The DnaK chaperone modulates the heat shock response of Escherichia coli by binding to the sigma super(32) transcription factor

The heat shock response and the heat shock proteins have been conserved across evolution. In Escherichia coli , the heat shock response is positively regulated by the sigma super(32) transcriptional factor and negatively regulated by a subset of the heat shock proteins themselves. In an effort to understand the regulation of the heat shock response, we have purified the sigma super(32) polypeptide to homogeneity. During the purification procedure, we found that a large fraction of the overexpressed sigma super(32) polypeptide copurified with the universally conserved DnaK heat shock protein (the prokaryotic equivalent of the 70-kDa heat shock protein, HSP70). Further experiments established that purified sigma super(32) bound to DnaK and that this complex was disrupted in the presence of ATP. Consistent with the fact that dnaK756 mutant bacteria overexpress heat shock proteins at all temperatures, purified DnaK756 mutant protein did not appreciably bind to sigma super(32).