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