title: Structural characterization of a K-antigen capsular polysaccharide essential for normal symbiotic infection in Rhizobium sp. NGR234: deletion of the rkpMNO locus prevents synthesis of 5,7-diacetamido-3,5,7,9-tetradeoxy-non-2-ulosonic acid.
abstract: Many early molecular events in symbiotic infection have been documented, although factors enabling Rhizobium to progress within the plant-derived infection thread and ultimately survive within the intracellular symbiosome compartment as mature nitrogen-fixing bacteroids are poorly understood. Rhizobial surface polysaccharides (SPS), including the capsular polysaccharides (K-antigens), exist in close proximity to plant-derived membranes throughout the infection process. SPSs are essential for bacterial survival, adaptation, and as potential determinants of nodulation and/or host specificity. Relatively few studies have examined the role of K-antigens in these events. However, we constructed a mutant that lacks genes essential for the production of the K-antigen strain-specific sugar precursor, pseudaminic acid, in the broad host range Rhizobium sp. NGR234. The complete structure of the K-antigen of strain NGR234 was established, and it consists of disaccharide repeating units of glucuronic and pseudaminic acid having the structure -->4)-beta-d-glucuronic acid-(1-->4)-beta-5,7-diacetamido-3,5,7,9-tetradeoxy-l-glycero-l-manno-nonulosonic acid-(2-->. Deletion of three genes located in the rkp-3 gene cluster, rkpM, rkpN, and part of rkpO, abolished pseudaminic acid synthesis, yielding a mutant in which the strain-specific K-antigen was totally absent: other surface glycoconjugates, including the lipopolysaccharides, exopolysaccharides, and flagellin glycoprotein appeared unaffected. The NGRDeltarkpMNO mutant was symbiotically defective, showing reduced nodulation efficiency on several legumes. K-antigen production was found to decline after rhizobia were exposed to plant flavonoids, and the decrease coincided with induction of a symbiotically active (bacteroid-specific) rhamnan-LPS, suggesting an exchange of SPS occurs during bacterial differentiation in the developing nodule.
authors.affiliation: Laboratoire de Biologie Moléculaire des Plantes Supérieures (LBMPS), Université de Genève, 1292 Genève, Switzerland.
authors.forename: Antoine J-L
authors.initials: AJ
authors.lastname: Le Quéré
authors.forename: William J
authors.initials: WJ
authors.lastname: Deakin
authors.forename: Christel
authors.initials: C
authors.lastname: Schmeisser
authors.forename: Russell W
authors.initials: RW
authors.lastname: Carlson
authors.forename: Wolfgang R
authors.initials: WR
authors.lastname: Streit
authors.forename: William J
authors.initials: WJ
authors.lastname: Broughton
authors.forename: L Scott
authors.initials: LS
authors.lastname: Forsberg
mesh_terms: D000942:Antigens, Bacterial / Q000737:chemistry* / Q000378:metabolism; D000954:Antigens, Surface / Q000737:chemistry* / Q000378:metabolism; D001426:Bacterial Proteins / Q000737:chemistry*; D002240:Carbohydrate Sequence; D004187:Disaccharides / Q000737:chemistry; D004926:Escherichia coli / Q000378:metabolism; D017353:Gene Deletion; D008957:Models, Genetic; D008969:Molecular Sequence Data; D011134:Polysaccharides / Q000737:chemistry*; D012231:Rhizobium / Q000378:metabolism*; D012794:Sialic Acids / Q000096:biosynthesis* / Q000737:chemistry
chemical_list: C502287:5,7-diacetamido-3,5,7,9-tetradeoxynon-2-ulosonic acid; D000942:Antigens, Bacterial; D000954:Antigens, Surface; D001426:Bacterial Proteins; D004187:Disaccharides; C030659:K antigens; D011134:Polysaccharides; D012794:Sialic Acids