Application of two new methods for cleavage of polysaccharides into specific oligosaccharide fragments. Structure of the capsular and extracellular polysaccharides of Rhizobium japonicum that bind soybean lectin.

The extracellular polysaccharide produced by the bacterium Rhizobium japonicum has been implicated in the recognition between symbionts which takes place in the association of R. japonicum with soybean. The complete primary structure of the polysaccharide produced by R. japonicum strain 3I1b 138 has been determined by a combination of conventional and unconventional methods. The polymer contains glucose, mannose, and galacturonic acid in the molar ratio of 2:1:1 and contains a varying proportion of galactose and 4-O-methylgalactose, the sum of these two being equivalent to the amount of mannose (Mort, A. J., and Bauer, W. D. (1980) Plant Physiol, 66, 158-163). The polymer was specifically degraded and its galacturonic acid residues by treatment with lithium in ethylenediamine to a tetrasaccharide. By sequential specific glycosidase hydrolysis and methylation analysis the tetrasaccharide was shown to have Structure 1: (formula: see text). Anhydrous liquid hydrogen fluoride at -23 degrees C degraded the polymer to a trisaccharide and monosaccharides. The galactose and 4-O-methylgalactose released by the HF as monosaccharides were found to be partially acetylated. By 1H NMR spectrometry and methylation analysis the trisaccharide was found to have Structure 2: (formula: see text). From the composition and the structural analyses of the tetra- and trisaccharides produced in high yields by specific degradations of the polysaccharide, the polymer is deduced to have a pentasaccharide repeating unit with Structure 3: (formula: see text).