Heat shock regulation of sigmaS turnover: a role for DnaK and relationship between stress responses mediated by sigmaS and sigma32 in Escherichia coli.

The cellular level of the rpoS-encoded sigmaS subunit of RNA polymerase increases in response to various stress situations that include starvation, high osmolarity, and shift to acid pH, and these different stress signals differentially affect rpoS translation and/or sigmaS stability. Here we demonstrate that sigmaS is also induced by heat shock and that this induction is exclusively due to an interference with sigmaS turnover. Some sigmaS-dependent genes exhibit similar heat shock induction, whereas others are not induced probably because they need additional regulatory factors that might not be present under conditions of heat shock or exponential growth. Despite its induction, sigmaS does not seem to contribute to heat adaptation but may induce cross-protection against different stresses. While sigmaS is not involved in the regulation of the heat shock sigma factor sigma32, the heat shock protein DnaK has a positive role in the posttranscriptional control of sigmaS. The present evidence suggests that DnaK is involved in the transduction of two of the signals that result in reduced sigmaS turnover, i.e., heat shock and carbon starvation. Heat shock induction of sigmaS also clearly indicates that a cessation of growth or even a reduction of the growth rate is not a prerequisite for the induction of sigmaS and sigmaS-dependent genes and underscores the importance of sigmaS as a general stress sigma factor.