The structure of a polysaccharide from Sinorhizobium frediiSVQ293, a thiamine auxotrophic mutant of S. fredii HH103, has been determined. This polysaccharide was isolated following the protocol for lipopolysaccharide extraction. On the basis of monosaccharide analysis, methylation analysis, fast atom bombardment MS, collision-induced dissociation tandem MS, one-dimensional 1H and 13C NMR and two-dimensional NMR experiments, the structure was shown to consist of the following trisaccharide repeating unit → 2)-α-d-Galp-(1 → 2)-β-d-Ribf-(1 → 9)-α-5-O-Me-Kdnp-(2 →, in which Kdn stands for deaminated neuraminic acid; 25% of the Kdn residues are not methylated. The structure of this polysaccharide is novel and this is the first report of the presence of Kdn in a rhizobial polysaccharide, as well as being the first structure described containing 5-O-Me-Kdn. This Kdn-containing polysaccharide is not present in the wild-type strain HH103, which produces a 3-deoxy-d-manno-2-octulosonic acid (Kdo)-rich polysaccharide. We conclude that it is likely that the appearance of this new Kdn-containing polysaccharide is a consequence of the mutation.

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