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 HMLalpha and the active MATalpha shows that unique organized chromatin structure characterizes the silent state of HMLalpha. Precisely positioned nucleosomes abutting the silencers extend over the alpha1 and alpha2 coding regions. The HO endonuclease recognition site, nuclease hypersensitive at MATalpha, is protected at HMLalpha. Although two precisely positioned nucleosomes incorporate transcription start sites at HMLalpha, the promoter region of the alpha1 and alpha2 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 HMLalpha in Sir protein and histone H4 N-terminal deletion mutants is identical to that of the transcriptionally active MATalpha. The discontinuous chromatin structure of HMLalpha contrasts with the continuous array of nucleosomes found at repressed a-cell-specific genes and the recombination enhancer. Punctuation at HMLalpha may be necessary for higher-order structure or karyoskeleton interactions. The unique chromatin architecture of HMLalpha may relate to the combined requirements of transcriptional repression and recombinational competence.
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