HEAT-SHOCK FACTOR GAINS ACCESS TO THE YEAST HSC82 PROMOTER INDEPENDENTLY OF OTHER SEQUENCE-SPECIFIC FACTORS AND ANTAGONIZES NUCLEOSOMAL REPRESSION OF BASAL AND INDUCED TRANSCRIPTION

Transcription in eukaryotic cells occurs in the context of chromatin. Binding of sequence-specific regulatory factors must contend with the presence of nucleosomes for establishment of a committed preinitiation complex. Here we demonstrate that the high-affinity binding site for heat shock transcription factor (HSF) is occupied independently of oth er cis-regulatory elements and is critically required for preventing n ucleosomal assembly over the yeast HSC82 core promoter under both noni nducing (basal) and inducing conditions, Chromosomal mutation of this sequence, termed HSE1, erases the HSF footprint and abolishes both tra nscription and in vivo occupancy of the TATA box. Moreover, it dramati cally reduces promoter chromatin accessibility to DNase I and TaqI as the nuclease-hypersensitive region is replaced by a localized nucleoso me. By comparison, in situ mutagenesis of two other promoter elements engaged in stable protein-DNA interactions in vivo, the GRF2/REB1 site and the TATA. bos, despite reducing transcription three- to fivefold, does not compromise the nucleosome-free state of the promoter. The GR F2-binding factor appears to facilitate the binding of proteins to bot h HSE1 and TATA, as these sequences, while still occupied, are less pr otected from in vivo dimethyl sulfate methylation in a Delta GRF2 stra in. Finally, deletion of a consensus upstream repressor sequence (URS1 ), positioned immediately upstream of the GRF2-HSE1 region and only we akly occupied in chromatin, has no expression phenotype, even under me iotic conditions, However, deletion of URS1, like mutation of GRF2, sh ifts the translational setting of an upstream nucleosomal array flanki ng the promoter region, Taken together, our results argue that HSF, in dependent of and dominant among sequence-specific factors binding to t he HSC82 upstream region, antagonizes nucleosomal repression and creat es an accessible chromatin structure conducive to preinitiation comple x assembly and transcriptional activation.