Conserved Region 2.1 of Escherichia coli Heat Shock Transcription Factor capital sigma super(32) Is Required for Modulating both Metabolic Stability and Transcriptional Activity

Escherichia coli heat shock transcription factor capital sigma super(32) 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 capital sigma super(32). To gain further insight into the regulation of capital sigma super(32) stability, we isolated capital sigma super(32) 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 capital 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 capital sigma super(32). Detailed analysis showed that some stable capital sigma super(32) 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 capital sigma super(32). The evidence suggests that capital sigma super(32) 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.