Molecular and Cellular Biology, September 1998, p. 5392-5403, Vol. 18, No. 9
0270-7306/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.
Department of Biochemistry and Molecular Biology, The Center for Gene Regulation, The Pennsylvania State University, University Park, Pennsylvania 16802
Received 5 March 1998/Returned for modification 21 April 1998/Accepted 4 June 1998
Genetic studies have suggested that chromatin structure is involved
in repression of the silent mating type loci in Saccharomyces cerevisiae. Chromatin mapping at nucleotide resolution of the transcriptionally silent HML
and the active
MAT
shows that unique organized chromatin structure
characterizes the silent state of HML
. Precisely
positioned nucleosomes abutting the silencers extend over the
1 and
2 coding regions. The HO endonuclease recognition site, nuclease
hypersensitive at MAT
, is protected at
HML
. Although two precisely positioned nucleosomes
incorporate transcription start sites at HML
, the
promoter region of the
1 and
2 genes is nucleosome free and more
nuclease sensitive in the repressed than in the transcribed locus.
Mutations in genes essential for HML silencing disrupt the
nucleosome array near HML-I but not in the vicinity of HML-E, which is
closer to the telomere of chromosome III. At the promoter and the HO
site, the structure of HML
in Sir protein and histone H4
N-terminal deletion mutants is identical to that of the
transcriptionally active MAT
. The discontinuous
chromatin structure of HML
contrasts with the continuous
array of nucleosomes found at repressed a-cell-specific genes and the
recombination enhancer. Punctuation at HML
may be
necessary for higher-order structure or karyoskeleton interactions. The
unique chromatin architecture of HML
may relate to the
combined requirements of transcriptional repression and recombinational competence.
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