| Summary: | The Mediator complex is a conserved coregulator of RNA polymerase II transcription. Whereas some Mediator subunits are universally essential for transcription, others regulate specialized gene programs by interacting with sequence-specific transcription factors (TFs). Mediator’s Cyclin dependent kinase 8 (CDK8) kinase module (CKM) consists of four subunits (CDK8, Cyclin C, MED12, MED13) and regulates transcription downstream of multiple cell signaling pathways. In addition, the CKM regulates other Mediator subunits, as CDK8-mediated phosphorylation promotes Mediator subunit turnover, at least in yeast. CKM subunits have been identified as human oncogenes or tumor suppressors, indicating that the CKM can modulate transcription in tumorigenesis. However, the roles of the CKM in animal development and physiology are less well understood, as its target TFs often remain undefined. Furthermore, whether the CKM regulates the activity of other Mediator subunits in metazoans remains unknown. In this dissertation, I investigated CKM interactions with TFs and other Mediator subunits in Caenorhabditis elegans development and physiology. Gene expression profiling of C. elegans cdk-8 mutants implicated CDK-8 in regulation of epidermal growth factor receptor (EGFR)-Ras-extracellular signal-regulated kinase (ERK)-driven transcription and cadmium-responsive transcription. I showed that the CKM inhibits ectopic vulval cell fates downstream of the EGFR-Ras-ERK pathway, dependent on CDK-8 kinase activity. Mechanistically, the CKM inhibits EGFR-Ras-ERK pathway output by promoting transcriptional repression by the LIN-1/Elk1 TF, and by inhibiting transcriptional activation by the Mediator subunit MDT-15. Furthermore, cdk-8 is required for post-transcriptional regulation of MDT-15. Therefore, the CKM restrains EGFR-Ras-ERK signaling in C. elegans development by regulating TF and Mediator activity. I also studied cdk-8 in the cadmium response. I showed that cdk-8 is required for cadmium-inducible transcription and organismal cadmium resistance. Dissecting a modular cadmium-responsive promoter, cdr-1, I showed that cdk-8 may cooperate with other factors known to regulate cadmium-responsive transcription: mdt-15, GATA-family TF elt-2 and GATA elements, and a high zinc-activated (HZA) element. I speculate that CDK-8 promotes cadmium-inducible transcription by activating MDT-15, ELT-2, or an HZA-binding TF. In sum, cdk-8 cooperates with distinct TFs, and can oppose or cooperate with the Mediator subunit mdt-15, to regulate EGFR-Ras-ERK-inducible vs. cadmium-inducible transcription. === Medicine, Faculty of === Medical Genetics, Department of === Graduate
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