SIN3-DEPENDENT TRANSCRIPTIONAL REPRESSION BY INTERACTION WITH THE MAD1 DNA-BINDING PROTEIN

The SIN3 gene in Saccharomyces cerevisiae encodes a negative regulator of transcription of a large number of genes. Mouse homologs of SIN3 h ave been identified through screens for proteins interacting with the mammalian Mad1 protein, a transcriptional repressor. We find that yeas t Sin3 (ySin3) interacts with Mad1 and that, as for mouse Sin3, the N terminus of Mad1 interacts with the PAH2 domain of ySin3. Although Mad 1 (a basic helix-loop-helix leucine zipper [bHLH-Zip] protein) forms a heterodimer with the Max bBLH-Zip protein, LexA-Mad1 and VP16-Max do not activate transcription of a reporter gene in a two-hybrid assay. T his failure in activation is due to direct repression by ySin3, as Lex A-Mad1 and VP16-Max are able to activate the two-hybrid reporter in a sin3 mutant. This inhibition of activation by LexA-Mad1 and VP16-Max r equires the PAH2 domain of ySin3 and the N-terminal interaction region of Mad1. These data demonstrate that ySin3 functions as a transcripti onal repressor by being brought to promoters by interacting with prote ins bound to DNA.