REGULATION OF A HEAT-SHOCK SIGMA(32) HOMOLOG IN CAULOBACTER-CRESCENTUS

High temperature and other environmental stresses induce the expressio n of several heat shock proteins in Caulobacter crescentus, including the molecular chaperones DnaJ, DnaK, GrpE, and GroEL and the Lon prote ase, We report here the isolation of the rpoH gene encoding a homolog of the Escherichia coli RNA polymerase sigma(32) subunit, the sigma fa ctor responsible for the transcription of heat shock promoters, The C. crescentus sigma(32) homolog, predicted to be a 33.7-kDa protein, is 42% identical to E. coli sigma(32) and cross-reacts with a monoclonal antibody to E. coli sigma(32). Functional homology was demonstrated by complementing the temperature-sensitive growth defect of an E. coli r poH deletion mutant with the C. crescentus rpoH gene. Immunoblot analy sis showed a transient rise in sigma(32) levels after a temperature sh ift from 30 to 42 degrees C similar to that described for E. coli. In addition, increasing the cellular content of sigma(32) by introducing a plasmid-encoded copy of rpoH induced DnaK expression in C. crescentu s cultures grown at 30 degrees C. The C. crescentus rpoH gene was tran scribed from either of two heat shock consensus promoters. rpoH transc ription and sigma(32) levels increased coordinately following heat sho ck, indicating that transcriptional regulation contributes to sigma(32 ) expression in this organism, Both the rpoH gene and sigma(32) protei n were expressed constitutively throughout the cell cycle at 30 degree s C. The isolation of rpoH provides an important tool for future studi es of the role of sigma(32) in the normal physiology of C. crescentus.