Negative regulation of bacterial heat shock genes.

The expression of eubacterial heat shock genes is efficiently controlled at the transcriptional level by both positive and negative mechanisms. Positive control operates by the use of alternative sigma factors that target RNA polymerase to heat shock gene promoters. Alternatively, bacteria apply repressor-dependent mechanisms, in which transcription of heat shock genes is initiated from a classical housekeeping promoter and cis-acting DNA elements are used in concert with a cognate repressor protein to limit transcription under physiological conditions. Eight examples of negative regulation will be presented, among them the widespread CIRCE/HrcA system and the control by HspR in Streptomyces. Both mechanisms are designed to permit simple feedback control at the level of gene expression. Many bacteria have established sophisticated regulatory networks, often combining positive and negative mechanisms, in order to allow fine-tuned heat shock gene expression in an environmentally responsive way.