Promoter recruitment of corepressors Sin3 and Cyc8 by activator proteins of the yeast Saccharomyces cerevisiae.

It is generally assumed that pathway-specific transcriptional activators recruit pleiotropic coactivators (such as chromatin-modifying complexes or general transcription factors), while specific repressors contact pleiotropic corepressors creating an inaccessible chromatin by the action of histone deacetylases. We have previously shown that the negative regulator Opi1 of yeast phospholipid biosynthesis inhibits transcription by recruiting corepressors Sin3 and Cyc8 in the presence of precursor molecules inositol and choline. To get access to its target genes, Opi1 physically contacts and counteracts DNA-bound activator Ino2. By using chromatin immunoprecipitation, we show that Sin3 and Cyc8 can be detected at Opi1 target promoters INO1 and CHO2 under repressing and derepressing conditions and that corepressor binding is effective even in the absence of Opi1, while Ino2 is absolutely required. Thus, corepressors may be recruited not only by repressors but also by activators such as Ino2. Indeed, we could demonstrate direct interaction of Ino2 with Sin3 and Cyc8. The Opi1 repressor interaction domain within Ino2 is also able to contact Sin3 and Cyc8. Recruitment of corepressors by an activator is not a regulatory exception as we could show that activators Pho4 and Hac1 also contain domains being able to interact with Sin3 and Cyc8.