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Sinorhizobium fredii HH103 rkp-3 Genes Are Required for K-Antigen Polysaccharide Biosynthesis, Affect Lipopolysaccharide Structure and Are Essential for Infection of Legumes Forming Determinate Nodules

June 2012 , Volume 25 , Number  6
Pages  825 - 838

Isabel Margaret,1 Juan C. Crespo-Rivas,1 Sebastián Acosta-Jurado,1 Ana M. Buendía-Clavería,1 María T. Cubo,1 Antonio Gil-Serrano,2 Javier Moreno,3 Piedad S. Murdoch,4 Miguel A. Rodríguez-Carvajal,2 Dulce N. Rodríguez-Navarro,5 José E. Ruiz-Sainz,1 Juan Sanjuán,6 María J. Soto,6 and José M. Vinardell1

1Departamento de Microbiología, Facultad de Biología, Universidad de Sevilla, Avda. Reina Mercedes 6, C.P. 41012, Sevilla, Spain; 2Departamento de Química Orgánica, Facultad de Química, Universidad de Sevilla, Apdo. 553, 41071-Sevilla, Spain; 3Departamento de Biología Celular y Bioquímica Vegetal and 4Departamento de Biología Molecular, Facultad de Biología, Universidad de Sevilla; 5IFAPA, Centro Las Torres-Tomejil. Apartado Oficial 41200, Alcalá del Río, Sevilla, Spain; 6Departamento de Microbiología del Suelo y Sistemas Simbióticos, Estación Experimental del Zaidín, CSIC, Apartado 419, 18080 Granada, Spain

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Accepted 26 February 2012.

The Sinorhizobium fredii HH103 rkp-3 region has been isolated and sequenced. Based on the similarities between the S. fredii HH103 rkpL, rkpM, rkpN, rkpO, rkpP, and rkpQ genes and their corresponding orthologues in Helicobacter pylori, we propose a possible pathway for the biosynthesis of the S. fredii HH103 K-antigen polysaccharide (KPS) repeating unit. Three rkp-3 genes (rkpM, rkpP, and rkpQ) involved in the biosynthesis of the HH103 KPS repeating unit (a derivative of the pseudaminic acid) have been mutated and analyzed. All the rkp-3 mutants failed to produce KPS and their lipopolysaccharide (LPS) profiles were altered. These mutants showed reduced motility and auto-agglutinated when early-stationary cultures were further incubated under static conditions. Glycine max, Vigna unguiculata (determinate nodule–forming legumes), and Cajanus cajan (indeterminate nodules) plants inoculated with mutants in rkpM, rkpQ, or rkpP only formed pseudonodules that did not fix nitrogen and were devoid of bacteria. In contrast, another indeterminate nodule–forming legume, Glycyrrhiza uralensis, was still able to form some nitrogen-fixing nodules with the three S. fredii HH103 rifampicin-resistant rkp-3 mutants tested. Our results suggest that the severe symbiotic impairment of the S. fredii rkp-3 mutants with soybean, V. unguiculata, and C. cajan is mainly due to the LPS alterations rather than to the incapacity to produce KPS.

© 2012 The American Phytopathological Society