{"gene":"MED21","run_date":"2026-06-10T02:59:50","timeline":{"discoveries":[{"year":1996,"finding":"Human SRB7 (MED21) protein co-purifies exclusively with high molecular weight forms of RNA polymerase II, establishing it as a component of a mammalian RNA polymerase II holoenzyme. Antibodies against human SRB7 were used to purify a holoenzyme containing general transcription factors TFIIE and TFIIH, which is more responsive to transcriptional activators than core Pol II in the presence of coactivators.","method":"Antibody-based purification, immunoprecipitation, in vitro transcription assay","journal":"Nature","confidence":"High","confidence_rationale":"Tier 2 / Strong — reciprocal biochemical purification and functional transcription assay; replicated by multiple subsequent labs","pmids":["8598913"],"is_preprint":false},{"year":1998,"finding":"A murine counterpart of yeast Mediator was identified; the mouse complex contains homologs of yeast Mediator subunits including Srb7 (MED21) and Med7, binds to the RNA polymerase II CTD, and stimulates phosphorylation of the CTD by TFIIH.","method":"Protein purification, peptide sequencing, CTD binding assay, CTD phosphorylation assay (in vitro)","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 1–2 / Strong — direct in vitro biochemical assays (CTD binding, phosphorylation stimulation), replicated across labs","pmids":["9671713"],"is_preprint":false},{"year":1998,"finding":"Human SRB7 (MED21) is a component of the NAT complex, which represses activator-dependent transcription and phosphorylates the CTD of RNA Pol II at residues distinct from those phosphorylated by TFIIH. The complex interacts specifically with RNA Pol II in a manner that is not mediated by the CTD but is precluded by CTD phosphorylation.","method":"Biochemical purification, in vitro transcription repression assay, kinase assay, co-immunoprecipitation with RNA Pol II","journal":"Molecular cell","confidence":"High","confidence_rationale":"Tier 1–2 / Strong — in vitro kinase assay and transcription assay plus co-IP, multiple orthogonal methods in single study","pmids":["9734358"],"is_preprint":false},{"year":1999,"finding":"Human SRB7 (MED21) is a component of the SMCC complex that can repress activator-dependent transcription mediated by PC4 or, at limiting TFIIH, synergistically enhance activator-dependent transcription. The complex shows direct activator interactions and can act independently of the RNA Pol II CTD.","method":"Biochemical purification, in vitro transcription assay, activator interaction assay","journal":"Molecular cell","confidence":"High","confidence_rationale":"Tier 1–2 / Strong — in vitro transcription assays demonstrating both positive and negative functions, multiple orthogonal methods","pmids":["10024883"],"is_preprint":false},{"year":1999,"finding":"Mouse Srb7 (MED21) gene is single copy, expressed in all tissues examined, and its disruption in embryonic stem cells is lethal, demonstrating an essential role in cell viability and murine embryonic development. Murine Srb7 associates exclusively with high molecular weight forms of RNA polymerase II in extracts.","method":"Northern blot, gene knockout in embryonic stem cells, co-fractionation","journal":"Genes & development","confidence":"High","confidence_rationale":"Tier 2 / Strong — clean gene knockout with defined lethal phenotype, co-fractionation, replicated by independent study","pmids":["10500093"],"is_preprint":false},{"year":1999,"finding":"Human SRB7 (MED21) is an integral component of an RNA Pol II-SRB complex; anti-SRB7 antibody immunoprecipitates hTRFP and RNA Pol II, and anti-hTRFP antibody reciprocally immunoprecipitates RNA Pol II and SRB7. The SRB7-containing complex supports basal transcription and enhances transcriptional activation by Gal4-VP16.","method":"Reciprocal co-immunoprecipitation, in vitro transcription assay","journal":"The Journal of biological chemistry","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — reciprocal co-IP and transcription assay, single lab","pmids":["9933582"],"is_preprint":false},{"year":2000,"finding":"Artificial recruitment of Srb7 (MED21) fused to a DNA-binding domain to a promoter in mammalian cells does not activate transcription, despite being associated with the Pol II holoenzyme and being directly recruited to the promoter.","method":"Transfection assay with fusion protein recruitment, reporter gene assay","journal":"Molecular and cellular biology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — direct functional recruitment assay in mammalian cells; negative finding reported as a mechanistic result","pmids":["10825198"],"is_preprint":false},{"year":2002,"finding":"In yeast, lexA-Srb7 (Med21) fusion acts as a cryptic transcriptional activator that becomes active in the absence of Srb8, Srb10, Srb11, or Sin4, revealing a functional interaction with the CDK8 module. lexA-Srb7 is stably associated with Med4 and Med8, indicating incorporation into Mediator.","method":"Transcription reporter assay with lexA fusion proteins, co-immunoprecipitation","journal":"The Journal of biological chemistry","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — functional reporter assay with genetic dependencies and co-IP evidence, single lab","pmids":["12468546"],"is_preprint":false},{"year":2005,"finding":"Crystal structure of the MED7·MED21 (Med7·Srb7) heterodimer at 3.0 Å resolution reveals a highly extended structure with a four-helix bundle domain and a coiled-coil protrusion connected by a flexible hinge. Four putative protein-binding surfaces allow assembly of the Mediator middle module and binding of MED6, which bridges to the head module.","method":"X-ray crystallography (3.0 Å), structure-function analysis","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 / Strong — atomic-resolution crystal structure with identification of functional surfaces; foundational structural study","pmids":["15710619"],"is_preprint":false},{"year":2006,"finding":"In yeast, Med21 (Srb7) physically interacts with Med7, Med10, Med6, and Tup1. Interactions with Med7 and Med10 were confirmed by co-immunoprecipitation of tagged proteins in insect cells and E. coli and were found to depend strongly on amino acid residues 2–8 of Med21. A Med21 self-interaction was also detected by two-hybrid assay.","method":"Yeast two-hybrid, co-immunoprecipitation (tagged proteins in insect cells and E. coli), high-copy suppressor screen of med21-ts mutant","journal":"Molecular genetics and genomics","confidence":"High","confidence_rationale":"Tier 2 / Strong — interactions confirmed by two orthogonal methods (yeast 2-hybrid and co-IP in heterologous systems) with mutational delineation of critical residues","pmids":["16758199"],"is_preprint":false},{"year":2012,"finding":"In yeast, Med21 mutations within or adjacent to the Mediator middle module severely diminish heat-shock-induced expression of HSP82 without impeding RNA Pol II recruitment to the promoter, but instead impairing Pol II transit through the coding region. The med21 mutant also impairs nucleosome displacement from promoter and coding regions and reduces Pol II processivity on a GAL1-regulated reporter.","method":"Genetic mutation analysis, chromatin immunoprecipitation (ChIP), 6-azauracil sensitivity assay, reporter gene assay","journal":"Genetics","confidence":"High","confidence_rationale":"Tier 2 / Strong — multiple orthogonal methods (ChIP, drug sensitivity, reporter assay) with defined post-initiation phenotype; genetic loss-of-function","pmids":["22377631"],"is_preprint":false},{"year":2013,"finding":"A cross-linking mass spectrometry-based structural model of the yeast Mediator middle module reveals that the Med7/Med21 heterodimer forms part of a central tetramer with Med4/Med9, flanked by Med10 and Med31, indicating the structural organization of the middle module.","method":"Lysine-lysine protein cross-linking, mass spectrometry, 3D modeling from crystal and homology structures","journal":"Nucleic acids research","confidence":"Medium","confidence_rationale":"Tier 1 / Moderate — structural model from cross-linking MS; single study, model not independently validated by crystallography","pmids":["23939621"],"is_preprint":false},{"year":2016,"finding":"The integrity of the flexible hinge in the human MED21-MED7 heterodimer is required for binding of Mediator to RNA Pol II to form the holoenzyme. Point mutations in the hinge region leave core Mediator intact but cause increased disorder of the middle module and markedly reduced affinity for Pol II.","method":"Biochemistry (Mediator-Pol II binding assay), point mutagenesis, negative-stain electron microscopy","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1–2 / Strong — mutagenesis of defined structural element with quantitative binding assay and EM structural analysis; directly tests the hinge function","pmids":["27821593"],"is_preprint":false},{"year":2010,"finding":"Knockdown of MED21 in human keratinocytes causes hyperproliferation and defects in calcium-induced differentiation, including decreased expression of differentiation markers and decreased translocation of E-cadherin to the membrane, with increased cyclin D1 and glioma-associated oncogene homolog mRNA. MED21 co-purified with the DRIP/Mediator complex from keratinocytes using VDR affinity beads.","method":"siRNA knockdown, VDR affinity purification, mass spectrometry, RT-PCR, immunofluorescence","journal":"The Journal of investigative dermatology","confidence":"Medium","confidence_rationale":"Tier 2–3 / Moderate — siRNA knockdown with defined cellular phenotypes and biochemical purification; single lab, multiple readouts","pmids":["20520624"],"is_preprint":false},{"year":2014,"finding":"siRNA-mediated knockdown of MED21 in human cells significantly impairs HIV-1 viral replication at a post-integration step, placing MED21 as a host factor required for HIV-1 transcription.","method":"siRNA knockdown, viral replication assay, measurement of HIV transcripts","journal":"The Journal of biological chemistry","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — siRNA knockdown with defined step of viral cycle; single lab, phenotypic readout for transcription","pmids":["25100719"],"is_preprint":false}],"current_model":"MED21 (SRB7/SURB7) is an essential, ubiquitously expressed subunit of the Mediator middle module whose conserved MED7·MED21 heterodimer forms an extended structure with a flexible hinge that is required for Mediator binding to RNA Pol II to form the holoenzyme; it physically contacts MED6 (bridging to the head module), MED10, Med4, and the co-repressor Tup1 via its N-terminal residues 2–8, and its integrity is necessary for both transcriptional activation and Pol II elongation through coding regions, while loss of function is lethal in both yeast and mammals."},"narrative":{"mechanistic_narrative":"MED21 (SRB7/SURB7) is an essential subunit of the Mediator complex that physically couples Mediator to RNA polymerase II and is required for transcriptional regulation by this complex [PMID:8598913, PMID:9671713]. It co-purifies exclusively with high-molecular-weight Pol II holoenzyme forms containing TFIIE and TFIIH and confers heightened responsiveness to transcriptional activators [PMID:8598913], and orthologous Mediator complexes containing MED21 bind the Pol II CTD and stimulate its phosphorylation by TFIIH [PMID:9671713]. MED21 resides in complexes with dual regulatory output, capable of both repressing and synergistically enhancing activator-dependent transcription depending on TFIIH availability [PMID:9734358, PMID:10024883]. Structurally, MED21 forms an extended, hinged heterodimer with MED7 comprising a four-helix bundle and a coiled-coil protrusion, presenting protein-binding surfaces that nucleate assembly of the Mediator middle module and engage MED6 to bridge toward the head module [PMID:15710619]; within yeast Mediator the MED7·MED21 heterodimer forms a central tetramer with Med4/Med9 flanked by Med10 and Med31 [PMID:23939621]. MED21 directly contacts MED7, MED10, MED6 and the co-repressor Tup1, with interactions critically dependent on its N-terminal residues 2–8 [PMID:16758199], and the integrity of the MED7-MED21 flexible hinge is required for Mediator to bind Pol II and form the holoenzyme [PMID:27821593]. Beyond holoenzyme assembly, MED21 acts after initiation: yeast med21 mutations impair Pol II transit and processivity through coding regions and nucleosome displacement without blocking Pol II recruitment [PMID:22377631]. MED21 is single-copy, ubiquitously expressed, and essential for cell viability and embryonic development [PMID:10500093], and it is required for epidermal differentiation [PMID:20520624] and HIV-1 transcription [PMID:25100719].","teleology":[{"year":1996,"claim":"Established that human SRB7/MED21 is a bona fide component of a mammalian Pol II holoenzyme, extending the yeast Mediator paradigm to human transcription and tying MED21 to activator-responsive transcription.","evidence":"Antibody-based purification and in vitro transcription assays of a holoenzyme containing TFIIE/TFIIH","pmids":["8598913"],"confidence":"High","gaps":["Did not resolve MED21's position or direct contacts within the complex","Mechanism of enhanced activator responsiveness not defined"]},{"year":1998,"claim":"Showed that MED21-containing Mediator engages the Pol II CTD and stimulates its phosphorylation by TFIIH, linking MED21 complexes to CTD-dependent regulation, while a distinct NAT complex repressed transcription and phosphorylated the CTD at sites distinct from TFIIH.","evidence":"Protein purification with CTD binding and phosphorylation assays; biochemical purification with in vitro repression and kinase assays","pmids":["9671713","9734358"],"confidence":"High","gaps":["MED21's specific contribution to CTD binding versus other subunits unclear","Relationship between the activating and repressive complexes not resolved"]},{"year":1999,"claim":"Defined the dual, context-dependent output of MED21-containing complexes, showing the SMCC/Mediator can either repress or synergistically enhance activator-dependent transcription depending on TFIIH levels.","evidence":"Biochemical purification with in vitro transcription and activator-interaction assays; reciprocal co-IP with hTRFP and Pol II","pmids":["10024883","9933582"],"confidence":"Medium","gaps":["Molecular switch controlling repression versus activation not defined","MED21's direct role in these opposing activities not isolated"]},{"year":1999,"claim":"Demonstrated that MED21 is single-copy, ubiquitously expressed, and essential, establishing it as required for cell viability and embryonic development.","evidence":"Northern blot, ES cell gene knockout, and co-fractionation in mouse","pmids":["10500093"],"confidence":"High","gaps":["Did not distinguish whether lethality reflects loss of Mediator integrity versus a MED21-specific function","Cell-type-specific requirements not addressed"]},{"year":2000,"claim":"Tested whether MED21 alone can drive transcription when recruited to a promoter, showing that direct recruitment is insufficient for activation despite holoenzyme association.","evidence":"Fusion-protein recruitment and reporter assays in mammalian cells","pmids":["10825198"],"confidence":"Medium","gaps":["Does not exclude activation in other promoter or cellular contexts","Negative result; mechanism of why recruitment fails not defined"]},{"year":2002,"claim":"Revealed a functional interaction between MED21 and the CDK8 (Srb8/10/11/Sin4) module, showing that lexA-Srb7 acts as a cryptic activator unmasked when the repressive module is absent.","evidence":"lexA-fusion reporter assays with genetic dependencies and co-IP in yeast","pmids":["12468546"],"confidence":"Medium","gaps":["Direct physical link between MED21 and CDK8 module not established","Whether cryptic activation reflects a native regulatory mechanism unclear"]},{"year":2005,"claim":"Provided the atomic-resolution architecture of the MED7·MED21 heterodimer, defining an extended hinged structure with protein-binding surfaces for middle-module assembly and MED6 bridging to the head module.","evidence":"3.0 Å X-ray crystallography with structure-function analysis","pmids":["15710619"],"confidence":"High","gaps":["Functional role of the flexible hinge not yet tested","Structure of the full middle module within intact Mediator not resolved"]},{"year":2006,"claim":"Mapped MED21's direct interaction network and the residues that drive it, identifying contacts with MED7, MED10, MED6 and Tup1 and showing dependence on N-terminal residues 2–8.","evidence":"Yeast two-hybrid, co-IP of tagged proteins in heterologous systems, and high-copy suppressor screen of a med21-ts mutant","pmids":["16758199"],"confidence":"High","gaps":["Functional consequence of the Tup1 contact not defined","Whether all interactions occur simultaneously within assembled Mediator unclear"]},{"year":2012,"claim":"Uncovered a post-initiation function for MED21, showing mutations impair Pol II transit, processivity, and nucleosome displacement through coding regions without affecting Pol II recruitment.","evidence":"Genetic mutation analysis with ChIP, 6-azauracil sensitivity, and reporter assays in yeast","pmids":["22377631"],"confidence":"High","gaps":["Molecular mechanism linking MED21 to elongation/nucleosome dynamics not defined","Whether this elongation role is conserved in mammals not tested here"]},{"year":2013,"claim":"Placed the MED7/MED21 heterodimer within the topology of the middle module, defining its position in a central tetramer with Med4/Med9 flanked by Med10 and Med31.","evidence":"Lysine cross-linking mass spectrometry with 3D modeling in yeast","pmids":["23939621"],"confidence":"Medium","gaps":["Model derived from cross-linking, not independently validated by high-resolution structure","Conformational dynamics within intact Mediator not captured"]},{"year":2016,"claim":"Established the functional importance of the MED7-MED21 hinge, showing hinge integrity is required for Mediator-Pol II binding and holoenzyme formation.","evidence":"Point mutagenesis with Mediator-Pol II binding assays and negative-stain EM on human Mediator","pmids":["27821593"],"confidence":"High","gaps":["Precise conformational change at the hinge during Pol II engagement not resolved","How hinge flexibility is regulated in vivo unknown"]},{"year":2014,"claim":"Identified MED21 as a host factor co-opted for pathogen transcription, showing its knockdown impairs HIV-1 replication at a post-integration transcriptional step.","evidence":"siRNA knockdown with viral replication and HIV transcript assays in human cells","pmids":["25100719"],"confidence":"Medium","gaps":["Whether MED21 acts directly at the HIV promoter or via general Mediator function unclear","Mechanistic link to Tat/HIV transcription machinery not defined"]},{"year":2010,"claim":"Demonstrated a tissue-specific developmental requirement, showing MED21 knockdown causes keratinocyte hyperproliferation and blocks calcium-induced differentiation.","evidence":"siRNA knockdown, VDR affinity purification, RT-PCR and immunofluorescence in human keratinocytes","pmids":["20520624"],"confidence":"Medium","gaps":["Direct target genes through which MED21 controls differentiation not identified","Whether phenotype reflects MED21-specific versus general Mediator loss unclear"]},{"year":null,"claim":"How the conformational flexibility of the MED7-MED21 hinge is regulated to control Pol II engagement, and how MED21's middle-module function is coupled to elongation and gene-specific regulation in mammalian cells, remain unresolved.","evidence":"","pmids":[],"confidence":"High","gaps":["No mechanism connecting MED21 to elongation defined at molecular resolution in mammals","Regulatory inputs controlling hinge conformation unknown","Direct gene targets of MED21-dependent regulation in differentiation not mapped"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0140110","term_label":"transcription regulator activity","supporting_discovery_ids":[0,2,3]},{"term_id":"GO:0005198","term_label":"structural molecule activity","supporting_discovery_ids":[8,11]},{"term_id":"GO:0060090","term_label":"molecular adaptor activity","supporting_discovery_ids":[8,9,12]}],"localization":[{"term_id":"GO:0005634","term_label":"nucleus","supporting_discovery_ids":[0,4]}],"pathway":[{"term_id":"R-HSA-74160","term_label":"Gene expression (Transcription)","supporting_discovery_ids":[0,1,10]}],"complexes":["Mediator complex","RNA Pol II holoenzyme","Mediator middle module","MED7·MED21 heterodimer"],"partners":["MED7","MED6","MED10","MED4","TUP1","POLR2A"],"other_free_text":[]}},"prefetch_data":{"uniprot":{"accession":"Q13503","full_name":"Mediator of RNA polymerase II transcription subunit 21","aliases":["Mediator complex subunit 21","RNA polymerase II holoenzyme component SRB7","RNAPII complex component SRB7","hSrb7"],"length_aa":144,"mass_kda":15.6,"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/Q13503/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":true,"resolved_as":"","url":"https://depmap.org/portal/gene/MED21","classification":"Common Essential","n_dependent_lines":1034,"n_total_lines":1208,"dependency_fraction":0.8559602649006622},"opencell":{"profiled":true,"resolved_as":"","ensg_id":"ENSG00000152944","cell_line_id":"CID000233","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":"MED19","stoichiometry":10.0},{"gene":"MED7","stoichiometry":10.0},{"gene":"IXL;MED29","stoichiometry":10.0},{"gene":"MED24","stoichiometry":10.0},{"gene":"MED6","stoichiometry":10.0},{"gene":"MED22","stoichiometry":10.0},{"gene":"MED17","stoichiometry":10.0}],"url":"https://opencell.sf.czbiohub.org/target/CID000233","total_profiled":1310},"omim":[{"mim_id":"612915","title":"MEDIATOR COMPLEX SUBUNIT 20; MED20","url":"https://www.omim.org/entry/612915"},{"mim_id":"603800","title":"MEDIATOR COMPLEX SUBUNIT 21; MED21","url":"https://www.omim.org/entry/603800"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Approved","locations":[{"location":"Nucleoplasm","reliability":"Approved"},{"location":"Nucleoli","reliability":"Additional"},{"location":"Cytosol","reliability":"Additional"}],"tissue_specificity":"Low tissue specificity","tissue_distribution":"Detected in all","driving_tissues":[],"url":"https://www.proteinatlas.org/search/MED21"},"hgnc":{"alias_symbol":["SRB7"],"prev_symbol":["SURB7"]},"alphafold":{"accession":"Q13503","domains":[{"cath_id":"1.10.287","chopping":"1-32_47-75","consensus_level":"medium","plddt":88.8457,"start":1,"end":75},{"cath_id":"1.20.5","chopping":"82-135","consensus_level":"medium","plddt":93.1996,"start":82,"end":135}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/Q13503","model_url":"https://alphafold.ebi.ac.uk/files/AF-Q13503-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-Q13503-F1-predicted_aligned_error_v6.png","plddt_mean":85.0},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=MED21","jax_strain_url":"https://www.jax.org/strain/search?query=MED21"},"sequence":{"accession":"Q13503","fasta_url":"https://rest.uniprot.org/uniprotkb/Q13503.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/Q13503/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/Q13503"}},"corpus_meta":[{"pmid":"9671713","id":"PMC_9671713","title":"Mammalian 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Antibodies against human SRB7 were used to purify a holoenzyme containing general transcription factors TFIIE and TFIIH, which is more responsive to transcriptional activators than core Pol II in the presence of coactivators.\",\n      \"method\": \"Antibody-based purification, immunoprecipitation, in vitro transcription assay\",\n      \"journal\": \"Nature\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — reciprocal biochemical purification and functional transcription assay; replicated by multiple subsequent labs\",\n      \"pmids\": [\"8598913\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1998,\n      \"finding\": \"A murine counterpart of yeast Mediator was identified; the mouse complex contains homologs of yeast Mediator subunits including Srb7 (MED21) and Med7, binds to the RNA polymerase II CTD, and stimulates phosphorylation of the CTD by TFIIH.\",\n      \"method\": \"Protein purification, peptide sequencing, CTD binding assay, CTD phosphorylation assay (in vitro)\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 / Strong — direct in vitro biochemical assays (CTD binding, phosphorylation stimulation), replicated across labs\",\n      \"pmids\": [\"9671713\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1998,\n      \"finding\": \"Human SRB7 (MED21) is a component of the NAT complex, which represses activator-dependent transcription and phosphorylates the CTD of RNA Pol II at residues distinct from those phosphorylated by TFIIH. The complex interacts specifically with RNA Pol II in a manner that is not mediated by the CTD but is precluded by CTD phosphorylation.\",\n      \"method\": \"Biochemical purification, in vitro transcription repression assay, kinase assay, co-immunoprecipitation with RNA Pol II\",\n      \"journal\": \"Molecular cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 / Strong — in vitro kinase assay and transcription assay plus co-IP, multiple orthogonal methods in single study\",\n      \"pmids\": [\"9734358\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1999,\n      \"finding\": \"Human SRB7 (MED21) is a component of the SMCC complex that can repress activator-dependent transcription mediated by PC4 or, at limiting TFIIH, synergistically enhance activator-dependent transcription. The complex shows direct activator interactions and can act independently of the RNA Pol II CTD.\",\n      \"method\": \"Biochemical purification, in vitro transcription assay, activator interaction assay\",\n      \"journal\": \"Molecular cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 / Strong — in vitro transcription assays demonstrating both positive and negative functions, multiple orthogonal methods\",\n      \"pmids\": [\"10024883\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1999,\n      \"finding\": \"Mouse Srb7 (MED21) gene is single copy, expressed in all tissues examined, and its disruption in embryonic stem cells is lethal, demonstrating an essential role in cell viability and murine embryonic development. Murine Srb7 associates exclusively with high molecular weight forms of RNA polymerase II in extracts.\",\n      \"method\": \"Northern blot, gene knockout in embryonic stem cells, co-fractionation\",\n      \"journal\": \"Genes & development\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — clean gene knockout with defined lethal phenotype, co-fractionation, replicated by independent study\",\n      \"pmids\": [\"10500093\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1999,\n      \"finding\": \"Human SRB7 (MED21) is an integral component of an RNA Pol II-SRB complex; anti-SRB7 antibody immunoprecipitates hTRFP and RNA Pol II, and anti-hTRFP antibody reciprocally immunoprecipitates RNA Pol II and SRB7. The SRB7-containing complex supports basal transcription and enhances transcriptional activation by Gal4-VP16.\",\n      \"method\": \"Reciprocal co-immunoprecipitation, in vitro transcription assay\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — reciprocal co-IP and transcription assay, single lab\",\n      \"pmids\": [\"9933582\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2000,\n      \"finding\": \"Artificial recruitment of Srb7 (MED21) fused to a DNA-binding domain to a promoter in mammalian cells does not activate transcription, despite being associated with the Pol II holoenzyme and being directly recruited to the promoter.\",\n      \"method\": \"Transfection assay with fusion protein recruitment, reporter gene assay\",\n      \"journal\": \"Molecular and cellular biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — direct functional recruitment assay in mammalian cells; negative finding reported as a mechanistic result\",\n      \"pmids\": [\"10825198\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"In yeast, lexA-Srb7 (Med21) fusion acts as a cryptic transcriptional activator that becomes active in the absence of Srb8, Srb10, Srb11, or Sin4, revealing a functional interaction with the CDK8 module. lexA-Srb7 is stably associated with Med4 and Med8, indicating incorporation into Mediator.\",\n      \"method\": \"Transcription reporter assay with lexA fusion proteins, co-immunoprecipitation\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — functional reporter assay with genetic dependencies and co-IP evidence, single lab\",\n      \"pmids\": [\"12468546\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"Crystal structure of the MED7·MED21 (Med7·Srb7) heterodimer at 3.0 Å resolution reveals a highly extended structure with a four-helix bundle domain and a coiled-coil protrusion connected by a flexible hinge. Four putative protein-binding surfaces allow assembly of the Mediator middle module and binding of MED6, which bridges to the head module.\",\n      \"method\": \"X-ray crystallography (3.0 Å), structure-function analysis\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — atomic-resolution crystal structure with identification of functional surfaces; foundational structural study\",\n      \"pmids\": [\"15710619\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"In yeast, Med21 (Srb7) physically interacts with Med7, Med10, Med6, and Tup1. Interactions with Med7 and Med10 were confirmed by co-immunoprecipitation of tagged proteins in insect cells and E. coli and were found to depend strongly on amino acid residues 2–8 of Med21. A Med21 self-interaction was also detected by two-hybrid assay.\",\n      \"method\": \"Yeast two-hybrid, co-immunoprecipitation (tagged proteins in insect cells and E. coli), high-copy suppressor screen of med21-ts mutant\",\n      \"journal\": \"Molecular genetics and genomics\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — interactions confirmed by two orthogonal methods (yeast 2-hybrid and co-IP in heterologous systems) with mutational delineation of critical residues\",\n      \"pmids\": [\"16758199\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"In yeast, Med21 mutations within or adjacent to the Mediator middle module severely diminish heat-shock-induced expression of HSP82 without impeding RNA Pol II recruitment to the promoter, but instead impairing Pol II transit through the coding region. The med21 mutant also impairs nucleosome displacement from promoter and coding regions and reduces Pol II processivity on a GAL1-regulated reporter.\",\n      \"method\": \"Genetic mutation analysis, chromatin immunoprecipitation (ChIP), 6-azauracil sensitivity assay, reporter gene assay\",\n      \"journal\": \"Genetics\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — multiple orthogonal methods (ChIP, drug sensitivity, reporter assay) with defined post-initiation phenotype; genetic loss-of-function\",\n      \"pmids\": [\"22377631\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"A cross-linking mass spectrometry-based structural model of the yeast Mediator middle module reveals that the Med7/Med21 heterodimer forms part of a central tetramer with Med4/Med9, flanked by Med10 and Med31, indicating the structural organization of the middle module.\",\n      \"method\": \"Lysine-lysine protein cross-linking, mass spectrometry, 3D modeling from crystal and homology structures\",\n      \"journal\": \"Nucleic acids research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — structural model from cross-linking MS; single study, model not independently validated by crystallography\",\n      \"pmids\": [\"23939621\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"The integrity of the flexible hinge in the human MED21-MED7 heterodimer is required for binding of Mediator to RNA Pol II to form the holoenzyme. Point mutations in the hinge region leave core Mediator intact but cause increased disorder of the middle module and markedly reduced affinity for Pol II.\",\n      \"method\": \"Biochemistry (Mediator-Pol II binding assay), point mutagenesis, negative-stain electron microscopy\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 / Strong — mutagenesis of defined structural element with quantitative binding assay and EM structural analysis; directly tests the hinge function\",\n      \"pmids\": [\"27821593\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"Knockdown of MED21 in human keratinocytes causes hyperproliferation and defects in calcium-induced differentiation, including decreased expression of differentiation markers and decreased translocation of E-cadherin to the membrane, with increased cyclin D1 and glioma-associated oncogene homolog mRNA. MED21 co-purified with the DRIP/Mediator complex from keratinocytes using VDR affinity beads.\",\n      \"method\": \"siRNA knockdown, VDR affinity purification, mass spectrometry, RT-PCR, immunofluorescence\",\n      \"journal\": \"The Journal of investigative dermatology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2–3 / Moderate — siRNA knockdown with defined cellular phenotypes and biochemical purification; single lab, multiple readouts\",\n      \"pmids\": [\"20520624\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"siRNA-mediated knockdown of MED21 in human cells significantly impairs HIV-1 viral replication at a post-integration step, placing MED21 as a host factor required for HIV-1 transcription.\",\n      \"method\": \"siRNA knockdown, viral replication assay, measurement of HIV transcripts\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — siRNA knockdown with defined step of viral cycle; single lab, phenotypic readout for transcription\",\n      \"pmids\": [\"25100719\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"MED21 (SRB7/SURB7) is an essential, ubiquitously expressed subunit of the Mediator middle module whose conserved MED7·MED21 heterodimer forms an extended structure with a flexible hinge that is required for Mediator binding to RNA Pol II to form the holoenzyme; it physically contacts MED6 (bridging to the head module), MED10, Med4, and the co-repressor Tup1 via its N-terminal residues 2–8, and its integrity is necessary for both transcriptional activation and Pol II elongation through coding regions, while loss of function is lethal in both yeast and mammals.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"MED21 (SRB7/SURB7) is an essential subunit of the Mediator complex that physically couples Mediator to RNA polymerase II and is required for transcriptional regulation by this complex [#0, #1]. It co-purifies exclusively with high-molecular-weight Pol II holoenzyme forms containing TFIIE and TFIIH and confers heightened responsiveness to transcriptional activators [#0], and orthologous Mediator complexes containing MED21 bind the Pol II CTD and stimulate its phosphorylation by TFIIH [#1]. MED21 resides in complexes with dual regulatory output, capable of both repressing and synergistically enhancing activator-dependent transcription depending on TFIIH availability [#2, #3]. Structurally, MED21 forms an extended, hinged heterodimer with MED7 comprising a four-helix bundle and a coiled-coil protrusion, presenting protein-binding surfaces that nucleate assembly of the Mediator middle module and engage MED6 to bridge toward the head module [#8]; within yeast Mediator the MED7\\u00b7MED21 heterodimer forms a central tetramer with Med4/Med9 flanked by Med10 and Med31 [#11]. MED21 directly contacts MED7, MED10, MED6 and the co-repressor Tup1, with interactions critically dependent on its N-terminal residues 2\\u20138 [#9], and the integrity of the MED7-MED21 flexible hinge is required for Mediator to bind Pol II and form the holoenzyme [#12]. Beyond holoenzyme assembly, MED21 acts after initiation: yeast med21 mutations impair Pol II transit and processivity through coding regions and nucleosome displacement without blocking Pol II recruitment [#10]. MED21 is single-copy, ubiquitously expressed, and essential for cell viability and embryonic development [#4], and it is required for epidermal differentiation [#13] and HIV-1 transcription [#14].\",\n  \"teleology\": [\n    {\n      \"year\": 1996,\n      \"claim\": \"Established that human SRB7/MED21 is a bona fide component of a mammalian Pol II holoenzyme, extending the yeast Mediator paradigm to human transcription and tying MED21 to activator-responsive transcription.\",\n      \"evidence\": \"Antibody-based purification and in vitro transcription assays of a holoenzyme containing TFIIE/TFIIH\",\n      \"pmids\": [\"8598913\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Did not resolve MED21's position or direct contacts within the complex\", \"Mechanism of enhanced activator responsiveness not defined\"]\n    },\n    {\n      \"year\": 1998,\n      \"claim\": \"Showed that MED21-containing Mediator engages the Pol II CTD and stimulates its phosphorylation by TFIIH, linking MED21 complexes to CTD-dependent regulation, while a distinct NAT complex repressed transcription and phosphorylated the CTD at sites distinct from TFIIH.\",\n      \"evidence\": \"Protein purification with CTD binding and phosphorylation assays; biochemical purification with in vitro repression and kinase assays\",\n      \"pmids\": [\"9671713\", \"9734358\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"MED21's specific contribution to CTD binding versus other subunits unclear\", \"Relationship between the activating and repressive complexes not resolved\"]\n    },\n    {\n      \"year\": 1999,\n      \"claim\": \"Defined the dual, context-dependent output of MED21-containing complexes, showing the SMCC/Mediator can either repress or synergistically enhance activator-dependent transcription depending on TFIIH levels.\",\n      \"evidence\": \"Biochemical purification with in vitro transcription and activator-interaction assays; reciprocal co-IP with hTRFP and Pol II\",\n      \"pmids\": [\"10024883\", \"9933582\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Molecular switch controlling repression versus activation not defined\", \"MED21's direct role in these opposing activities not isolated\"]\n    },\n    {\n      \"year\": 1999,\n      \"claim\": \"Demonstrated that MED21 is single-copy, ubiquitously expressed, and essential, establishing it as required for cell viability and embryonic development.\",\n      \"evidence\": \"Northern blot, ES cell gene knockout, and co-fractionation in mouse\",\n      \"pmids\": [\"10500093\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Did not distinguish whether lethality reflects loss of Mediator integrity versus a MED21-specific function\", \"Cell-type-specific requirements not addressed\"]\n    },\n    {\n      \"year\": 2000,\n      \"claim\": \"Tested whether MED21 alone can drive transcription when recruited to a promoter, showing that direct recruitment is insufficient for activation despite holoenzyme association.\",\n      \"evidence\": \"Fusion-protein recruitment and reporter assays in mammalian cells\",\n      \"pmids\": [\"10825198\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Does not exclude activation in other promoter or cellular contexts\", \"Negative result; mechanism of why recruitment fails not defined\"]\n    },\n    {\n      \"year\": 2002,\n      \"claim\": \"Revealed a functional interaction between MED21 and the CDK8 (Srb8/10/11/Sin4) module, showing that lexA-Srb7 acts as a cryptic activator unmasked when the repressive module is absent.\",\n      \"evidence\": \"lexA-fusion reporter assays with genetic dependencies and co-IP in yeast\",\n      \"pmids\": [\"12468546\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Direct physical link between MED21 and CDK8 module not established\", \"Whether cryptic activation reflects a native regulatory mechanism unclear\"]\n    },\n    {\n      \"year\": 2005,\n      \"claim\": \"Provided the atomic-resolution architecture of the MED7\\u00b7MED21 heterodimer, defining an extended hinged structure with protein-binding surfaces for middle-module assembly and MED6 bridging to the head module.\",\n      \"evidence\": \"3.0 \\u00c5 X-ray crystallography with structure-function analysis\",\n      \"pmids\": [\"15710619\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Functional role of the flexible hinge not yet tested\", \"Structure of the full middle module within intact Mediator not resolved\"]\n    },\n    {\n      \"year\": 2006,\n      \"claim\": \"Mapped MED21's direct interaction network and the residues that drive it, identifying contacts with MED7, MED10, MED6 and Tup1 and showing dependence on N-terminal residues 2\\u20138.\",\n      \"evidence\": \"Yeast two-hybrid, co-IP of tagged proteins in heterologous systems, and high-copy suppressor screen of a med21-ts mutant\",\n      \"pmids\": [\"16758199\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Functional consequence of the Tup1 contact not defined\", \"Whether all interactions occur simultaneously within assembled Mediator unclear\"]\n    },\n    {\n      \"year\": 2012,\n      \"claim\": \"Uncovered a post-initiation function for MED21, showing mutations impair Pol II transit, processivity, and nucleosome displacement through coding regions without affecting Pol II recruitment.\",\n      \"evidence\": \"Genetic mutation analysis with ChIP, 6-azauracil sensitivity, and reporter assays in yeast\",\n      \"pmids\": [\"22377631\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Molecular mechanism linking MED21 to elongation/nucleosome dynamics not defined\", \"Whether this elongation role is conserved in mammals not tested here\"]\n    },\n    {\n      \"year\": 2013,\n      \"claim\": \"Placed the MED7/MED21 heterodimer within the topology of the middle module, defining its position in a central tetramer with Med4/Med9 flanked by Med10 and Med31.\",\n      \"evidence\": \"Lysine cross-linking mass spectrometry with 3D modeling in yeast\",\n      \"pmids\": [\"23939621\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Model derived from cross-linking, not independently validated by high-resolution structure\", \"Conformational dynamics within intact Mediator not captured\"]\n    },\n    {\n      \"year\": 2016,\n      \"claim\": \"Established the functional importance of the MED7-MED21 hinge, showing hinge integrity is required for Mediator-Pol II binding and holoenzyme formation.\",\n      \"evidence\": \"Point mutagenesis with Mediator-Pol II binding assays and negative-stain EM on human Mediator\",\n      \"pmids\": [\"27821593\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Precise conformational change at the hinge during Pol II engagement not resolved\", \"How hinge flexibility is regulated in vivo unknown\"]\n    },\n    {\n      \"year\": 2014,\n      \"claim\": \"Identified MED21 as a host factor co-opted for pathogen transcription, showing its knockdown impairs HIV-1 replication at a post-integration transcriptional step.\",\n      \"evidence\": \"siRNA knockdown with viral replication and HIV transcript assays in human cells\",\n      \"pmids\": [\"25100719\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Whether MED21 acts directly at the HIV promoter or via general Mediator function unclear\", \"Mechanistic link to Tat/HIV transcription machinery not defined\"]\n    },\n    {\n      \"year\": 2010,\n      \"claim\": \"Demonstrated a tissue-specific developmental requirement, showing MED21 knockdown causes keratinocyte hyperproliferation and blocks calcium-induced differentiation.\",\n      \"evidence\": \"siRNA knockdown, VDR affinity purification, RT-PCR and immunofluorescence in human keratinocytes\",\n      \"pmids\": [\"20520624\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Direct target genes through which MED21 controls differentiation not identified\", \"Whether phenotype reflects MED21-specific versus general Mediator loss unclear\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"How the conformational flexibility of the MED7-MED21 hinge is regulated to control Pol II engagement, and how MED21's middle-module function is coupled to elongation and gene-specific regulation in mammalian cells, remain unresolved.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"High\",\n      \"gaps\": [\"No mechanism connecting MED21 to elongation defined at molecular resolution in mammals\", \"Regulatory inputs controlling hinge conformation unknown\", \"Direct gene targets of MED21-dependent regulation in differentiation not mapped\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0140110\", \"supporting_discovery_ids\": [0, 2, 3]},\n      {\"term_id\": \"GO:0005198\", \"supporting_discovery_ids\": [8, 11]},\n      {\"term_id\": \"GO:0060090\", \"supporting_discovery_ids\": [8, 9, 12]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005634\", \"supporting_discovery_ids\": [0, 4]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-74160\", \"supporting_discovery_ids\": [0, 1, 10]}\n    ],\n    \"complexes\": [\"Mediator complex\", \"RNA Pol II holoenzyme\", \"Mediator middle module\", \"MED7\\u00b7MED21 heterodimer\"],\n    \"partners\": [\"MED7\", \"MED6\", \"MED10\", \"MED4\", \"TUP1\", \"POLR2A\"],\n    \"other_free_text\": []\n  }\n}","audit_flag":null,"evaluation":{"pairwise":"win","faith_supported":7,"faith_total":7,"faith_pct":100.0}}