A large protein complex containing the yeast Sin3p and Rpd3p transcriptional regulators.

The SIN3 gene is required for the transcriptional repression of diverse genes in Saccharomyces cerevisiae. Sin3p does not bind directly to DNA but is thought to be targeted to promoters by interacting with sequence-specific DNA-binding proteins. We show here that Sin3p is present in a large multiprotein complex with an apparent molecular mass, estimated by gel filtration chromatography, of greater than 2 million Da. Genetic studies have shown that the yeast RPD3 gene has a function similar to that of SIN3 in transcriptional regulation, as SIN3 and RPD3 negatively regulate the same set of genes. The SIN3 and RPD3 genes are conserved from yeasts to mammals, and recent work suggests that RPD3 may encode a histone deacetylase. We show that Rpd3p is present in the Sin3p complex and that an rpd3 mutation eliminates SIN3-dependent repression. Thus, Sin3p may function as a bridge to recruit the Rpd3p histone deacetylase to specific promoters.