Loss of the response regulator CtrA causes pleiotropic effects on gene expression but does not affect growth phase regulation in Rhodobacter capsulatus.

The purple nonsulfur photosynthetic bacterium Rhodobacter capsulatus has been extensively studied for its metabolic versatility as well as for production of a gene transfer agent called RcGTA. Production of RcGTA is highest in the stationary phase of growth and requires the response regulator protein CtrA. The CtrA protein in Caulobacter crescentus has been thoroughly studied for its role as an essential, master regulator of the cell cycle. Although the CtrA protein in R. capsulatus shares a high degree of sequence similarity with the C. crescentus protein, it is nonessential and clearly plays a different role in this bacterium. We have used transcriptomic and proteomic analyses of wild-type and ctrA mutant cultures to identify the genes dysregulated by the loss of CtrA in R. capsulatus. We have also characterized gene expression differences between the logarithmic and stationary phases of growth. Loss of CtrA has pleiotropic effects, with dysregulation of expression of approximately 6% of genes in the R. capsulatus genome. This includes all flagellar motility genes and a number of other putative regulatory proteins but does not appear to include any genes involved in the cell cycle. Quantitative proteomic data supported 88% of the CtrA transcriptome results. Phylogenetic analysis of CtrA sequences supports the hypothesis of an ancestral ctrA gene within the alphaproteobacteria, with subsequent diversification of function in the major alphaproteobacterial lineages.