34 Heat Shock Factor

Abstract

OVERVIEW
The transcriptional activation of genes encoding the heat shock proteins is mediated by the binding of heat shock factor (HSF) to specific DNA sequence elements (heat shock elements [HSEs]). The canonical HSE is a continuous array of three, alternately oriented nGAAn units. However, the number of repeating pentanucleotide units in an HSE can vary, and the unusual multimeric nature of HSF allows it to bind efficiently HSEs of different lengths. HSF activity is regulated by heat shock at the level of both DNA binding and transcriptional activation. The binding activity of HSF is dependent on its multimeric state, whereas transcriptional activity correlates with its heat-shock-induced phosphorylation. Complementary DNAs for yeast and Drosophila HSF have been cloned. Although heat shock proteins exhibit strong sequence conservation among many species, the sequences of Drosophila and yeast HSF are divergent, except in regions important for DNA binding and multimerization.

In uninduced cells, heat shock promoters are organized in nuclease-hypersensitive chromatin structures. These hypersensitive sites apparently possess bound transcription factor IID (TFIID) and a transcriptionally engaged but arrested RNA polymerase molecule. The heat shock promoter is thus primed or potentiated for activity in the absence of heat shock. Binding of HSF upon heat induction leads to increased transcription by unknown mechanisms, possibly involving both the release of the arrested RNA polymerase and the recruitment of additional polymerase molecules to the promoter itself.

INTRODUCTION
The heat shock gene regulatory circuit has several features that make it an attractive model for investigating the activation of transcription.