Downregulation of the heat shock response is independent of DnaK and σ32 levels in Caulobacter crescentus

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Abstract
Expression of heat shock genes in Gram-negative proteobacteria is positively modulated by the transcriptional regulator RpoH, the sigma(32) subunit of RNA polymerase (RNAP). In this study we investigated the chaperones DnaK/DnaJ and GroES/GroEL as possible modulators of the heat response in Caulobacter crescentus. We have shown that cells overexpressing DnaK show poor induction of heat shock protein (HSP) synthesis, even though sigma(32) levels present a normal transient increase upon heat stress. On the other hand, depletion of DnaK led to higher levels of sigma(32) and increased transcription of HSP genes, at normal growth temperature. In contrast, changes in the amount of GroES/EL had little effect on sigma(32) levels and HSP gene transcription. Despite the strong effect of DnaK levels on the induction phase of the heat shock response, downregulation of HSP synthesis was not affected by changes in the amount this chaperone. Thus, we propose that competition between sigma(32) and sigma(73), the major sigma factor, for the core RNAP could be the most important factor controlling the shut-off of HSP synthesis during recovery phase. In agreement with this hypothesis, we have shown that expression of sigma(73) gene is heat shock inducible.