An essential regulatory function of the DnaK chaperone dictates the decision between proliferation and maintenance in Caulobacter crescentus

Hsp70 chaperones are well known for their important functions in maintaining protein homeostasis during thermal stress conditions. In many bacteria the Hsp70 homolog DnaK is also required for growth in the absence of stress. The molecular reasons underlying Hsp70 essentiality remain in most cases unclear. Here, we demonstrate that DnaK is essential in the α-proteobacterium Caulobacter crescentus due to its regulatory function in gene expression. Using a suppressor screen we identified mutations in genes coding for different components of the transcriptional machinery as well as an ATP dependent protease that allow growth in the absence of DnaK. These mutations reduced the activity of the heat shock sigma factor σ32 by different mechanisms, suggesting that DnaK's sole essential function in the absence of stress is the inactivation of σ32. We found that σ32 inhibits growth and that its unleashed activity leads to an extensive reprogramming of global gene expression, which re-allocates cellular resources from proliferative to maintenance functions. While this re-allocation provides an advantage during heat stress, it leads to detrimental growth defects under favorable conditions. We conclude that Caulobacter has co-opted the DnaK chaperone system as an essential regulator of gene expression under conditions when its folding activity is dispensable. RNA-sequencing was performed to determine gene expression changes after 6 and 9 h of depletion of DnaKJ in the PxylX-dnaKJ and PxylX-dnaKJ, rpoHD252G background, after 6 h of depletion in the PxylX-dnaKJ, ∆rpoH background as well as 2 h of overexpression of rpoH(V56A) in a PdnaK::Tn background. The RNA abundance of each sample was compared to undepleted PxylX-dnaKJ cells.