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 that allow growth in the absence of DnaK. All mutations reduced the activity of the heat shock sigma factor σ(32), demonstrating that the DnaK-dependent inactivation of σ(32) is a growth requirement. While most mutations occurred in the rpoH gene encoding σ(32), we also identified mutations affecting σ(32) activity or stability in trans, providing important new insight into the regulatory mechanisms controlling σ(32) activity. Most notably, we describe a mutation in the ATP dependent protease HslUV that induces rapid degradation of σ(32), and a mutation leading to increased levels of the house keeping σ(70) that outcompete σ(32) for binding to the RNA polymerase. We demonstrate that σ(32) inhibits growth and that its unrestrained activity leads to an extensive reprogramming of global gene expression, resulting in upregulation of repair and maintenance functions and downregulation of the growth-promoting functions of protein translation, DNA replication and certain metabolic processes. While this re-allocation from proliferative to maintenance functions could provide an advantage during heat stress, it leads to 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.