Mechanisms of transcriptional repression

Eukaryotic transcriptional repressors utilize a variety of mechanisms to inhibit transcription and thereby regulate cellular growth. In this thesis study, I have investigated the involvement of two repressors and their distinct repression mechanisms in animal development and cellular differentiation.; The stage-specific expression of the Drosophila alcohol dehydrogenase (Adh) gene is achieved through the alternate activation of two tandem promoters. The proximal promoter is active primarily during embryonic development and early larval stages, while the distal promoter is active in third instar larvae and adults. I demonstrate that this Adh promoter switch is regulated by a zinc finger repressor protein (AEF-1) that is expressed predominantly in adult flies and targets the initiator region of the proximal promoter. Specifically, I show that AEF-1 binds with high affinity to the initiator (Inr) region of the proximal promoter, and inhibits transcription both in vivo and in vitro. Mutations in the proximal promoter that disrupt AEF-1 binding result in elevated proximal promoter activity in adult flies. Finally, when the Inr elements of the distal and proximal promoters were exchanged, the activity of the proximal promoter increased in adults while that of the distal promoter decreased.; The differentiation of mammalian B cells into plasma cells involves inhibition of the c-myc gene transcription by a repressor protein PRDI-BF1/Blimp-1. The mechanism of this PRDI-BF1/Blimp-1-mediated repression was not clear. I present evidence that PRDI-BF1 represses transcription by recruiting corepressors of the groucho family. Specifically, I show that PRDI-BF1 can function as a long-range repressor. It contains a transferable repression domain that is both necessary and sufficient for its repression. PRDI-BF1 interacts specifically through the repression domain with the groucho family proteins, particularly TLE1, TLE2 and the human groucho related gene (hGrg). I demonstrate that hGrg, like other groucho family proteins, represses transcription when tethered to a promoter. In addition, the N terminal Q domains of hGrg and TLE1, which are involved in protein-protein interactions with PRDI-BF1 as well as in dimerization between groucho family proteins, act as dominant negative inhibitors of the PRDI-BF1 repression in cultured cells.