Global alterations in chromatin accessibility associated with loss of SIN4 function

Sin4p is a component of a mediator complex associated with the C-terminal domain of RNA polymerase II and SIN4 is required for proper regulation of several genes in yeast, including the HO endonuclease gene, glucose repressible genes and MATa cell-specific genes. Previous studies indicated that SIN4 may influence transcription through changes in the organization of chromatin. We have examined a specific chromatin structure associated with MATa cell-specific repression in sin4 MATalpha cells to determine if SIN4 is required for nucleosome positioning. Although the loss of SIN4 has no effect on nucleosome location, we find that the sensitivity of bulk chromatin from sin4 cells to micrococcal nuclease digestion is strikingly increased relative to chromatin from isogenic wild-type cells. The nuclease hypersensitivity of chromatin from sin4 cells is not related to gross alterations in histone gene expression or to bulk increases in histone modification. Our experiments suggest that SIN4 directly or indirectly regulates a global aspect of chromatin accessibility, providing a molecular basis for phenotypic similarities between sin4 mutations and mutations in histones.