Location and identity of the acyl substituents on the extracellular polysaccharides of Rhizobium trifolii and Rhizobium leguminosarum.

The basic structures of the extracellular polysaccharides of Rhizobium leguminosarum and Rhizobium trifolii were found to be identical, but their acylation patterns differ. Liquid hydrogen fluoride at -40 degrees degrades the two polysaccharides to a series of oligosaccharides representing the repeating units of the polysaccharides and their higher homologs. At -23 degrees, it degrades the polymers to a mixture of oligosaccharides from which a tetrasaccharide constituting a unit of the backbone of the polysaccharide, and a trisaccharide representing all but the non-reducing terminus of the side chain, could be readily purified. The location and identity of the acyl substituents were determined by 1H-n.m.r. spectroscopy, methylation analysis, and f.a.b. mass spectrometry. The unusual substituent D-3-hydroxybutanoate was found esterified to O-3 of a terminal 4,6-O-pyruvic acetylated D-galactose in both strains of R. leguminosarum, and in one of the three strains of R. trifolii tested. All of the strains tested contained a 3-O-acetyl substituent on the (1----4)-beta-D-glucopyranosyl residues in the backbone of the polysaccharide. Only the R. leguminosarum polysaccharides contained a combination of 2- and 3-O-acetyl groups on the branching sugar of the backbone of the polymer.