In the previous grant period, we made the unexpected discovery that members of the epidermal growth factor receptor family differentially regulate expression of the central transcription initiation factor, TBP, and defined new signaling pathways and transcription factors that regulate TBP expression. In addition to identifying positive regulators of TBP, we discovered a novel human protein, Maf1, which represses TBP transcription. Maf1 is an important transcriptional repressor that uniquely targets both RNA pol II- and pol III-transcribed genes that promote oncogenic transformation. Our new results support the idea that Maf1 is a key target of PTEN that is critical for its tumor suppressor function. Loss of PTEN results in a marked decrease in Maf1 expression; increased Maf1 expression suppresses cellular transformation in PTEN- deficient cells; and nuclear Maf1 expression is diminished in both mouse and human prostate cancers that are PTEN-deficient. Our overall goal is to understand the molecular and biological function of Maf1, and to use mouse models to determine whether the resultant decrease in Maf1 expression, by loss of PTEN, contributes to the development of prostate cancer. Aim 1 will identify Maf1 occupied regions genome- wide in primary human prostate epithelial cells. Given our newly identified interaction between Maf1 and the transcription factor Mediator CDK8 subcomplex, we will further test the novel hypothesis that Maf1 blocks the ability of this subcomplex to induce both RNA pol II- and III-dependent gene expression. These studies will define new paradigms by which transcription from different RNA polymerases are co-repressed, thus elucidating important gene repression pathways. Given that CDK8 is a potent oncoprotein, Aim 2 will further assess whether Maf1 abrogates CDK8-mediated oncogenic transformation. A transgenic mouse model will be established to test the idea that restoring Maf1 expression in PTEN-deficient mouse prostate will block or delay prostate intraepithelial neoplasia and tumorigenesis. Characterization of Maf1 will provide a new paradigm for how cells suppress a transformed phenotype and define a novel PTEN target whose loss is critical to the development of prostate cancer.