Cells lacking ClpB display a prolonged shutoff phase of the heat shock response in Caulobacter crescentus

Abstract

The heat shock response in Caulobacter crescentus was previously shown to be positively regulated by the alternative sigma factor of RNA polymerase (RNAP) σ32, and negatively modulated by DnaK during the induction phase of the heat shock response but not during the recovery phase. In the present work we have investigated the involvement of the chaperone ClpB in the control of the heat shock response in C. crescentus. Data obtained indicated a role of ClpB in downregulation of heat shock protein (HSP) synthesis, as cells lacking this chaperone showed a prolonged shutoff phase of the heat shock response. In Escherichia coli, it has been proposed that the DnaK chaperone system switches transcription back to constitutively expressed genes through simultaneous reactivation of heat-aggregated σ70, as well as sequestration of σ32 away from RNAP. In C. crescentus, results obtained with a clpB null mutant indicate that ClpB could be involved in the reactivation of the major sigma factor σ73. In support of this hypothesis, we showed that transcription directed from σ73- dependent promoters is not switched back in the clpB null mutant during the recovery phase. Furthermore, we observed that resolubilization of heat-aggregated σ73 is dependent on the presence of ClpB. Our findings also indicated that the absence of ClpB made cells more sensitive to heat shock and ethanol but not to other stresses, and unable to acquire thermotolerance. © 2005 Blackwell Publishing Ltd.