Abstract
We have utilized DNase I and micrococcal nuclease (MNase) to map the chromatin structure of the HSC82 heat shock gene of Saccharomyces cerevisiae. The gene is expressed at a high basal level which is enhanced 2-3-fold by thermal stress. A single, heat-shock invariant DNase I hypersensitive domain is found within the HSC82 chromosomal locus; it maps to the gene's 5' end and spans 250 bp of promoter sequence. DNase I genomic footprinting reveals that within this hypersensitive region are four constitutive protein-DNA interactions. These map to the transcription initiation site, the TATA box, the promoter-distal heat shock element (HSE1) and a consensus GRF2 (REB1/Factor Y) sequence. However, two other potential regulatory sites, the promoter-proximal heat shock element (HSE0) and a consensus upstream repressor sequence (URS1), are not detectably occupied under either transcriptional state. In contrast to its sensitivity to DNAase I, the nucleosome-free promoter region is relatively protected from MNase; the enzyme excises a stable nucleoprotein fragment of approximately 210 bp. As detected by MNase, there are at least two sequence-positioned nucleosomes arrayed 5' of the promoter; regularly spaced nucleosomes exhibiting an average repeat length of 160-170 bp span several kilobases of both upstream and downstream regions. Similarly, the body of the gene, which exhibits heightened sensitivity to DNase I, displays a nucleosomal organization under both basal and induced states, but these nucleosomes are not detectably positioned with respect to the underlying DNA sequence and may be irregularly spaced and/or structurally altered. We present a model of the chromatin structure of HSC82 and compare it to one previously derived for the closely related, but differentially regulated, HSP82 heat shock gene.