Regulation of HIS4 expression by the Saccharomyces cerevisiae SIN4 transcriptional regulator.

The yeast SIN4 gene functions in the transcriptional activation and repression of diverse yeast genes. Previous experiments suggest a sin4 mutation affects chromatin structure and thus alters transcriptional regulation. In this report we show that SIN4 is required for full expression of the HIS4, Ty1, and MAT alpha genes, in addition to the previously described SIN4-dependence of CTS1 expression. All of these genes contain within their promoters a binding site for the Rap1p transcriptional regulator. However, SIN4 does not play a direct role either in transcriptional activation or repression by Rap1p. The HIS4 gene can be activated by either of two pathways, the basal or the inducible pathway, and experiments are described that show that a sin4 mutation affects both pathways. It was shown previously that mutation of the Rap1p binding site in the HIS4 promoter causes a similar effect on HIS4 expression and that this promoter mutation also causes a change in chromatin structure. The SNF2/SWI2 gene is also required for full HIS4 expression, and we show that a sin4 snf2 double mutant is not synergistic compared 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 plays a distinct role in transcriptional regulation.