The production of the Sinorhizobium meliloti exopolysaccharide, succinoglycan, is required for the formation of infection threads inside root hairs, a critical step during the nodulation of alfalfa (Medicago sativa) by S. meliloti. Two bacterial mutations, exoR95::Tn5 and exoS96::Tn5, resulted in the overproduction of succinoglycan and a reduction in symbiosis. Systematic analyses of the symbiotic phenotypes of the two mutants demonstrated their reduced efficiency of root hair colonization. In addition, both the exoR95 and exoS96 mutations caused a marked reduction in the biosynthesis of flagella and consequent loss of ability of the cells to swarm and swim. Succinoglycan overproduction did not appear to be the cause of the suppression of flagellum biosynthesis. Further analysis indicated that both the exoR95 and exoS96 mutations affected the expression of the flagellum biosynthesis genes. These findings suggest that both the ExoR protein and the ExoS/ChvI two-component regulatory system are involved in the regulation of both succinoglycan and flagellum biosynthesis. These findings provide new avenues of understanding of the physiological changes S. meliloti cells go through during the early stages of symbiosis and of the signal transduction pathways that mediate such changes.Sinorhizobium meliloti and its legume host, alfalfa (Medicago sativa), establish an effective nitrogen-fixing symbiosis through a series of signal exchanges that starts with the exchange of Nod (nodulation) factors and flavonoids, which results in the formation of curled alfalfa root hairs that are colonized by S. meliloti cells (13,20,32). The colonized curled root hairs develop infection threads within the root hairs, which allow S. meliloti cells to invade the developing root nodules (14,16,31). A successful invasion of nodules by S. meliloti will result in the formation of pink nitrogen-fixing nodules. The pink color is due to the presence of leghemoglobin. Nodules that are not occupied by S. meliloti and/or not capable of fixing nitrogen are most often white due to the lack of leghemoglobin (20).The formation of infection threads inside root hairs requires the presence of an S. meliloti exopolysaccharide, succinoglycan (9), in addition to the Nod factor (33). Succinoglycan is a polymer that consists of different numbers of a repeating unit consisting of one galactose and seven glucoses with three modification groups: acetyl, pyruvyl, and succinyl (17, 24). All three modifications must be present in order for the S. meliloti succinoglycan to be active in eliciting infection thread formation (9). Surprisingly, overproduction of succinoglycan appears to reduce efficiency of nodulation (12).Two S. meliloti mutants, exoR95::Tn5 and exoS96::Tn5, were isolated based on their ability to overproduce succinoglycan (12). The exoR gene encodes a protein of 268 amino acids that shares no significant homology with any other protein in currently available databases (23). The exoS gene encodes the membrane-bound sensor of the ExoS/ChvI two-component r...

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