Independent Recruitment of the Mediator Tail Module to theHOPromoter Suggests Mediator Core Limits Coactivator Recruitment inSaccharomyces cerevisiae

Abstract

ABSTRACTMediator is an essential, multisubunit complex that functions as a transcriptional coactivator in yeast and other eukaryotic organisms. Mediator has four conserved modules, Head, Middle, Tail, and Kinase, and has been implicated in nearly all aspects of gene regulation. The Tail module has been shown to recruit the Mediator complex to the enhancer or UAS regions of genes via interactions with transcription factors, and the Kinase domain facilitates the transition of Mediator from the UAS/enhancer to the preinitiation complex via protein phosphorylation. Here we analyze expression of theSaccharomyces cerevisiae HOgene using asin4Mediator Tail mutation that separates the Tail module from the rest of the complex; thesin4mutation permits independent recruitment of the Tail module to promoters without the rest of Mediator. Significant increases in recruitment of the SWI/SNF and SAGA coactivators to theHOpromoter UAS were observed in asin4mutant, along with increased gene activation. These results are consistent with recent studies that have suggested the Kinase module functions negatively to inhibit activation by the Tail. However, we found that Kinase module mutations did not mimic the effect of asin4mutation onHOexpression. This suggests that atHOthe core Mediator complex (Middle and Head modules) must play a role in limiting Tail binding to the promoter UAS and gene activation. We propose that the core Mediator complex helps modulate Mediator binding to the UAS regions of genes to limit coactivator recruitment and ensure proper regulation of gene transcription.