The role of CtsR and ClpX in the regulation of the heat stress response in Bacillus subtilis

B. subtilis possesses several mechanisms to adapt to and withstand different kinds of stress. If the bacteria are exposed to heat stress, among others, stress specific heat shock proteins of the CtsR regulon are synthesized at increased rate. The members of this regulon, ATPases, proteolytic subunits and their adaptors, inherit a crucial role in the cellular proteostasis under stress conditions [Miethke M. et al., J Bacteriol. 2006.]. During vegetative growth CtsR (class three stress repressor) represses the transcription of the clpC operon (ctsR-mcsA-mcsB-clpC), clpE and clpP gene [Krüger E. et al., J Bacteriol. 1998.]. Upon heat shock CtsR detaches from its operator caused by a conformational change in a thermosensing glycine rich loop leading to the transcription of the regulon [Elsholz A.K. et al. EMBO J. 2010.]. Novel data of clpX deficient strains revealed a positive effect of the ATPase ClpX on the induction of clpE upon heat stress. Hence, we constructed several strains of deletional and complemental mutants of clpX and ctsR to uncouple the induction of these genes from their natural stimuli. The results demonstrated that full induction of the CtsR regulon and especially clpE, so far believed to be solely regulated by CtsR and Spx, is dependent on the presence of heat and ClpX. This observation was made on transcriptional as well as protein level indicating a key role of ClpX in the transcriptional activation. Additionally, the data suggests that ClpX might act as a chaperone on CtsR to achieve full induction of clpE under heat shock conditions. Taken together we are able to show a so far unknown influence of ClpX on the induction of the CtsR regulon, including an interaction of ClpX and CtsR and thereby adding a new layer of complexity to the understanding how B. subtilis responds to heat stress.