Regulation of the heat shock response in Escherichia coli and Bacillus subtilis

A detailed study of the heat shock response and its regulation has only been carried out in Escherichia coli and to some extent in Bacillus subtilis. In E. coli, the induction of heat shock genes is obtained through two different alternative sigma factors, sigma-32 and sigma-24 which modify the promoter recognition specificity of the RNA polymerase to enable the expression of heat shock genes. Whereas sigma-32 controls the expression of some 31 heat shock genes, sigma-24 activates at least ten different genes of which three have been identified so far. In contrast, in Bacillus subtilis, three classes of heat shock genes have been described, all three regulated at the level of transcription. Whereas class I genes are regulated by a repressor, class II are under the control of an alternate sigma factor, and the regulation mechanism of class III heat shock genes is unknown. The repressor protein of class I genes interacts with an operator sequence designated CIRCE which is widespread among eubacteria. This novel heat shock element has been described more than 50 times occurring in 28 different eubacterial species suggesting that the underlying regulation mechanism is also widespread. Preliminary results suggest that there are bacterial species which express both the sigma-32 and the CIRCE mechanism.