| Rhizobium and related genera are soil bacteria with great metabolic plasticity. These microorganisms survive in many different environments and are capable of eliciting the formation of nitrogen-fixing nodules on legume plants. The successful establishment of symbiosis is precisely regulated and requires a series of signal exchanges between the two partners. Quorum sensing (QS) is a prevalent form of population density-dependent gene regulation. Recently, increasing evidence indicates that rhizobial quorum sensing is a pervasive regulatory network, which might play a more generalized role in bacterial physiology in free-living rhizobia as well as during symbiosis. Among rhizobia, Rhizobium sp. strain NGR234 has the broadest host range yet determined. The complete sequence of the NGR234 symbiotic plasmid pNGR234a revealed the presence of a luxR/luxI-type quorum sensing circuit. I have studied the role of the quorum sensing system encoded by pNGR234 as a regulator of rhizobial physiology and growth rate, using a combination of genetics, molecular biology and proteomic analyses. The study revealed that (1) the traR/traI quorum sensing circuit encoded by pNGR234a controls multiple functions encoded on all three NGR234 replicons pNGR234a, pNGR234b and the chromosome); (2) there is a potential regulatory link between population density sensing and genetic diversification mechanisms in NGR234; and (3) quorum sensing regulates NGR234 growth rate via several mechanistically different pathways. |
