Edgardo Jofré1 and
1Departamento de Ciencias Naturales, Facultad de Ciencias Exactas, Físico-Químicas y Naturales, Universidad Nacional de Río Cuarto, Ruta 36 Km 601 (X5804BYA) Río Cuarto, Córdoba, Argentina; 2Institute of Biology III, Faculty of Biology, University of Freiburg, Schänzlestr 1, 79104 Freiburg, Germany
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Accepted 10 August 2009.
The protein tyrosine kinase ExoP, consisting of an N-terminal periplasmic and a C-terminal cytoplasmic domain, is important for polymerization of the exopolysaccharide succinoglycan (EPS I) in Sinorhizobium meliloti. We analyzed the contribution of the ExoP paralogs ExoP2 and SMb21506 to the production of the high molecular weight (HMW) form of EPS I. ExoP2, though not contributing to EPS I or lipopolysaccharide biosynthesis, showed increased expression at high osmolarity and was expressed in Medicago sativa nodules, suggesting an involvement in the synthesis of an as-yet-unidentified polysaccharide. Furthermore, a mutation in SMb21506 affected the production of HMW EPS I, particularly in the absence of the C-terminal ExoP domain. High salinity induced the production of HMW EPS I by the wild type and mutants whereas high osmolarity had the opposite effect. It was shown that ExoP localizes at the inner membrane of S. meliloti cells. Tyrosine phosphorylation of the C-terminal domain was strongly increased by amino acid substitutions in the polysaccharide co-polymerase motif (formerly proline-rich motif) located in the N-terminal domain, suggesting that this phosphorylation could be modulated by conformational changes of the N-terminal domain. Moreover, deletion of a coiled-coil motif present in the N-terminal domain abolished phosphorylation and EPS I production and, consequently, the ability to nodulate M. sativa.
© 2009 The American Phytopathological Society