PhyK, PhyR, σT and σU regulons

The extracytoplasmic function sigma factor σT is the master regulator of general stress response in Caulobacter crescentus, and controls the expression of its paralogue σU. This work showed that PhyR and NepR act as positive and negative regulators, respectively, of σT expression and function. Biochemical data also indicated that NepR directly binds σT and the phosphorylated form of PhyR. We also demonstrated the essential role of the histidine kinase gene CC3474, here denominated phyK, for expression of σT-dependent genes and for resistance to stress conditions. Additionally, in vivo evidence of PhyK-dependent phosphorylation of PhyR is presented. This study also identified a conserved cysteine residue (C95) present in the periplasmic portion of PhyK that is crucial for the function of the protein. Furthermore, we showed that PhyK, PhyR and σT regulate the same set of genes and that σT is apparently responsible for the direct control of its complete regulon. In contrast, σU seems to have a very modest contribution for expression of a subset of σT-dependent genes. In conclusion, this work describes the mechanism involved in the control of general stress response in C. crescentus. Five experimetal procedures, each of them performed in three replicates. A total of fifteen independent biological samples were used