Repression by Imitation

Abstract

RNA polymerase II (RNA pol II) has a heptapeptide repeat motif in the COOH-terminal domain (CTD) that serves as a docking site for positive transcription elongation factor b (P-TEFb). The P-TEFb complex contains the cyclin T1 and cyclin-dependent kinase 9 (CDK9) and interacts with RNA pol II through the cyclin T1 subunit. Upon phosphorylation of the heptapeptide motif by CDK9, RNA pol II is released from P-TEFb and proceeds to catalyze transcription. Zhang et al. showed that CTD-like sequences that cannot be phosphorylated were effective repressors of expression in a reporter assay when targeted to the promoter region of the reporter gene. The Caenorhabditis elegans transcriptional repressor PIE-1 has a CTD-like motif that cannot be phosphorylated, and this motif also inhibited expression of the reporter. The PIE-1 CTD-like and synthetic CTD-like sequences interacted with cyclin T1 in a glutathione S -transferase pull-down assay. Mutation of the CTD-like sequences so that they contain acidic or phosphorylatable residues blocked the interaction with cyclin T1 and the ability to repress transcription. These results provide a molecular mechanism by which transcriptional repressors containing pseudo-CTD sequences inhibit transcription may act as pseudosubstrates for the P-TEFb, thus preventing P-TEFb from phosphorylating and activating RNA pol II. F. Zhang, M. Barboric, T. K. Blackwell, B. M. Peterlin, A model of repression: CTD analogs and PIE-1 inhibit transcriptional elongation by P-TEFb. Genes Dev. 17 , 748-758 (2003). [Abstract] [Full Text]