Structure of the unusual trisaccharide lipopolysaccharide component produced by a symbiotically defective mutant of Rhizobium leguminosarum biovar viciae

The structure of an unusual trisaccharide component isolated from the lipopolysaccharide (LPS) of a Tn5 mutant of Rhizobium leguminosarum biovar viciae VF39 which is defective in infection of its host plant has been elucidated. This mutant also appears to be defective in the synthesis of a tetrasaccharide component normally synthesized by the wild-type organism. The three glycosyl components are galactose, mannose, and 3-deoxy-d-manno-2-octulosonic acid (Kdo). Mannose is linked to the 5-position and galactose to the 7-position of the 3-deoxy-2-octulosonic acid residue (Kdo). Both hexosyl components are in the α-pyranosyl form. In the isolated molecule the octulosonic acid appears to be present as its γ-lactone. However, in the lipopolysaccharide molecule, it is most likely present in the pyranosyl form. The structure was determined by 1H NMR spectroscopy and methylation analysis as well as by fast-atom-bombardment mass spectrometry of the peracetylated and per(trideuterio) acetylated oligosaccharides. Small amounts of the methylation analysis product of another tetrasaccharide different to normal tetrasaccharide component made by the wild-type organisms were detected. This indicates that in this mutant, there is a block in the synthesis of the normal tetrasaccharide component in addition to a switch in the synthesis of the LPS type.