In both induced and basally transcribed states, the chromatin structure of the yeast HSP82 heat-shock locus exhibits a remarkable degree of organization with respect to DNA sequence. The promoter region contains a constitutive DNase I hypersensitive site. The transcription unit is markedly sensitive to DNase I, and exhibits a sharp transition from a phased half- to a whole nucleosomal cleavage periodicity at the 3′ end. Distant upstream and downstream regions are also organized into distinct arrays of phased nucleosomes. Each array is demarcated by DNase I hypersensitive sites that display internal protected regions, suggesting the presence of DNA binding proteins. In addition, since these sites are of mononucleosomal DNA length, they may acquire a nucleosomal structure under certain environmental conditions without disrupting flanking nucleosomal phasing frames. Thus, the HSP82 locus is organized into specific, phased, chromatin structures that appear to function in transcriptional initiation, RNA polymerase passage, transcriptional termination, and the establishment of chromatin-domain microenvironments.
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