Conserved region 2.1 of Escherichia coli heat shock transcription factor sigma32 is required for modulating both metabolic stability and transcriptional activity.

Escherichia coli heat shock transcription factor sigma32 is rapidly degraded in vivo, with a half-life of about 1 min. A set of proteins that includes the DnaK chaperone team (DnaK, DnaJ, GrpE) and ATP-dependent proteases (FtsH, HslUV, etc.) are involved in degradation of sigma32. To gain further insight into the regulation of sigma32 stability, we isolated sigma32 mutants that were markedly stabilized. Many of the mutants had amino acid substitutions in the N-terminal half (residues 47 to 55) of region 2.1, a region highly conserved among bacterial sigma factors. The half-lives ranged from about 2-fold to more than 10-fold longer than that of the wild-type protein. Besides greater stability, the levels of heat shock proteins, such as DnaK and GroEL, increased in cells producing stable sigma32. Detailed analysis showed that some stable sigma32 mutants have higher transcriptional activity than the wild type. These results indicate that the N-terminal half of region 2.1 is required for modulating both metabolic stability and the activity of sigma32. The evidence suggests that sigma32 stabilization does not result from an elevated affinity for core RNA polymerase. Region 2.1 may, therefore, be involved in interactions with the proteolytic machinery, including molecular chaperones.