Effects of Sin(-) versions of histone H4 on yeast chromatin structure and function

Previous studies have identified single amino acid changes within either histone H3 or H4 (Sin(-) versions) that allow transcription in the absence of the yeast SWI-SNF complex, The histone H4 mutants are competent for nucleosome assembly in vivo, and the residues that are altered appear to define a discrete domain on the surface of the histone octamer, We have analyzed the effects of the Sin(-) versions of histone H4 on transcription and chromatin structure in vivo. These histone H4 mutants cause an increased accessibility of nucleosomal DNA to Dam methyltransferase and to micrococcal nuclease, Sin(-) derivatives of histone H4 also grossly impair the ability of nucleosomes to constrain supercoils in vivo. Nucleosome-mediated repression of the PH05 gene is severely impaired by these histone H4 mutants; PH05 expression is derepressed to 31% of the wild-type induced level, In contrast to the induction caused by nucleosome depletion, full PH05 derepression by Sin(-) versions of histone H4 requires upstream regulatory elements. In addition, Sin(-) derivatives of histone H4 do not activate expression from CYC1 or GAL1 promoters that lack UAS elements, We propose that these Sin(-) mutations alter histone-DNA contact residues that play key roles in restricting the accessibility of nucleosomal DNA to transcription factors.