Quorum-Sensing and Growth Control in Rhizobium sp. NGR234

Indiana University, Bloomington IN

Investigators

Abstract

Gram-negative bacteria within the Proteobacteria group commonly use acylated homoserine lactones (acyl-HSLs) as molecular signals in the process of quorum sensing (QS). Quorum-sensing bacteria release diffusible signal molecules that accumulate with increasing cell number, eventually triggering adaptive responses. Regulatory proteins of the LuxI and LuxR families are usually required for synthesis of, and response to acyl-HSLs, respectively. Diffuse populations of cells generally produce a constant, low level of acyl-HSLs, and these rapidly diffuse out of cells down their concentration gradient. Elevated population density increases the relative acyl-HSL concentration, eventually fostering interaction of the signals with LuxR-type proteins, which in turn, control the transcription of target genes. Although this basic mechanism is well conserved, the context of QS regulation and the cellular functions under its control are highly variable among different bacteria. This project focuses on the regulatory context of a LuxI-LuxR-type QS system employed by the nitrogen-fixing plant symbiont Rhizobium sp. NGR234, its mechanism of action, and its effect on cellular growth rate. NGR234 incites the formation of nitrogen-fixing, symbiotic nodules on the roots of a wide range of leguminous plants. The molecular basis of this promiscuous host interaction has been extensively studied. Many of the functions that orchestrate the plant interaction are carried on the 536 kb pNGR234a plasmid. The pNGR234a plasmid also carries a large cluster of genes homologous to plasmid replication (rep) and conjugal transfer (trb/tra) genes from other bacteria. The rep/trb/tra cluster includes a LuxI-LuxR-type regulatory pair, TraI and TraR, and the additional QS regulator TraM. TraI synthesizes 3-oxo-octanoyl-L-homoserine lactone and TraR interacts with this acyl HSL to regulate tra/trb and rep operon expression. TraM acts to inhibit TraR through formation of an anti-activation complex. A molecular genetic approach is being employed to study the QS mechanism in NGR234. The regulatory signals and pathways that control traR expression will be investigated to determine the conditions that foster QS. Based on analogous systems, the host plant is likely to play a role in this regulation. The QS-regulated pNGR234a genes under TraR control will be identified and the mechanism by which TraR controls their expression elucidated. Lastly, control of cellular growth rate by QS, a common QS regulatory target among several Rhizobium species, will be examined. Findings generated from the research project will provide fundamental information on cell-to-cell communication in an important plant symbiont, and the role of this communication in its highly plastic host interactions. More generally, these studies will add to the understanding of how cell-to-cell communication directly and indirectly influences interactions with host organisms, and enables quorum-sensing microbes to balance their physiological activity with the host environment. The bacterium Rhizobium sp. NGR234 uses cell-to-cell communication during its symbiotic relationships with higher plants. The research project examines the biochemical and genetic mechanisms underlying this intercellular communication, and its influence on the interaction of this bacterium and host plants. The findings generated from this work will provide fundamental knowledge required to take advantage of these bacterial communication systems, for combating infectious disease in plants and animals, as well as promoting beneficial microbial interactions.