EFFICIENT TRANSCRIPTIONAL SILENCING IN SACCHAROMYCES-CEREVISIAE REQUIRES A HETEROCHROMATIN HISTONE ACETYLATION PATTERN

Heterochromatin in metazoans induces transcriptional silencing, as exe mplified by position effect variegation in Drosophila melanogaster and X-chromosome inactivation in mammals. Heterochromatic DNA is packaged in nucleosomes that are distinct in their acetylation pattern from th ose present in euchromatin, although the role these differences play i n the structure of heterochromatin or in the effects of heterochromati n on transcriptional activity is unclear. Here we report that, as obse rved in the facultative heterochromatin of the inactive X chromosome i n female mammalian cells, histones H3 and H4 in chromatin spanning the transcriptionally silenced mating-type cassettes of the yeast Sacchar omyces cerevisiae are hypoacetylated relative to histones H3 and H4 of transcriptionally active regions of the genome. By immunoprecipitatio n of chromatin fragments, with antibodies specific for H4 acetylated a t particular lysine residues, we found that only three of the four lys ine residues in the amino-terminal domain of histone H4 spanning the s ilent cassettes are hypoacetylated. Lysine 12 shows significant acetyl ation levels. This is identical to the pattern of histone H4 acetylati on observed in centric heterochromatin of D. melanogaster. These two o bservations provide additional evidence that the silent cassettes are encompassed in the yeast equivalent of metazoan heterochromatin. Furth er, mutational analysis of the amino-terminal domain of histone H4 in S. cerevisiae demonstrated that this observed pattern of histone H4 ac etylation is required for transcriptional Silencing. This result, in c onjunction with prior mutational analyses of yeast histones H3 and H4, indicates that the particular pattern of nucleosome acetylation found in heterochromatin is required for its effects on transcription and i s not simply a side effect of heterochromatin formation.