Surface carbohydrates of Rhizobium. I. Beta-1, 2-glucans

Because of increased interest in surface carbohydrates of Rhizobium in relation to host specificity, phenol-water extractions were carried out of whole cells of Rhizobium strains of the species R. leguminosarum, R. phaseoli, R. trifolii and R. meliloti. Fractionation of the crude extracts with cetavlon afforded polysaccharide mixtures, which were essentially free of RNA and acidic exopolysaccharide (EPS). They could be separated into a high molecular weight heteropolysaccharide fraction of lipopolysaccharide (LPS) nature and a low molecular weight glucan fraction. Glucan turned out to be the principal polysaccharide component of the cells (up to 10% of the dry cell weight), when cultivated in carbohydrate-rich media, and to be present as firmly attached capsular material. Glucan (mol wt 3000) structure was elucidated by methylation and periodate oxidation techniques. Methylation yielded 3, 4, 6-tri-O-methyl-D-glucose, characterized by GLC-MS, as the only product of hydrolysis of the fully methylated glucan. The glucan consumed 1 mole of periodate per mole anhydroglucose unit and gave sophorose on partial hydrolysis. From these data a linear beta-1,2-linked glucan structure was deduced. The occurrence of beta-1,2-glucan and the implications for the specific binding properties of Rhizobium cells are discussed.