The GroE chaperonin machine is the major modulator of the CIRCE heat shock regulon ofBacillus subtilis

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Abstract

Regulation of the heat shock response in bacteria has been studied extensively in Escherichia coli where heat shock genes are classified into three classes and where each class is regulated by a different alternate sigma factor. Bacillus subtilis serves as a second model bacterium to study regulation of the heat shock response in detail. Here, four classes of heat shock genes have been described so far where two are controlled by two different repressor proteins and the third by the alternate sigma factor σB. Class I heat shock genes consists of two operons, the heptacistronic dnaK and the bicistronic groE operon. Transcription of the dnaK operon is complex involving two promoters, premature termination of transcription, mRNA processing and different stabilities of the processed transcripts to ensure the appropriate amounts of heat shock proteins under different growth conditions. The translation product of the hrcA gene, the first gene of the dnaK operon, binds to an operator designated CIRCE element, and its activity is modulated by the GroE chaperonin system. We assume that the HrcA protein, upon de novo synthesis and upon dissociation from its operator, is present in an inactive form and has to be activated by the GroE chaperonin system resulting in an HrcA-GroE reaction cycle. Induction of class I heat shock genes occurs by the appearance of denatured proteins within the cytoplasm which titrate the GroE system. This results in accumulation of inactive HrcA repressor and thereby in induction of class I heat shock genes. Upon removal of the non-native proteins from the cytoplasm, the GroE chaperonin will interact with HrcA and promote folding into its active conformation resulting in turning off of class I heat shock genes. This mechanism ensures adequate adjustment of class I heat shock proteins depending on their actual need.

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