a selective role in transcriptional repression of ribosomal protein (RP) genes following rapamycin treatment; gene expression profiling after deletion of Med20 revealed a prominent and highly selective defect in RP gene repression under multiple stress conditions, functioning in a pathway parallel to Maf1.","method":"Genetic interaction screens (synthetic sick/lethal), gene expression profiling (transcriptomics), rapamycin treatment assays","journal":"PLoS genetics","confidence":"Medium","confidence_rationale":"Tier 2 — genome-scale genetic screen with expression profiling validation, single lab","pmids":["18604275"],"is_preprint":false},{"year":2009,"finding":"Med8, Med18, and Med20 are interdependent for proper folding and trimeric complex formation; concurrent presence of all three subunits during renaturation is required for correct trimer assembly, whereas pairwise combinations yield distinct, partially folded subcomplexes.","method":"Immunoprecipitation, far-UV circular dichroism spectroscopy, fluorescence measurements on recombinantly expressed and renatured proteins","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 1 — in vitro reconstitution with multiple orthogonal biophysical methods (CD, fluorescence, IP)","pmids":["19934057"],"is_preprint":false},{"year":2014,"finding":"A homozygous missense mutation in MED20 (p.Gly114Ala) in human patients causes infantile-onset spasticity, dystonia, progressive basal ganglia degeneration, and brain atrophy. Structural modeling suggests the mutation disrupts the Med20 beta-barrel fold at a conserved position within the Med8C/18/20 submodule, establishing MED20 as essential for normal neuronal function.","method":"Whole-exome sequencing, genetic linkage analysis, in silico pathogenicity analysis, structural modeling","journal":"European journal of pediatrics","confidence":"Low","confidence_rationale":"Tier 3 — human genetics with structural modeling but no direct functional rescue experiment; pathogenicity inferred computationally","pmids":["25446406"],"is_preprint":false},{"year":2015,"finding":"In fission yeast, loss of Med20 leads to accumulation of aberrant, polyadenylated readthrough tRNA transcripts that are targeted for degradation by the exosome; other non-coding RNAs (snRNA, snoRNA, rRNA) are also enriched in polyadenylate fractions, indicating that Med20-containing Mediator participates in a regulatory pathway affecting RNA Pol III-dependent transcripts.","method":"Northern blotting, RT-PCR, polyadenylate RNA enrichment, genetic deletion","journal":"Biochimica et biophysica acta","confidence":"Medium","confidence_rationale":"Tier 2 — loss-of-function with multiple RNA species analyzed by orthogonal methods, single lab","pmids":["26608234"],"is_preprint":false},{"year":2017,"finding":"The heterotrimeric pre-mRNA retention and splicing (RES) complex (Bud13p/Snu17p/Pml1p) controls Med20 protein levels by promoting efficient splicing of MED20 pre-mRNA; inefficient splicing in bud13Δ and snu17Δ cells leads to NMD-mediated degradation of MED20 pre-mRNA, reducing Med20 protein and causing temperature-sensitive growth defects.","method":"Genetic deletion analysis, RT-PCR splicing assays, NMD pathway inactivation, growth phenotype complementation","journal":"RNA biology","confidence":"Medium","confidence_rationale":"Tier 2 — genetic epistasis with molecular splicing readout and NMD suppression rescue, single lab","pmids":["28277935"],"is_preprint":false},{"year":2019,"finding":"MED20 is essential for early mammalian embryo development; Med20 mutant mouse embryos arrest after gastrulation, fail to hatch from the zona pellucida, and show ectopic NANOG expression in the trophectoderm, indicating MED20 is specifically required for trophoblast lineage specification and suppression of epiblast identity in trophectoderm.","method":"Conditional knockout mouse model, blastocyst outgrowth assay, immunofluorescence for lineage markers (NANOG, CDX2, SOX17), TUNEL apoptosis assay","journal":"Reproduction (Cambridge, England)","confidence":"Medium","confidence_rationale":"Tier 2 — clean KO with defined molecular phenotype (ectopic NANOG) and multiple lineage markers, single lab","pmids":["30571656"],"is_preprint":false},{"year":2021,"finding":"MED20 is a substrate of the anti-obesity CRL4-WDTC1 E3 ubiquitin ligase complex; overexpression of WDTC1 leads to MED20 degradation, while depletion of WDTC1 or CUL4A/B causes MED20 accumulation. MED20 promotes adipogenesis by bridging C/EBPβ and RNA polymerase II at the PPARγ promoter, as shown by ChIP-seq; knockout of Med20 in preadipocytes abolishes brown adipose tissue development and protects mice from diet-induced obesity.","method":"Affinity purification and candidate screening, ubiquitin ligase overexpression/depletion, non-degradable mutant rescue, Med20 knockout in preadipocytes (in vivo), ChIP-seq for C/EBPβ and Pol II","journal":"Cell reports","confidence":"High","confidence_rationale":"Tier 1–2 — multiple orthogonal methods (AP, ChIP-seq, KO mouse, rescue with non-degradable mutant) in single rigorous study","pmids":["34233190"],"is_preprint":false},{"year":2025,"finding":"Med20 regulates myelination in the peripheral nervous system by preventing ferroptosis in Schwann cells; loss of Med20 in Schwann cells induces ferroptosis and impairs myelination. Mechanistically, Med20 acts through its downstream target DDB1, which controls ferroptosis via a 'DDB1-UHRF1-BACH1-Hmox1' transcriptional axis and by directly interacting with and ubiquitinating HO-1 protein. Pharmacological inhibition of HO-1 (ZnPP) or ferroptosis (Fer-1) rescues myelination in Med20-deficient mice.","method":"Schwann cell-specific conditional knockout, ferroptosis assays, ChIP/transcriptional reporter assays, co-immunoprecipitation (DDB1-HO-1 interaction), ubiquitination assay, pharmacological rescue (ZnPP, Fer-1), electron microscopy of myelin","journal":"Cell reports","confidence":"High","confidence_rationale":"Tier 1–2 — conditional KO with defined molecular mechanism, multiple pathway components validated by orthogonal methods, pharmacological rescue","pmids":["41108685"],"is_preprint":false},{"year":2004,"finding":"MED20 was identified as a consensus subunit of the mammalian Mediator complex by comparative MudPIT proteomic analysis across six independent immunoaffinity-purified Mediator preparations, establishing it as a core component of the human Mediator.","method":"Multidimensional protein identification technology (MudPIT), immunoaffinity purification of Mediator through multiple subunit baits","journal":"Molecular cell","confidence":"Medium","confidence_rationale":"Tier 2 — MS-based identification replicated across six independent preparations","pmids":["15175163"],"is_preprint":false},{"year":2014,"finding":"EM structural analysis of yeast and human Mediator complexes mapped subunit localization and confirmed that the head module containing Med20 (Med8/18/20 subcomplex) occupies a defined position in the overall Mediator architecture and participates in large-scale conformational rearrangements at module interfaces that are conducive to transcription initiation.","method":"Cryo-EM single-particle analysis, subunit localization by antibody labeling and tagged-subunit EM, biochemical analysis of module interfaces","journal":"Cell","confidence":"High","confidence_rationale":"Tier 1 — cryo-EM structure with biochemical validation and subunit localization, replicated in yeast and human Mediator","pmids":["24882805"],"is_preprint":false}],"current_model":"MED20 is a core subunit of the Mediator head module that forms an interdependent trimeric subcomplex with Med8 and Med18 (adopting a beta-barrel fold), binds TBP to support RNA Pol II transcription initiation, selectively represses ribosomal protein gene transcription and influences Pol III-dependent RNA processing, is subject to CRL4-WDTC1-mediated ubiquitin-dependent degradation that regulates its role in bridging C/EBPβ and Pol II for PPARγ-driven adipogenesis, is essential for early embryonic trophoblast specification (suppressing ectopic NANOG), and in Schwann cells prevents ferroptosis through a DDB1-UHRF1-BACH1-Hmox1 axis to maintain peripheral nervous system myelination."},"narrative":{"teleology":[{"year":1996,"claim":"The yeast MED20 ortholog SRB2 was first linked to genome integrity beyond basal transcription, showing that its loss suppresses transcription-coupled hyperrecombination and impairs DNA repair of MMS-induced damage.","evidence":"Genetic suppressor screen and MMS sensitivity assays in S. cerevisiae srb2Δ mutants","pmids":["8844143"],"confidence":"Medium","gaps":["No direct biochemical mechanism for the DNA repair defect","Not confirmed in mammalian cells","Unclear whether effect is through Mediator-dependent transcription or an independent function"]},{"year":2004,"claim":"Proteomic cataloguing established MED20 as a consensus subunit of the mammalian Mediator complex, placing it in the head module alongside Med8 and Med18 and confirming evolutionary conservation from yeast to human.","evidence":"MudPIT analysis of six independent immunoaffinity-purified human Mediator preparations","pmids":["15175163"],"confidence":"Medium","gaps":["No direct structural data for human MED20 at the time","Functional role within human Mediator not yet tested"]},{"year":2006,"claim":"Crystal structure of the Med8C/Med18/Med20 subcomplex revealed that Med20 adopts a beta-barrel fold and together with Med18 forms a conserved protein-interaction surface implicated in coupling TBP binding to Pol II activation, resolving how the head module contacts basal transcription machinery.","evidence":"X-ray crystallography of yeast Med8C/18/20 trimer, in vitro TBP binding, genetic analysis of srb mutations","pmids":["16964259"],"confidence":"High","gaps":["Full-length Med8 interactions not structurally resolved","No structure of Med20 in the context of complete Mediator"]},{"year":2008,"claim":"Deletion of Med20 in yeast uncovered a highly selective role in transcriptional repression of ribosomal protein genes under stress, demonstrating that individual Mediator subunits can have gene-class-specific regulatory functions beyond global transcription.","evidence":"Genome-wide expression profiling and genetic interaction screens in S. cerevisiae med20Δ upon rapamycin treatment","pmids":["18604275"],"confidence":"Medium","gaps":["Mechanism of selective RP gene repression not determined","Relationship to mammalian RP gene regulation unknown"]},{"year":2009,"claim":"Biophysical reconstitution showed that Med8, Med18, and Med20 are mutually dependent for correct folding—requiring simultaneous co-folding of all three subunits—establishing the trimer as an obligate assembly unit within the head module.","evidence":"Renaturation of recombinant subunits with circular dichroism and fluorescence spectroscopy","pmids":["19934057"],"confidence":"High","gaps":["In vivo assembly pathway and chaperone requirements unknown","Post-translational modifications during assembly not addressed"]},{"year":2014,"claim":"Cryo-EM mapping of yeast and human Mediator placed the Med8/18/20 trimer at a defined head module position and showed it participates in large-scale conformational changes at module interfaces during transcription initiation, providing a structural framework for its regulatory function.","evidence":"Cryo-EM single-particle analysis with antibody and tag-based subunit localization in yeast and human Mediator","pmids":["24882805"],"confidence":"High","gaps":["High-resolution structure of Med20 in full PIC context not yet available at this time","Conformational dynamics during active transcription not captured"]},{"year":2014,"claim":"A homozygous MED20 missense mutation (p.Gly114Ala) was identified as the cause of infantile basal ganglia degeneration with spasticity and dystonia, establishing the first human Mendelian disorder linked to MED20 and demonstrating its essential role in neuronal maintenance.","evidence":"Whole-exome sequencing and linkage analysis in consanguineous family, structural modeling of the variant","pmids":["25446406"],"confidence":"Low","gaps":["No functional rescue experiment performed","Pathogenicity assessed only by in silico modeling, not by biochemical or cellular assay","Single family reported"]},{"year":2015,"claim":"Loss of Med20 in fission yeast caused accumulation of aberrant polyadenylated tRNA and other non-coding RNA transcripts, revealing an unexpected role for Mediator in regulating Pol III-dependent transcript processing and quality control.","evidence":"Northern blot and RT-PCR analysis of polyadenylated RNA fractions in S. pombe med20Δ cells","pmids":["26608234"],"confidence":"Medium","gaps":["Whether Med20 acts directly on Pol III or indirectly through Pol II-dependent factors unclear","Not tested in mammalian systems"]},{"year":2017,"claim":"The RES splicing complex was shown to control Med20 protein levels by promoting efficient splicing of MED20 pre-mRNA, with defective splicing leading to NMD-mediated mRNA degradation—revealing a post-transcriptional regulatory input into Mediator composition.","evidence":"Splicing assays and NMD pathway inactivation in S. cerevisiae bud13Δ and snu17Δ mutants","pmids":["28277935"],"confidence":"Medium","gaps":["Whether this regulatory mechanism is conserved in mammals unknown","Impact on specific Med20-dependent target genes not profiled"]},{"year":2019,"claim":"Mouse knockout studies demonstrated that MED20 is essential for early embryogenesis, specifically for trophoblast lineage specification: Med20-null embryos arrest after gastrulation with ectopic NANOG in the trophectoderm, establishing MED20 as a lineage gatekeeper.","evidence":"Conditional knockout mouse, blastocyst outgrowth, immunofluorescence for NANOG, CDX2, SOX17","pmids":["30571656"],"confidence":"Medium","gaps":["Direct transcriptional targets of Med20 in trophoblast specification not identified","Whether Med20 acts through specific transcription factor partnerships in this context is unknown"]},{"year":2021,"claim":"MED20 was identified as a direct substrate of the CRL4-WDTC1 E3 ligase, whose ubiquitin-dependent degradation limits MED20 availability at the PPARγ promoter; MED20 bridges C/EBPβ and Pol II to drive adipogenesis, and preadipocyte-specific Med20 knockout abolishes brown fat and protects against obesity.","evidence":"Affinity purification, ChIP-seq, Med20 knockout in preadipocytes in vivo, non-degradable mutant rescue","pmids":["34233190"],"confidence":"High","gaps":["Ubiquitination sites on MED20 not mapped","Whether WDTC1-MED20 axis operates in white adipogenesis or other lineages not tested"]},{"year":2025,"claim":"In Schwann cells, MED20 prevents ferroptosis through transcriptional regulation of DDB1, which controls a UHRF1–BACH1–Hmox1 axis and directly ubiquitinates HO-1; pharmacological inhibition of HO-1 or ferroptosis rescues myelination in Med20-deficient mice, establishing MED20 as a survival factor for peripheral nerve myelination.","evidence":"Schwann cell-specific conditional knockout, ferroptosis assays, Co-IP and ubiquitination assays for DDB1–HO-1, pharmacological rescue with ZnPP and Fer-1","pmids":["41108685"],"confidence":"High","gaps":["Whether other Mediator subunits contribute to ferroptosis regulation in Schwann cells not tested","Relevance to human peripheral neuropathies not established"]},{"year":null,"claim":"Key unresolved questions include the high-resolution structure of MED20 within the complete human pre-initiation complex, the full repertoire of transcription factors that use MED20 as their primary Mediator contact, and whether the ferroptosis-protective and adipogenic functions reflect a unified gene-regulatory logic or distinct context-dependent mechanisms.","evidence":"","pmids":[],"confidence":"High","gaps":["No high-resolution structure of human Med20 within the full PIC","Genome-wide direct target genes of Med20 in differentiated mammalian cell types not systematically defined","Functional validation of the human disease mutation (p.Gly114Ala) in cellular or biochemical assays still lacking"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0140223","term_label":"general transcription initiation factor activity","supporting_discovery_ids":[0,11]},{"term_id":"GO:0140110","term_label":"transcription regulator activity","supporting_discovery_ids":[2,8,9]},{"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":[0,8,11]}],"pathway":[{"term_id":"R-HSA-74160","term_label":"Gene expression (Transcription)","supporting_discovery_ids":[0,2,8,11]},{"term_id":"R-HSA-1266738","term_label":"Developmental Biology","supporting_discovery_ids":[7]},{"term_id":"R-HSA-392499","term_label":"Metabolism of proteins","supporting_discovery_ids":[8]},{"term_id":"R-HSA-5357801","term_label":"Programmed Cell Death","supporting_discovery_ids":[9]}],"complexes":["Mediator complex","Med8/Med18/Med20 head module trimer"],"partners":["MED8","MED18","WDTC1","CUL4A","DDB1","CEBPB","TBP"],"other_free_text":[]},"mechanistic_narrative":"MED20 is a core subunit of the Mediator head module that functions as a structural scaffold and transcriptional co-regulator essential for RNA polymerase II-dependent gene expression, lineage specification, and cell survival. It forms an obligate trimeric subcomplex with Med8 and Med18, in which Med18 and Med20 adopt related beta-barrel folds and present a conserved protein-interaction surface that supports TBP binding and pre-initiation complex assembly [PMID:16964259, PMID:19934057, PMID:24882805]. Beyond general transcription, MED20 selectively represses ribosomal protein genes under stress, regulates Pol III-dependent RNA processing, and bridges C/EBPβ with Pol II at the PPARγ promoter to drive adipogenesis—a function controlled by CRL4-WDTC1-mediated ubiquitin-dependent degradation of MED20 [PMID:18604275, PMID:26608234, PMID:34233190]. In mice, MED20 is required for trophoblast specification during early embryogenesis and for Schwann cell myelination, where it prevents ferroptosis through a DDB1–UHRF1–BACH1–Hmox1 axis; in humans, a homozygous MED20 missense mutation (p.Gly114Ala) causes infantile basal ganglia degeneration with spasticity and dystonia [PMID:30571656, PMID:41108685, PMID:25446406]."},"prefetch_data":{"uniprot":{"accession":"Q9H944","full_name":"Mediator of RNA polymerase II transcription subunit 20","aliases":["Mediator complex subunit 20","TRF-proximal protein homolog","hTRFP"],"length_aa":212,"mass_kda":23.2,"function":"Component of the Mediator complex, a coactivator involved in the regulated transcription of nearly all RNA polymerase II-dependent genes. Mediator functions as a bridge to convey information from gene-specific regulatory proteins to the basal RNA polymerase II transcription machinery. Mediator is recruited to promoters by direct interactions with regulatory proteins and serves as a scaffold for the assembly of a functional preinitiation complex with RNA polymerase II and the general transcription factors","subcellular_location":"Nucleus","url":"https://www.uniprot.org/uniprotkb/Q9H944/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":true,"resolved_as":"","url":"https://depmap.org/portal/gene/MED20","classification":"Common Essential","n_dependent_lines":1163,"n_total_lines":1208,"dependency_fraction":0.9627483443708609},"opencell":{"profiled":true,"resolved_as":"","ensg_id":"ENSG00000124641","cell_line_id":"CID000240","localizations":[{"compartment":"nuclear_punctae","grade":3},{"compartment":"nucleoplasm","grade":3}],"interactors":[{"gene":"MED10","stoichiometry":10.0},{"gene":"MED11","stoichiometry":10.0},{"gene":"MED14","stoichiometry":10.0},{"gene":"MED17","stoichiometry":10.0},{"gene":"MED18","stoichiometry":10.0},{"gene":"MED19","stoichiometry":10.0},{"gene":"MED27","stoichiometry":10.0},{"gene":"MED9","stoichiometry":10.0},{"gene":"MED31","stoichiometry":10.0},{"gene":"MED30","stoichiometry":10.0}],"url":"https://opencell.sf.czbiohub.org/target/CID000240","total_profiled":1310},"omim":[{"mim_id":"612915","title":"MEDIATOR COMPLEX SUBUNIT 20; MED20","url":"https://www.omim.org/entry/612915"},{"mim_id":"612914","title":"MEDIATOR COMPLEX SUBUNIT 29; MED29","url":"https://www.omim.org/entry/612914"},{"mim_id":"612384","title":"MEDIATOR COMPLEX SUBUNIT 18; MED18","url":"https://www.omim.org/entry/612384"},{"mim_id":"600045","title":"DNA DAMAGE-BINDING PROTEIN 1; DDB1","url":"https://www.omim.org/entry/600045"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"","locations":[],"tissue_specificity":"Low tissue specificity","tissue_distribution":"Detected in all","driving_tissues":[],"url":"https://www.proteinatlas.org/search/MED20"},"hgnc":{"alias_symbol":["DKFZp586D2223","PRO0213","SRB2"],"prev_symbol":["TRFP"]},"alphafold":{"accession":"Q9H944","domains":[{"cath_id":"2.40.320","chopping":"2-203","consensus_level":"medium","plddt":93.7687,"start":2,"end":203}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/Q9H944","model_url":"https://alphafold.ebi.ac.uk/files/AF-Q9H944-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-Q9H944-F1-predicted_aligned_error_v6.png","plddt_mean":91.62},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=MED20","jax_strain_url":"https://www.jax.org/strain/search?query=MED20"},"sequence":{"accession":"Q9H944","fasta_url":"https://rest.uniprot.org/uniprotkb/Q9H944.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/Q9H944/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/Q9H944"}},"corpus_meta":[{"pmid":"29627764","id":"PMC_29627764","title":"Dynamic 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Med8C/Med18/Med20 form a second submodule where Med18 and Med20 adopt related beta-barrel folds. Conserved surfaces on this submodule include sites altered by srb mutations that counteract defects from Pol II truncation, supporting a role in initiation-complex formation.\",\n      \"method\": \"X-ray crystallography, in vitro TBP-binding assay, genetic analysis of srb mutations\",\n      \"journal\": \"Nature structural & molecular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — crystal structure with in vitro binding assay and mutagenesis/genetic validation\",\n      \"pmids\": [\"16964259\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"Med8, Med18, and Med20 are interdependent for proper folding and trimeric complex formation; the concurrent presence of all three subunits during renaturation is required for correct folding and trimer assembly, although pairwise subcomplexes can form independently.\",\n      \"method\": \"Immunoprecipitation, far-UV circular dichroism, fluorescence spectroscopy of recombinant proteins under renaturation conditions\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — in vitro reconstitution with multiple orthogonal biophysical methods\",\n      \"pmids\": [\"19934057\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1996,\n      \"finding\": \"Yeast SRB2 (Med20 ortholog) functions as a component of the RNA Pol II holoenzyme; loss-of-function (srb2Δ) suppresses hyperrecombination between direct repeats in hpr1Δ cells, and srb2 mutations impair DNA repair of MMS-induced damage, establishing a connection between SRB2-dependent transcription and recombination/DNA repair.\",\n      \"method\": \"Genetic suppressor screen, null allele construction, MMS sensitivity assay, deletion analysis\",\n      \"journal\": \"Genetics\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — genetic epistasis with multiple mutant alleles but no in vitro biochemistry\",\n      \"pmids\": [\"8844143\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"Yeast Med20 (head module subunit) is required for transcriptional repression of ribosomal protein (RP) genes under multiple stress conditions (rapamycin, etc.); med20Δ is synthetically sick/lethal with maf1Δ, placing Med20 in a parallel pathway to Maf1 for coordinated negative regulation of RP mRNA and tRNA synthesis.\",\n      \"method\": \"Genome-wide synthetic sick/lethal screen, gene expression profiling, rapamycin treatment assays\",\n      \"journal\": \"PLoS genetics\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — epistasis screen with expression profiling, single lab\",\n      \"pmids\": [\"18604275\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"Loss of Med20 in fission yeast leads to accumulation of aberrant, polyadenylated read-through tRNA transcripts targeted for exosome degradation, and also affects snRNA, snoRNA, and rRNA levels, indicating Med20/Mediator participates in a pathway regulating Pol III-dependent transcripts.\",\n      \"method\": \"Deletion mutant analysis, Northern blotting, polyA-enrichment of non-coding RNAs\",\n      \"journal\": \"Biochimica et biophysica acta\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — clean KO with defined RNA phenotype, single lab\",\n      \"pmids\": [\"26608234\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"The RES complex (Bud13p/Snu17p/Pml1p) promotes splicing of MED20 pre-mRNA; RES mutants accumulate unspliced MED20 pre-mRNA that is degraded by NMD, reducing Med20p levels and causing temperature-sensitive growth defects that are partially rescued by NMD inactivation.\",\n      \"method\": \"Genetic suppressor analysis, RT-PCR splicing assays, NMD pathway inactivation, growth assays\",\n      \"journal\": \"RNA biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — epistasis with splicing assays and genetic rescue, single lab\",\n      \"pmids\": [\"28277935\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"MED20 is essential for early mouse embryogenesis; Med20 mutant blastocysts fail to hatch from the zona pellucida and show ectopic NANOG expression in the trophectoderm, indicating MED20 is required for trophoblast specification and proper epiblast/trophectoderm lineage restriction.\",\n      \"method\": \"Knockout mouse embryo analysis, blastocyst outgrowth assay, immunofluorescence for lineage markers, apoptosis assays\",\n      \"journal\": \"Reproduction (Cambridge, England)\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — clean KO with defined cellular phenotype, single lab\",\n      \"pmids\": [\"30571656\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"MED20 is a substrate of the anti-obesity CRL4-WDTC1 E3 ubiquitin ligase complex; WDTC1 overexpression causes MED20 degradation while depletion of WDTC1/CUL4A/B causes MED20 accumulation. MED20 promotes adipogenesis by bridging C/EBPβ and RNA Pol II at the PPARγ promoter; Med20 knockout in preadipocytes abolishes brown adipose tissue development.\",\n      \"method\": \"Affinity purification/candidate screening, co-immunoprecipitation, knockdown/overexpression, non-degradable mutant rescue, ChIP-seq, knockout mouse model\",\n      \"journal\": \"Cell reports\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — affinity purification identifying E3 substrate relationship, ChIP-seq, KO mouse, multiple orthogonal methods in one study\",\n      \"pmids\": [\"34233190\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"Med20 regulates peripheral nervous system myelination by controlling Schwann cell ferroptosis; loss of Med20 in Schwann cells induces ferroptosis via its downstream target DDB1, which regulates HO-1 (Hmox1) through a DDB1–UHRF1–BACH1–Hmox1 axis and by direct ubiquitination of HO-1 protein. HO-1 inhibitor ZnPP or ferroptosis inhibitor Fer-1 rescues myelination in Med20-deficient mice.\",\n      \"method\": \"Conditional knockout mouse, co-immunoprecipitation, ubiquitination assay, ChIP, pharmacological rescue with ferroptosis inhibitor\",\n      \"journal\": \"Cell reports\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — conditional KO with mechanistic pathway dissection using multiple orthogonal methods and pharmacological rescue\",\n      \"pmids\": [\"41108685\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"A homozygous missense mutation (p.Gly114Ala) in MED20 is associated with infantile-onset neurodegenerative movement disorder; structural modeling indicates this residue lies in the beta-barrel fold of Med20 within the Med8/18/20 subcomplex, suggesting the mutation disrupts complex integrity.\",\n      \"method\": \"Whole-exome sequencing, genetic linkage analysis, in silico pathogenicity analysis, structural modeling\",\n      \"journal\": \"European journal of pediatrics\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 4 — structural modeling and in silico analyses only, no direct biochemical experiment\",\n      \"pmids\": [\"25446406\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"MED20 is a head-module subunit of the Mediator complex that forms an obligate trimeric subcomplex with Med8 and Med18 (adopting a beta-barrel fold), participates in RNA Pol II transcription initiation by facilitating TBP binding and initiation-complex formation, represses ribosomal protein gene transcription, is ubiquitinated and degraded by the CRL4-WDTC1 E3 ligase to limit adipogenesis, bridges C/EBPβ and Pol II at the PPARγ promoter to drive adipogenic gene expression, and in Schwann cells controls myelination by suppressing ferroptosis through a DDB1–UHRF1–BACH1–HO-1 regulatory axis.\"\n}\n```\n\n--- ROUND 2 (R2/R3, augmented corpus) ---\n\nI'll classify each paper first, then extract discoveries from KEEP papers only.\n\n**Classification:**\n\n- Papers [1], [3], [4], [6], [16], [17] from retrieved: CD36/SR-B2 lipid transporter — EXCLUDE (alias collision: SR-B2 is an alias for CD36, not MED20)\n- Paper [5] from retrieved: SRB-2 RNA aptamer — EXCLUDE (alias collision)\n- Paper [2]: Med8-Med18-Med20 structure and TBP binding — KEEP\n- Paper [7]: Yeast SRB2 (=Med20 ortholog) mutations — KEEP (ortholog)\n- Paper [8]: MED20 mutation in human neurodegenerative disease — KEEP\n- Paper [9]: MED20 essential for early embryogenesis, NANOG regulation — KEEP\n- Paper [10]: MED20 in adipogenesis, CRL4-WDTC1 substrate — KEEP\n- Paper [11]: Med20 role in RP gene repression (yeast) — KEEP (ortholog)\n- Paper [12]: ARL13B-tRFP cilium reporter — EXCLUDE (unrelated)\n- Paper [13]: Fission yeast Med20, tRNA/ncRNA regulation — KEEP (ortholog)\n- Paper [14]: Med8/Med18/Med20 folding and complex formation — KEEP (ortholog)\n- Paper [15]: RES complex controls MED20 pre-mRNA splicing — KEEP (ortholog)\n- Paper [18]: Med20 regulates myelination via ferroptosis in Schwann cells — KEEP\n\n**Additional curated papers:** Most are large interactome/genomics datasets with no specific MED20 mechanistic findings, or unrelated (proteases, GWAS, BET inhibitors, HuR, miRNA, etc.). Papers specifically about Mediator:\n- [19] (PMID:15175163): Identifies consensus mammalian Mediator subunits including MED20 by MudPIT — KEEP (partial)\n- [26] (PMID:24882805): EM structure of Mediator with subunit localization — KEEP\n- [18] (PMID:21729782): MED26 paper — EXCLUDE (different subunit)\n- [2] (PMID:26186194), [3] (PMID:28514442), [8] (PMID:33961781): BioPlex interactome datasets — low MED20-specific content, but note interactions. EXCLUDE (no specific MED20 mechanism)\n\n```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 2006,\n      \"finding\": \"Med8, Med18, and Med20 form a head subcomplex (Med8/18/20) with two submodules: the N-terminal domain of Med8 binds TBP in vitro and is essential in vivo, while the C-terminal region of Med8 (Med8C), Med18, and Med20 form a second submodule. X-ray crystallography revealed that Med18 and Med20 adopt related beta-barrel folds, and a conserved putative protein-interaction face on the Med8C/18/20 submodule includes sites altered by srb mutations that counteract defects from RNA Pol II truncation.\",\n      \"method\": \"X-ray crystallography, in vitro TBP binding assay, genetic analysis of srb mutations\",\n      \"journal\": \"Nature structural & molecular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — crystal structure combined with in vitro binding and in vivo genetic validation in single rigorous study\",\n      \"pmids\": [\"16964259\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1996,\n      \"finding\": \"Yeast SRB2 (Med20 ortholog) is required for normal DNA repair of MMS-induced damage and modulates transcription-associated recombination; loss of SRB2 suppresses hyperrecombination between direct repeats in hpr1Δ cells, linking this basal transcription factor to both recombination and DNA repair pathways.\",\n      \"method\": \"Genetic suppressor screen, null allele construction, MMS sensitivity assays, recombination frequency measurement\",\n      \"journal\": \"Genetics\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — genetic epistasis with defined phenotypic readouts, single lab\",\n      \"pmids\": [\"8844143\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"In Saccharomyces cerevisiae, Med20 (head module subunit) plays a selective role in transcriptional repression of ribosomal protein (RP) genes following rapamycin treatment; gene expression profiling after deletion of Med20 revealed a prominent and highly selective defect in RP gene repression under multiple stress conditions, functioning in a pathway parallel to Maf1.\",\n      \"method\": \"Genetic interaction screens (synthetic sick/lethal), gene expression profiling (transcriptomics), rapamycin treatment assays\",\n      \"journal\": \"PLoS genetics\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — genome-scale genetic screen with expression profiling validation, single lab\",\n      \"pmids\": [\"18604275\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"Med8, Med18, and Med20 are interdependent for proper folding and trimeric complex formation; concurrent presence of all three subunits during renaturation is required for correct trimer assembly, whereas pairwise combinations yield distinct, partially folded subcomplexes.\",\n      \"method\": \"Immunoprecipitation, far-UV circular dichroism spectroscopy, fluorescence measurements on recombinantly expressed and renatured proteins\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — in vitro reconstitution with multiple orthogonal biophysical methods (CD, fluorescence, IP)\",\n      \"pmids\": [\"19934057\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"A homozygous missense mutation in MED20 (p.Gly114Ala) in human patients causes infantile-onset spasticity, dystonia, progressive basal ganglia degeneration, and brain atrophy. Structural modeling suggests the mutation disrupts the Med20 beta-barrel fold at a conserved position within the Med8C/18/20 submodule, establishing MED20 as essential for normal neuronal function.\",\n      \"method\": \"Whole-exome sequencing, genetic linkage analysis, in silico pathogenicity analysis, structural modeling\",\n      \"journal\": \"European journal of pediatrics\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 — human genetics with structural modeling but no direct functional rescue experiment; pathogenicity inferred computationally\",\n      \"pmids\": [\"25446406\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"In fission yeast, loss of Med20 leads to accumulation of aberrant, polyadenylated readthrough tRNA transcripts that are targeted for degradation by the exosome; other non-coding RNAs (snRNA, snoRNA, rRNA) are also enriched in polyadenylate fractions, indicating that Med20-containing Mediator participates in a regulatory pathway affecting RNA Pol III-dependent transcripts.\",\n      \"method\": \"Northern blotting, RT-PCR, polyadenylate RNA enrichment, genetic deletion\",\n      \"journal\": \"Biochimica et biophysica acta\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — loss-of-function with multiple RNA species analyzed by orthogonal methods, single lab\",\n      \"pmids\": [\"26608234\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"The heterotrimeric pre-mRNA retention and splicing (RES) complex (Bud13p/Snu17p/Pml1p) controls Med20 protein levels by promoting efficient splicing of MED20 pre-mRNA; inefficient splicing in bud13Δ and snu17Δ cells leads to NMD-mediated degradation of MED20 pre-mRNA, reducing Med20 protein and causing temperature-sensitive growth defects.\",\n      \"method\": \"Genetic deletion analysis, RT-PCR splicing assays, NMD pathway inactivation, growth phenotype complementation\",\n      \"journal\": \"RNA biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — genetic epistasis with molecular splicing readout and NMD suppression rescue, single lab\",\n      \"pmids\": [\"28277935\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"MED20 is essential for early mammalian embryo development; Med20 mutant mouse embryos arrest after gastrulation, fail to hatch from the zona pellucida, and show ectopic NANOG expression in the trophectoderm, indicating MED20 is specifically required for trophoblast lineage specification and suppression of epiblast identity in trophectoderm.\",\n      \"method\": \"Conditional knockout mouse model, blastocyst outgrowth assay, immunofluorescence for lineage markers (NANOG, CDX2, SOX17), TUNEL apoptosis assay\",\n      \"journal\": \"Reproduction (Cambridge, England)\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — clean KO with defined molecular phenotype (ectopic NANOG) and multiple lineage markers, single lab\",\n      \"pmids\": [\"30571656\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"MED20 is a substrate of the anti-obesity CRL4-WDTC1 E3 ubiquitin ligase complex; overexpression of WDTC1 leads to MED20 degradation, while depletion of WDTC1 or CUL4A/B causes MED20 accumulation. MED20 promotes adipogenesis by bridging C/EBPβ and RNA polymerase II at the PPARγ promoter, as shown by ChIP-seq; knockout of Med20 in preadipocytes abolishes brown adipose tissue development and protects mice from diet-induced obesity.\",\n      \"method\": \"Affinity purification and candidate screening, ubiquitin ligase overexpression/depletion, non-degradable mutant rescue, Med20 knockout in preadipocytes (in vivo), ChIP-seq for C/EBPβ and Pol II\",\n      \"journal\": \"Cell reports\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — multiple orthogonal methods (AP, ChIP-seq, KO mouse, rescue with non-degradable mutant) in single rigorous study\",\n      \"pmids\": [\"34233190\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"Med20 regulates myelination in the peripheral nervous system by preventing ferroptosis in Schwann cells; loss of Med20 in Schwann cells induces ferroptosis and impairs myelination. Mechanistically, Med20 acts through its downstream target DDB1, which controls ferroptosis via a 'DDB1-UHRF1-BACH1-Hmox1' transcriptional axis and by directly interacting with and ubiquitinating HO-1 protein. Pharmacological inhibition of HO-1 (ZnPP) or ferroptosis (Fer-1) rescues myelination in Med20-deficient mice.\",\n      \"method\": \"Schwann cell-specific conditional knockout, ferroptosis assays, ChIP/transcriptional reporter assays, co-immunoprecipitation (DDB1-HO-1 interaction), ubiquitination assay, pharmacological rescue (ZnPP, Fer-1), electron microscopy of myelin\",\n      \"journal\": \"Cell reports\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — conditional KO with defined molecular mechanism, multiple pathway components validated by orthogonal methods, pharmacological rescue\",\n      \"pmids\": [\"41108685\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"MED20 was identified as a consensus subunit of the mammalian Mediator complex by comparative MudPIT proteomic analysis across six independent immunoaffinity-purified Mediator preparations, establishing it as a core component of the human Mediator.\",\n      \"method\": \"Multidimensional protein identification technology (MudPIT), immunoaffinity purification of Mediator through multiple subunit baits\",\n      \"journal\": \"Molecular cell\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — MS-based identification replicated across six independent preparations\",\n      \"pmids\": [\"15175163\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"EM structural analysis of yeast and human Mediator complexes mapped subunit localization and confirmed that the head module containing Med20 (Med8/18/20 subcomplex) occupies a defined position in the overall Mediator architecture and participates in large-scale conformational rearrangements at module interfaces that are conducive to transcription initiation.\",\n      \"method\": \"Cryo-EM single-particle analysis, subunit localization by antibody labeling and tagged-subunit EM, biochemical analysis of module interfaces\",\n      \"journal\": \"Cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — cryo-EM structure with biochemical validation and subunit localization, replicated in yeast and human Mediator\",\n      \"pmids\": [\"24882805\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"MED20 is a core subunit of the Mediator head module that forms an interdependent trimeric subcomplex with Med8 and Med18 (adopting a beta-barrel fold), binds TBP to support RNA Pol II transcription initiation, selectively represses ribosomal protein gene transcription and influences Pol III-dependent RNA processing, is subject to CRL4-WDTC1-mediated ubiquitin-dependent degradation that regulates its role in bridging C/EBPβ and Pol II for PPARγ-driven adipogenesis, is essential for early embryonic trophoblast specification (suppressing ectopic NANOG), and in Schwann cells prevents ferroptosis through a DDB1-UHRF1-BACH1-Hmox1 axis to maintain peripheral nervous system myelination.\"\n}\n```","stage2_raw":"```json\n{\n  \"mechanistic_narrative\": \"MED20 is a head-module subunit of the Mediator complex that functions in RNA polymerase II transcription initiation and regulates diverse developmental and metabolic programs. Within the head module, MED20 forms an obligate trimeric subcomplex with Med8 and Med18, where Med18 and Med20 adopt related beta-barrel folds and the Med8 N-terminal domain contacts TBP, facilitating pre-initiation complex assembly [PMID:16964259, PMID:19934057]. MED20 promotes adipogenesis by bridging C/EBPβ and RNA Pol II at the PPARγ promoter, and its protein levels are negatively regulated through ubiquitination and degradation by the CRL4-WDTC1 E3 ligase; accordingly, Med20 knockout abolishes brown adipose tissue development [PMID:34233190]. In Schwann cells, Med20 controls peripheral myelination by suppressing ferroptosis through a DDB1–UHRF1–BACH1–HO-1 regulatory axis, and a homozygous missense mutation (p.Gly114Ala) in the beta-barrel domain is associated with infantile-onset neurodegenerative movement disorder [PMID:41108685, PMID:25446406].\",\n  \"teleology\": [\n    {\n      \"year\": 1996,\n      \"claim\": \"Establishing that the MED20 ortholog SRB2 is a functional component of the RNA Pol II holoenzyme answered whether this gene participates in general transcription or has a specialized role, linking it to both transcription and DNA damage responses.\",\n      \"evidence\": \"Genetic suppressor screen and MMS sensitivity assays in yeast srb2Δ mutants\",\n      \"pmids\": [\"8844143\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"No biochemical reconstitution of SRB2 role in transcription\", \"DNA repair connection not mechanistically dissected\", \"Relevance to metazoan MED20 not tested\"]\n    },\n    {\n      \"year\": 2006,\n      \"claim\": \"Solving the crystal structure of the Med8/18/20 subcomplex revealed that Med20 and Med18 adopt beta-barrel folds and that the trimeric module bridges TBP binding to Pol II initiation, providing the first atomic-resolution framework for MED20 function.\",\n      \"evidence\": \"X-ray crystallography of yeast Med8C/Med18/Med20, in vitro TBP-binding assays, and genetic analysis of srb mutations\",\n      \"pmids\": [\"16964259\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structure determined for partial subcomplex only; full Mediator context not resolved\", \"TBP binding measured for Med8 N-terminus, not the trimeric beta-barrel surface directly\"]\n    },\n    {\n      \"year\": 2008,\n      \"claim\": \"Demonstrating that Med20 is required for stress-induced repression of ribosomal protein genes revealed a gene-specific regulatory function beyond general transcription initiation.\",\n      \"evidence\": \"Genome-wide synthetic lethal screen and expression profiling in yeast med20Δ under rapamycin treatment\",\n      \"pmids\": [\"18604275\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Mechanism of RP gene repression by Med20 not biochemically defined\", \"Parallel pathway with Maf1 not tested for direct physical interaction\"]\n    },\n    {\n      \"year\": 2009,\n      \"claim\": \"Showing that Med8, Med18, and Med20 are interdependent for correct folding and that all three must be simultaneously present during renaturation established the obligate nature of the trimeric subcomplex.\",\n      \"evidence\": \"Reconstitution with recombinant subunits monitored by co-IP, CD, and fluorescence spectroscopy\",\n      \"pmids\": [\"19934057\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Folding pathway in vivo with chaperones not addressed\", \"Stoichiometry of assembly intermediates not fully characterized\"]\n    },\n    {\n      \"year\": 2014,\n      \"claim\": \"Identification of a homozygous MED20 missense mutation (p.Gly114Ala) in patients with infantile-onset neurodegeneration provided the first human disease link and suggested that disruption of the beta-barrel fold has pathological consequences.\",\n      \"evidence\": \"Whole-exome sequencing with linkage analysis and in silico structural modeling in a consanguineous family\",\n      \"pmids\": [\"25446406\"],\n      \"confidence\": \"Low\",\n      \"gaps\": [\"No functional biochemical validation of mutant protein\", \"No independent replication cohort\", \"Effect on Med8/18/20 complex integrity not experimentally tested\"]\n    },\n    {\n      \"year\": 2015,\n      \"claim\": \"Finding that Med20 loss causes accumulation of aberrant polyadenylated tRNA read-through transcripts extended Mediator function beyond Pol II to influence Pol III-dependent transcript quality control.\",\n      \"evidence\": \"Fission yeast med20Δ analysis with Northern blotting and polyA-enrichment of non-coding RNAs\",\n      \"pmids\": [\"26608234\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Whether Med20 acts directly on Pol III or indirectly through Pol II-dependent regulators is unknown\", \"Exosome targeting mechanism not defined\"]\n    },\n    {\n      \"year\": 2019,\n      \"claim\": \"Demonstrating that Med20 knockout causes early embryonic lethality with failure of trophoblast specification established MED20 as essential for mammalian lineage commitment.\",\n      \"evidence\": \"Med20 knockout mouse embryo analysis with blastocyst outgrowth assays and lineage marker immunofluorescence\",\n      \"pmids\": [\"30571656\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Target genes mediating trophoblast specification not identified\", \"Whether phenotype reflects general transcription failure or gene-specific regulation is unclear\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"Identifying MED20 as a CRL4-WDTC1 ubiquitin ligase substrate that bridges C/EBPβ to Pol II at the PPARγ promoter established a post-translational mechanism controlling adipogenic transcription through Mediator subunit turnover.\",\n      \"evidence\": \"Affinity purification, co-IP, ChIP-seq, non-degradable mutant rescue, and Med20 conditional knockout mouse with loss of brown adipose tissue\",\n      \"pmids\": [\"34233190\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Ubiquitination sites on MED20 not mapped\", \"Whether WDTC1-mediated degradation regulates MED20 in non-adipogenic contexts is unknown\"]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"Revealing that Med20 controls Schwann cell myelination by suppressing ferroptosis through a DDB1–UHRF1–BACH1–HO-1 axis uncovered a cell-type-specific non-canonical mechanism linking Mediator to lipid peroxidation and iron homeostasis.\",\n      \"evidence\": \"Schwann cell conditional knockout mouse, co-IP, ubiquitination assays, ChIP, and pharmacological rescue with ferroptosis and HO-1 inhibitors\",\n      \"pmids\": [\"41108685\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether Med20 transcriptionally regulates DDB1 or acts through protein-protein interaction is not fully resolved\", \"Generalizability of the ferroptosis-suppressive role to other Mediator subunits or cell types untested\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"The mechanism by which the MED20 beta-barrel surface communicates with Pol II during initiation, the identity of ubiquitination sites targeted by CRL4-WDTC1, and whether the ferroptosis-regulatory function extends beyond Schwann cells remain unresolved.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Low\",\n      \"gaps\": [\"No high-resolution cryo-EM structure of full human Mediator with MED20 contacts to Pol II mapped\", \"Ubiquitin acceptor lysines not identified\", \"Human disease mutation not functionally validated\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0005198\", \"supporting_discovery_ids\": [0, 1]},\n      {\"term_id\": \"GO:0140110\", \"supporting_discovery_ids\": [3, 7]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005634\", \"supporting_discovery_ids\": [0, 3, 7]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-74160\", \"supporting_discovery_ids\": [0, 3, 7]},\n      {\"term_id\": \"R-HSA-1266738\", \"supporting_discovery_ids\": [6, 8]},\n      {\"term_id\": \"R-HSA-392499\", \"supporting_discovery_ids\": [7]},\n      {\"term_id\": \"R-HSA-5357801\", \"supporting_discovery_ids\": [8]}\n    ],\n    \"complexes\": [\n      \"Mediator complex\",\n      \"Med8/Med18/Med20 trimeric subcomplex\"\n    ],\n    \"partners\": [\n      \"MED8\",\n      \"MED18\",\n      \"WDTC1\",\n      \"CUL4A\",\n      \"DDB1\",\n      \"CEBPB\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```\n\n--- ROUND 2 (R2/R3, augmented corpus) ---\n\n```json\n{\n  \"mechanistic_narrative\": \"MED20 is a core subunit of the Mediator head module that functions as a structural scaffold and transcriptional co-regulator essential for RNA polymerase II-dependent gene expression, lineage specification, and cell survival. It forms an obligate trimeric subcomplex with Med8 and Med18, in which Med18 and Med20 adopt related beta-barrel folds and present a conserved protein-interaction surface that supports TBP binding and pre-initiation complex assembly [PMID:16964259, PMID:19934057, PMID:24882805]. Beyond general transcription, MED20 selectively represses ribosomal protein genes under stress, regulates Pol III-dependent RNA processing, and bridges C/EBPβ with Pol II at the PPARγ promoter to drive adipogenesis—a function controlled by CRL4-WDTC1-mediated ubiquitin-dependent degradation of MED20 [PMID:18604275, PMID:26608234, PMID:34233190]. In mice, MED20 is required for trophoblast specification during early embryogenesis and for Schwann cell myelination, where it prevents ferroptosis through a DDB1–UHRF1–BACH1–Hmox1 axis; in humans, a homozygous MED20 missense mutation (p.Gly114Ala) causes infantile basal ganglia degeneration with spasticity and dystonia [PMID:30571656, PMID:41108685, PMID:25446406].\",\n  \"teleology\": [\n    {\n      \"year\": 1996,\n      \"claim\": \"The yeast MED20 ortholog SRB2 was first linked to genome integrity beyond basal transcription, showing that its loss suppresses transcription-coupled hyperrecombination and impairs DNA repair of MMS-induced damage.\",\n      \"evidence\": \"Genetic suppressor screen and MMS sensitivity assays in S. cerevisiae srb2Δ mutants\",\n      \"pmids\": [\"8844143\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"No direct biochemical mechanism for the DNA repair defect\", \"Not confirmed in mammalian cells\", \"Unclear whether effect is through Mediator-dependent transcription or an independent function\"]\n    },\n    {\n      \"year\": 2004,\n      \"claim\": \"Proteomic cataloguing established MED20 as a consensus subunit of the mammalian Mediator complex, placing it in the head module alongside Med8 and Med18 and confirming evolutionary conservation from yeast to human.\",\n      \"evidence\": \"MudPIT analysis of six independent immunoaffinity-purified human Mediator preparations\",\n      \"pmids\": [\"15175163\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"No direct structural data for human MED20 at the time\", \"Functional role within human Mediator not yet tested\"]\n    },\n    {\n      \"year\": 2006,\n      \"claim\": \"Crystal structure of the Med8C/Med18/Med20 subcomplex revealed that Med20 adopts a beta-barrel fold and together with Med18 forms a conserved protein-interaction surface implicated in coupling TBP binding to Pol II activation, resolving how the head module contacts basal transcription machinery.\",\n      \"evidence\": \"X-ray crystallography of yeast Med8C/18/20 trimer, in vitro TBP binding, genetic analysis of srb mutations\",\n      \"pmids\": [\"16964259\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Full-length Med8 interactions not structurally resolved\", \"No structure of Med20 in the context of complete Mediator\"]\n    },\n    {\n      \"year\": 2008,\n      \"claim\": \"Deletion of Med20 in yeast uncovered a highly selective role in transcriptional repression of ribosomal protein genes under stress, demonstrating that individual Mediator subunits can have gene-class-specific regulatory functions beyond global transcription.\",\n      \"evidence\": \"Genome-wide expression profiling and genetic interaction screens in S. cerevisiae med20Δ upon rapamycin treatment\",\n      \"pmids\": [\"18604275\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Mechanism of selective RP gene repression not determined\", \"Relationship to mammalian RP gene regulation unknown\"]\n    },\n    {\n      \"year\": 2009,\n      \"claim\": \"Biophysical reconstitution showed that Med8, Med18, and Med20 are mutually dependent for correct folding—requiring simultaneous co-folding of all three subunits—establishing the trimer as an obligate assembly unit within the head module.\",\n      \"evidence\": \"Renaturation of recombinant subunits with circular dichroism and fluorescence spectroscopy\",\n      \"pmids\": [\"19934057\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"In vivo assembly pathway and chaperone requirements unknown\", \"Post-translational modifications during assembly not addressed\"]\n    },\n    {\n      \"year\": 2014,\n      \"claim\": \"Cryo-EM mapping of yeast and human Mediator placed the Med8/18/20 trimer at a defined head module position and showed it participates in large-scale conformational changes at module interfaces during transcription initiation, providing a structural framework for its regulatory function.\",\n      \"evidence\": \"Cryo-EM single-particle analysis with antibody and tag-based subunit localization in yeast and human Mediator\",\n      \"pmids\": [\"24882805\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"High-resolution structure of Med20 in full PIC context not yet available at this time\", \"Conformational dynamics during active transcription not captured\"]\n    },\n    {\n      \"year\": 2014,\n      \"claim\": \"A homozygous MED20 missense mutation (p.Gly114Ala) was identified as the cause of infantile basal ganglia degeneration with spasticity and dystonia, establishing the first human Mendelian disorder linked to MED20 and demonstrating its essential role in neuronal maintenance.\",\n      \"evidence\": \"Whole-exome sequencing and linkage analysis in consanguineous family, structural modeling of the variant\",\n      \"pmids\": [\"25446406\"],\n      \"confidence\": \"Low\",\n      \"gaps\": [\"No functional rescue experiment performed\", \"Pathogenicity assessed only by in silico modeling, not by biochemical or cellular assay\", \"Single family reported\"]\n    },\n    {\n      \"year\": 2015,\n      \"claim\": \"Loss of Med20 in fission yeast caused accumulation of aberrant polyadenylated tRNA and other non-coding RNA transcripts, revealing an unexpected role for Mediator in regulating Pol III-dependent transcript processing and quality control.\",\n      \"evidence\": \"Northern blot and RT-PCR analysis of polyadenylated RNA fractions in S. pombe med20Δ cells\",\n      \"pmids\": [\"26608234\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Whether Med20 acts directly on Pol III or indirectly through Pol II-dependent factors unclear\", \"Not tested in mammalian systems\"]\n    },\n    {\n      \"year\": 2017,\n      \"claim\": \"The RES splicing complex was shown to control Med20 protein levels by promoting efficient splicing of MED20 pre-mRNA, with defective splicing leading to NMD-mediated mRNA degradation—revealing a post-transcriptional regulatory input into Mediator composition.\",\n      \"evidence\": \"Splicing assays and NMD pathway inactivation in S. cerevisiae bud13Δ and snu17Δ mutants\",\n      \"pmids\": [\"28277935\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Whether this regulatory mechanism is conserved in mammals unknown\", \"Impact on specific Med20-dependent target genes not profiled\"]\n    },\n    {\n      \"year\": 2019,\n      \"claim\": \"Mouse knockout studies demonstrated that MED20 is essential for early embryogenesis, specifically for trophoblast lineage specification: Med20-null embryos arrest after gastrulation with ectopic NANOG in the trophectoderm, establishing MED20 as a lineage gatekeeper.\",\n      \"evidence\": \"Conditional knockout mouse, blastocyst outgrowth, immunofluorescence for NANOG, CDX2, SOX17\",\n      \"pmids\": [\"30571656\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Direct transcriptional targets of Med20 in trophoblast specification not identified\", \"Whether Med20 acts through specific transcription factor partnerships in this context is unknown\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"MED20 was identified as a direct substrate of the CRL4-WDTC1 E3 ligase, whose ubiquitin-dependent degradation limits MED20 availability at the PPARγ promoter; MED20 bridges C/EBPβ and Pol II to drive adipogenesis, and preadipocyte-specific Med20 knockout abolishes brown fat and protects against obesity.\",\n      \"evidence\": \"Affinity purification, ChIP-seq, Med20 knockout in preadipocytes in vivo, non-degradable mutant rescue\",\n      \"pmids\": [\"34233190\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Ubiquitination sites on MED20 not mapped\", \"Whether WDTC1-MED20 axis operates in white adipogenesis or other lineages not tested\"]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"In Schwann cells, MED20 prevents ferroptosis through transcriptional regulation of DDB1, which controls a UHRF1–BACH1–Hmox1 axis and directly ubiquitinates HO-1; pharmacological inhibition of HO-1 or ferroptosis rescues myelination in Med20-deficient mice, establishing MED20 as a survival factor for peripheral nerve myelination.\",\n      \"evidence\": \"Schwann cell-specific conditional knockout, ferroptosis assays, Co-IP and ubiquitination assays for DDB1–HO-1, pharmacological rescue with ZnPP and Fer-1\",\n      \"pmids\": [\"41108685\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether other Mediator subunits contribute to ferroptosis regulation in Schwann cells not tested\", \"Relevance to human peripheral neuropathies not established\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"Key unresolved questions include the high-resolution structure of MED20 within the complete human pre-initiation complex, the full repertoire of transcription factors that use MED20 as their primary Mediator contact, and whether the ferroptosis-protective and adipogenic functions reflect a unified gene-regulatory logic or distinct context-dependent mechanisms.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"High\",\n      \"gaps\": [\"No high-resolution structure of human Med20 within the full PIC\", \"Genome-wide direct target genes of Med20 in differentiated mammalian cell types not systematically defined\", \"Functional validation of the human disease mutation (p.Gly114Ala) in cellular or biochemical assays still lacking\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0140223\", \"supporting_discovery_ids\": [0, 11]},\n      {\"term_id\": \"GO:0140110\", \"supporting_discovery_ids\": [2, 8, 9]},\n      {\"term_id\": \"GO:0005198\", \"supporting_discovery_ids\": [0, 3]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005634\", \"supporting_discovery_ids\": [0, 8, 11]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-74160\", \"supporting_discovery_ids\": [0, 2, 8, 11]},\n      {\"term_id\": \"R-HSA-1266738\", \"supporting_discovery_ids\": [7]},\n      {\"term_id\": \"R-HSA-392499\", \"supporting_discovery_ids\": [8]},\n      {\"term_id\": \"R-HSA-5357801\", \"supporting_discovery_ids\": [9]}\n    ],\n    \"complexes\": [\n      \"Mediator complex\",\n      \"Med8/Med18/Med20 head module trimer\"\n    ],\n    \"partners\": [\n      \"MED8\",\n      \"MED18\",\n      \"WDTC1\",\n      \"CUL4A\",\n      \"DDB1\",\n      \"CEBPB\",\n      \"TBP\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```"}