REGULATION OF HIS4 EXPRESSION BY THE SACCHAROMYCES-CEREVISIAE SIN4 TRANSCRIPTIONAL REGULATOR

The yeast SIN4 gene functions in the transcriptional activation and re pression of diverse yeast genes. Previous experiments suggest a sin4 m utation affects chromatin structure and thus alters transcriptional re gulation. In this report we show that SIN4 is required for full expres sion of the HIS4, Ty1, and MAT alpha genes, in addition to the previou sly described SIN4-dependence of CTS1 expression. All of these genes c ontain within their promoters a binding site for the Rap1p transcripti onal regulator. However, SIN4 does not play a direct role either in tr anscriptional activation or repression by Rap1p. The HIS4 gene can be activated by either of true pathways, the basal or the inducible pathw ay, and experiments are described that show that a sin4 mutation affec ts both pathways. It was shown previously that mutation of the Rap1p b inding site in the HIS4 promoter causes a similar effect on HIS4 expre ssion and that this promoter mutation also causes a change in chromati n structure. The SNF2/SW12 gene is also required for full HIS4 express ion, and we show that a sin4 snf2 double mutant is not synergistic com pared to either single mutant. We show that nucleosomes are positioned at the HIS4 promoter and that this positioning is disrupted in a snf2 mutant but not in a sin4 mutant. These findings suggest that SIN4 pla ys a distinct role in transcriptional regulation.