Regulatory systems controlling motility and gene transfer agent production and release in Rhodobacter capsulatus

Production of the gene transfer agent of Rhodobacter capsulatus, RcGTA, is dependent upon several cellular regulatory systems, including a putative phosphorelay involving the CtrA and CckA proteins. These proteins are also involved in flagellar motility in R. capsulatus. The interactions of proteins in this system are best understood in Caulobacter crescentus where CtrA is activated by phosphorylation by the CckA-ChpT phosphorelay. CtrA~P activity is further controlled by SciP, which represses ctrA transcription and CtrA activation of transcription. We show that R. capsulatus chpT and cckA mutants both have greatly reduced motility and RcGTA activity. Unlike the ctrA mutant where RcGTA gene transcription is absent, the decrease in RcGTA activity is because of reduced release of RcGTA from the cells. The sciP mutant is not affected for RcGTA production but our results support the C. crescentus model of SciP repression of flagellar motility genes. We show that both unphosphorylated and phosphorylated CtrA can activate RcGTA gene expression, while CtrA~P seems to be required for release of the particle and expression of motility genes. This has led us to a new model of how this regulatory system controls motility and production of RcGTA in R. capsulatus.