POSTERS - BLAST X - University of Utah

BLAST X Poster #20
THE ROLE OF QUORUM SENSING IN THE CONTROL OF MOTILITY AND PLANT INVASION
BY SINORHIZOBIUM MELILOTI
Nataliya Gurich* and Juan E. González
Department of Molecular and Cell Biology, University of Texas at Dallas, Richardson, Texas,
USA
Quorum sensing, a population density dependent regulation of gene expression, is used
by various bacteria to establish symbiotic or pathogenic bacterium-host associations. This
mechanism requires the production of signaling molecules called autoinducers. At high cell
population densities, autoinducer concentration in the surrounding area reaches a threshold
level, which leads to activation of specific transcriptional regulators and the control of numerous
phenotypes.
Sinorhizobium meliloti is a gram-negative soil bacterium that can form a nitrogen-fixing
symbiotic association with its host Medicago sativa. The quorum sensing system in S. meliloti is
comprised of a transcriptional regulator, SinR and an autoinducer synthase, SinI, which
specifies production of N-acyl homoserine lactone (AHL) signaling molecules. In conjunction
with AHL, an additional regulator, ExpR, controls expression of several hundred genes. Recent
microarray studies in our laboratory described the control of motility and chemotaxis in S.
meliloti by quorum sensing through the regulators VisN/VisR and Rem in a population-densitydependent
manner. The wild type strain is motile during the early log phase of growth but shuts
down flagella synthesis genes during the mid- and late log phases. In contrast, the sinI mutant
remains motile throughout all phases of growth, and at the late log phase, 35 motility and
chemotaxis genes are upregulated when compared to wild type.
This inability to repress flagella production by the sinI mutant leads to severe invasion
deficiency. Detailed analysis of microarray results suggested that the inability of the sinI mutant
to shut down flagella synthesis might be detrimental to successful plant invasion. Mutation of
flagella synthesis in the sinI strain restored invasion efficiency to wild type levels. Therefore,
control of motility and chemotaxis by the ExpR/Sin quorum sensing system plays an important
role in plant invasion and may provide a competitive edge for strains possessing it.
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