Abstract Body: The Mediator complex serves as a bridge to link transcriptional machinery and transcription factors (TFs) to control transcription, but detailed molecular mechanisms of transcriptional regulation by Mediator are not well-understood. Mediator is comprised of the head, middle, and tail submodules that make up the core Mediator, and the transiently associating kinase submodule. The CDK8 kinase submodule acts to regulate Mediator-RNA Polymerase II interaction. MED12 is an essential Mediator component within the kinase submodule and is required for CDK8 kinase activity. Human heart failure patients exhibit dysregulated levels of MED12 at both the mRNA and protein level, while we observe the progression of dilated cardiomyopathy (DCM) in mouse models of both Med12 deletion and overexpression. These observations are strengthened by a common group of genes, identified from RNA sequencing of ventricular tissue, involved in calcium handling and contraction that are expressed at atypical levels in our Med12 mouse models. While MED12 is established as a transcriptional regulator, it does not directly bind to DNA. To determine how MED12 regulates gene expression we screened for TFs that interact with MED12. We performed immunoprecipitation of MED12 and by mass spectrometry and determined that MED12 interacts with the TFs MEF2, CREB, and SRF. In follow-up experiments we demonstrated that MED12 interacts with the transcription factor (TF) MEF2 to regulate calcium handling genes. Current studies are focused on determining the domains of MED12 that are required for these TF interactions, and the molecular mechanisms that are disabled when those interactions do not occur. Collectively, our data demonstrate that MED12 acts as a key regulator of cardiac transcription, where dysregulated levels lead to the development of heart failure in part by dysregulation of gene expression. Therefore, determining the precise relationship between MED12 and its TF interacting partners will allow us to better understand how the regulation of gene expression progresses in heart failure.