Sinorhizobium meliloti is a soil bacterium that enters into a symbiotic association with the alfalfa plant, Medicago sativa , through the formation and subsequent invasion of nitrogen-fixing nodules. The process requires a complex exchange of signaling molecules between the bacterium and its host. More recently, it has been determined that the exchange of quorum-sensing signals within a population of S. meliloti contributes to important functions for establishing symbiosis. Quorum-sensing mutants have a delay in nodulation and a reduction in the number of nitrogen-fixing nodules on their host plant. We show that the ExpR/Sin quorum-sensing system is involved in the production of EPS II, one of the symbiotically important exopolysaccharides produced by S. meliloti. We extend the quorum-sensing regulation to the well-characterized exopolysaccharide, succinoglycan, where we demonstrate that the ExpR/Sin quorum-sensing system regulates multiple steps in the succinoglycan biosynthetic pathway. We also show that the production of the symbiotically important low molecular weight forms of the exopolysaccharide is controlled by quorum sensing. A role for quorum sensing is proposed in the export of cyclic beta-glucan, a third polymer necessary for successful nodule invasion. This work focuses on the global role of quorum sensing on the production of three polymers in S. meliloti, each of which is involved in the development of a nitrogen-fixing symbiosis. We also present the identification of a two-component system which is involved in regulation of succinoglycan production and the generation of the low molecular weight forms of the S. meliloti exopolysaccharide.
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