POSTERS - BLAST X - University of Utah

BLAST X Thurs. Morning Session
REGULATION OF MOTILITY BY QUORUM SENSING IN SINORHIZOBIUM MELILOTI AND
ITS ROLE IN SYMBIOSIS ESTABLISHMENT
Juan E. González*, Nataliya Gurich, Jennifer L. Morris, Konrad Mueller, and Arati V. Patankar
Department of Molecular and Cell Biology, University of Texas at Dallas, Richardson, Texas,
USA
Quorum sensing is a mechanism widely used by bacteria to coordinate their behavior in
response to a particular cell population density. Signal molecules, termed autoinducers, are
produced by bacteria, and at a high population density, accumulate in the environment. Once a
threshold level of autoinducer is reached, they bind to their cognate transcriptional regulators
and activate or repress expression of target genes, thereby preparing the bacteria for behaviors
associated with high cell density, such as interacting with eukaryotic hosts.
In Sinorhizobium meliloti, this mechanism is utilized to appropriately modulate gene
expression and permit the establishment of a nitrogen-fixing symbiosis with its host plant
Medicago sativa. S. meliloti possesses a quorum-sensing system composed of two
transcriptional regulators, SinR and ExpR, and the SinR-controlled autoinducer synthase SinI,
which is responsible for the biosynthesis of the signal molecule in the form of an N-acyl
homoserine lactone (AHL). These AHLs, in conjunction with the ExpR regulator, control a
variety of downstream genes. The concentration of AHLs varies with changes in population
density. As a result, expression of quorum-sensing-dependent genes may exhibit different
patterns during various stages of bacterial growth. Work in our laboratory has shown that the S.
meliloti ExpR/Sin quorum-sensing system regulates over 200 genes, including those involved in
exopolysaccharide synthesis, motility and chemotaxis, metal transport, and other metabolic
functions, thereby playing an important role during plant-bacteria interactions.
Inoculation of plants with a sinI-deficient strain results in a delay in invasion as well as a
significant reduction in the total number of nodules per plant when compared to the wild type,
resulting in plant development deficiencies. Concurrently, expression of most of the motility and
chemotaxis genes in the sinI mutant fail to be down-regulated by quorum sensing at high cell
population density. Microarray and real-time PCR analyses revealed that the ExpR/Sin system
adjusts the expression of the transcriptional regulators VisN/VisR and Rem, which in turn
modulate downstream motility genes in a population-density-dependent manner to decrease
motility. Recently we have shown that mutating flagellar production in a sinI mutant restores
bacterial competency for symbiosis establishment to wild type levels, suggesting that the
elimination of flagella during the invasion process is crucial. Therefore, down-regulation of
motility and chemotaxis by the ExpR/Sin quorum-sensing system plays an essential role in
successful plant invasion by S. meliloti.
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