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Journal of Bacteriology, October 2007, p. 7077-7088, Vol. 189, No. 19
0021-9193/07/$08.00+0 doi:10.1128/JB.00906-07
Copyright © 2007, American Society for Microbiology. All Rights Reserved.

and
Juan E. González*
Department of Molecular and Cell Biology, University of Texas at Dallas, Richardson, Texas 75083-0688
Received 8 June 2007/ Accepted 16 July 2007
Sinorhizobium meliloti is a gram-negative soil bacterium capable of forming a symbiotic nitrogen-fixing relationship with its plant host, Medicago sativa. Various bacterially produced factors are essential for successful nodulation. For example, at least one of two exopolysaccharides produced by S. meliloti (succinoglycan or EPS II) is required for nodule invasion. Both of these polymers are produced in high- and low-molecular-weight (HMW and LMW, respectively) fractions; however, only the LMW forms of either succinoglycan or EPS II are active in nodule invasion. The production of LMW succinoglycan can be generated by direct synthesis or through the depolymerization of HMW products by the action of two specific endoglycanases, ExsH and ExoK. Here, we show that the ExpR/Sin quorum-sensing system in S. meliloti is involved in the regulation of genes responsible for succinoglycan biosynthesis as well as in the production of LMW succinoglycan. Therefore, quorum sensing, which has been shown to regulate the production of EPS II, also plays an important role in succinoglycan biosynthesis.
Published ahead of print on 20 July 2007.
Present address: Department of Microbiology and Molecular Genetics, Harvard Medical School, 200 Longwood Avenue, Boston, MA 02115.
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