Bacterial polysaccharide which binds Rhizobium trifolii to clover root hairs

Immunofluorescence, quantitative immunoprecipitation, and inhibition of bacterial agglutination and passive hemagglutination indicate that cross-reactive antigenic determinants are present on the surface of Rhizobium trifolii and clover roots. These determinants are immunochemically unique to this Rhizobium-legume cross-inoculation group. The multivalent lectin trifoliin and antibody to the clover root antigenic determinants bind competitively to two acidic heteropolysaccharides isolated from capsular material of R. Trifolii 0403. The major polysaccharide is an antigen which lacks heptose, 2-keto-3-deoxyoctulosonic acid, and endotoxic lipid A. The minor polysaccharide in the capsular material of R. Trifolii 0403 contains the same antigen in addition to heptose, 2-keto-3-deoxyoctonate, and lipid A. The acidic polysaccharides of two strains of R. trifolii share the clover r-ot cross-reactive antigenic determinant despite other differences in their carbohydrate composition. Studies with monovalent antigen-binding fragments of anti-clover root antibody and Azotobacter vinelandii hybrid transformants carrying the unique antigenic determinant suggest that these polysaccharides bind R. trifolii to the clover root hair tips which contain trifoliin.