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Sinorhizobium meliloti ExoR and ExoS Proteins Regulate both Succinoglycan and Flagellum Production

Shi-Yi Yao,¹ Li Luo,² Katherine J. Har,^1, Anke Becker,³ Silvia Rüberg,³ Guan-Qiao Yu,² Jia-Bi Zhu,² and Hai-Ping Cheng^1*

Biological Sciences Department, Lehman College, The City University of New York, Bronx, New York,¹ National Laboratory of Molecular Genetics, Institute of Plant Physiology and Ecology, Shanghai Institute of Biological Sciences, Chinese Academia of Science, Shanghai, People's Republic of China,² Institut für Genomforschung, Universität Bielefeld, Bielefeld, Germany³

Received 1 August 2003/ Accepted 22 June 2004

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.

Present address: Greenwich High School, Greenwich, CT 06830.

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