STRUCTURAL DETERMINATION OF A 5-O-METHYL-DEAMINATED NEURAMINIC ACID (KDN)-CONTAINING POLYSACCHARIDE ISOLATED FROM SINORHIZOBIUM-FREDII

The structure of a polysaccharide from Sinorhizobium fredii SVQ293, a thiamine auxotrophic mutant of S. fredii HH103, has been determined. T his polysaccharide was isolated following the protocol for lipopolysac charide extraction. On the basis of monosaccharide analysis, methylati on analysis, fast atom bombardment MS, collision-induced dissociation tandem MS, one-dimensional H-1 and C-13 NMR and two-dimensional NMR ex periments, the structure was shown to consist of the following trisacc haride repeating unit --> 2)-alpha-D-Galp-(1 --> 2)-beta-D-Ribf-(1 --> 9)-alpha-5-O-Me-Kdnp-(2 -->,in which Kdn stands for deaminated neuram inic acid; 25 % of the Kdn residues are not methylated. The structure of this polysaccharide is novel and this is the first report of the pr esence of Kdn in a rhizobial polysaccharide. as well as being the firs t structure described containing 5-O-Me-Kdn. This Kdn-containing polys accharide is not present in the wild-type strain HH103, which produces a 3-deoxy-D-manno-2-octulosonic acid (Kdo)-rich polysaccharide. We co nclude that it is likely that the appearance of this new Kdn-containin g polysaccharide is a consequence of the mutation.