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 PHO5 gene is severely impaired by these histone H4 mutants; PHO5 expression is derepressed to 31% of the wild‐type induced level. In contrast to the induction caused by nucleosome depletion, full PHO5 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.