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Mol. Cell. Biol., 08 1996, 4349-4356, Vol 16, No. 8
M Braunstein, RE Sobel, CD Allis, BM Turner and JR Broach
Heterochromatin in metazoans induces transcriptional silencing, as
exemplified 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 those
present in euchromatin, although the role these differences play in the
structure of heterochromatin or in the effects of heterochromatin on
transcriptional activity is unclear. Here we report that, as observed in
the facultative heterochromatin of the inactive X chromosome in female
mammalian cells, histones H3 and H4 in chromatin spanning the
transcriptionally silenced mating-type cassettes of the yeast Saccharomyces
cerevisiae are hypoacetylated relative to histones H3 and H4 of
transcriptionally active regions of the genome. By immunoprecipitation of
chromatin fragments with antibodies specific for H4 acetylated at
particular lysine residues, we found that only three of the four lysine
residues in the amino-terminal domain of histone H4 spanning the silent
cassettes are hypoacetylated. Lysine 12 shows significant acetylation
levels. This is identical to the pattern of histone H4 acetylation observed
in centric heterochromatin of D. melanogaster. These two observations
provide additional evidence that the silent cassettes are encompassed in
the yeast equivalent of metazoan heterochromatin. Further, mutational
analysis of the amino- terminal domain of histone H4 in S. cerevisiae
demonstrated that this observed pattern of histone H4 acetylation is
required for transcriptional silencing. This result, in conjunction 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 is not simply a side effect
of heterochromatin formation.
Copyright © 1996, American Society for Microbiology
Efficient transcriptional silencing in Saccharomyces cerevisiae requires a heterochromatin histone acetylation pattern
Department of Molecular Biology, Princeton University, New Jersey 08544, USA.
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