The structure of an extracellular acidic polysaccharide (APS-I) of R. meliloti 201 was studied.Digestion of APS-I with APS-I depolymerase revealed that it consists of three repeating units of oligosaccharides, termed OP1, OP2 and OP3, in a ratio of 55:25:20 (by weight). 1H-NMR analysis, methylation analysis, Smith degradation and α-D-mannosidase digestion indicated that OP1 was a pentasaccharide with the following structure:
β-D-Glcp
(1→4)
α-D-Manp-(1→4)-α-D-GlcUAp-(1→3)-α-D-Manp-(1→3)-D-Glcp
The structures of OP2 and OP3 were not fully elucidated, but each oligosaccharide had some structural similarities with OP1, containing a (1→4)-linked D-glucuronosyl residue, (1→3)(1→4)-linked D-mannosyl residue, (1→3)-linked reducing-terminal D-glucose residue and non-reducing-terminal D-mannosyl and D-glucosyl residues.