Cyclic-di-GMP signalling proteins and gene transfer agent (GTA) production in Rhodobacter capsulatus

Gene transfer agents (GTAs) are bacteriophage-like particles produced by several bacterial and archaeal lineages that contain small pieces of the producing cells’ genomes that can be transferred to other cells in a process similar to transduction. One well-studied GTA is RcGTA, produced by the -proteobacterium Rhodobacter capsulatus. RcGTA gene expression is regulated by several cellular regulatory systems, including the CckA-ChpT-CtrA phosphorelay. A previous study on CtrA, a DNA-binding response regulator protein, showed that transcription of multiple other regulator-encoding genes was affected by its loss. These included genes encoding proteins predicted to be involved in the synthesis and hydrolysis of the second messenger bis-(3’-5’)-cyclic dimeric guanosine monophosphate (c-di-GMP). During my PhD work, I have investigated these genes and found that disruption of four of them (rcc00620, rcc00645, rcc02629, rcc02857) affected RcGTA production and flagellar motility, where c-di-GMP acts to inhibit RcGTA production and motility. I subsequently demonstrated that the enzymatic activity of one of the proteins, Rcc00620, is regulated through the phosphorylation status of its REC domain, which is controlled by a cognate histidine kinase protein, Rcc00621 in a two component system (TCS) signalling fashion. Interestingly, the rcc00620/rcc00621 genes were acquired via horizontal gene transfer from a distantly related -proteobacterium. All these results add c-di-GMP signaling to the collection of cellular regulatory systems controlling gene transfer in this bacterium and add gene transfer to the diverse list of activities affected by c-di-GMP in bacteria. This is also the first report on the involvement of c-di-GMP
signaling in gene transfer.