Regulatory Activity Switching by <scp>NtrX</scp> Rescues the Stress Phenotypes Caused by Loss of <scp>ChvIG</scp> in <i>Caulobacter crescentus</i>

Two-component systems, comprising a sensor kinase and a response regulator, are very important for bacteria to respond to stress in their environment. In alpha-proteobacteria, two such systems (ChvIG and NtrYX) help bacteria to respond to cell envelope and acid stress, respectively. In contrast to the majority of two-component systems, which do not cross talk with one another, ChvIG and NtrYX in Caulobacter are tightly functionally linked and regulate many of the same genes. Moreover, the antagonistic balance between phosphorylated ChvI and non-phosphorylated NtrX is essential for growth in defined medium. We show that the NtrX-ChvI balance is also critical for swimming motility in defined medium (M2G) stress conditions, and that loss of motility in M2G in ΔchvI or ΔchvG mutants is rescued by dominant gain-of-function mutations in ntrYX. These mutations promote a regulatory switch of NtrX away from the genes that are regulated by NtrX in its non-phosphorylated form, at least one of which is jointly bound by ChvI and NtrX at the same binding site. The data support a model where these two systems co-operate tightly as a stress response mediator, almost to the extent that they could be considered a four-component system, and that ChvI is required for NtrX to regulate some of its target genes.