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J. Bacteriol. doi:10.1128/JB.01310-07
Copyright (c) 2007, American Society for Microbiology and/or the Listed Authors/Institutions. All Rights Reserved.

Regulation of Motility by the ExpR/Sin Quorum Sensing System in Sinorhizobium meliloti

Department of Molecular and Cell Biology, University of Texas at Dallas, Richardson, TX 75083-0688

^* To whom correspondence should be addressed. Email: jgonzal{at}utdallas.edu.

	Abstract

A successful symbiotic relationship between Sinorhizobium meliloti and its host Medicago sativa (alfalfa) depends on several signaling mechanisms, such as the biosynthesis of exopolysaccharides (EPS) by S. meliloti. Previous work in our laboratory has shown that a quorum-sensing mechanism controls the production of the symbiotically active EPS II. Recent microarray analysis of the whole genome expression profile of S. meliloti reveals that the ExpR/Sin quorum sensing system regulates additional physiological processes which include low molecular weight succinoglycan production, nitrogen utilization, metal transport, motility and chemotaxis. Nearly half of the flagellar genes and their dependence on quorum sensing are prominently displayed in our microarray analyses. We extend those observations in this work and confirm the findings by real-time PCR expression analysis of selected genes involved in motility and chemotaxis, including the flaF, flbT, flaC, cheY1, and flgB genes. These genes code for regulators of flagella synthesis, the chemotactic response, or parts of the flagellar apparatus. Gene expression analyses and visualization of flagella by electron microscopy performed at different points in the growth phase support our proposed model in which quorum sensing down-regulates motility in S. meliloti. We demonstrate that the ExpR/Sin quorum sensing system controls motility gene expression through the VisN/VisR/Rem relay. This work also shows that the ExoS-dependent two-component system suppresses motility gene expression through VisN and Rem in parallel to quorum sensing. The current study contributes to our understanding of the mechanisms that govern motility in S. meliloti.