Sinorhizobium fredii and Sinorhizobium meliloti produce structurally conserved lipopolysaccharides and strain-specific K antigens

Lipopolysaccharides (LPS) and capsular polysaccharides (K antigens) may inf luence the interaction of rhizobia with their specific hosts; therefore, we conducted a comparative analysis of Sinorhizobium fredii and Sinorhizobium meliloti, which are genetically related, yet symbiotically distinct, nitro gen-fixing microsymbionts of legumes, We found that both species typically produce strain-specific K antigens that consist of 3-deoxy-D-manno-2-octulo sonic acid (Kdo), or other 1-carboxy-2-keto-3-deoxy sugars (such as sialic acid), and hexoses. The K antigens of each strain are distinguished by glyc osyl composition, anomeric configuration, acetylation, and molecular weight distribution. One consistent difference between the K antigens of S, fredi i and those of S, meliloti is the presence of N-acetyl groups in the polysa ccharides of the latter. In contrast to the K antigens, the LPS of Sinorhiz obium spp, are major common antigens, Rough (R) LPS is the predominant Form of LPS produced by cultured cells, and some strains release almost no dete ctable smooth (S) LPS upon extraction. Sinorhizobium spp, are delineated in to two major RLPS core serogroups, which do not correspond to species (i.e. , host range), The O antigens of the SLPS, when present, have similar degre es of, polymerization and appear to be structurally conserved throughout th e genus, Interestingly, one strain was found to be distinct from all others : S. fredii HH303 produces a unique K antigen, which contains galacturonic acid and rhamnose, and the RLPS did not fall into either of the RLPS core s erogroups. The results of this study indicate that the conserved S- and RLP S of Sinorhizobium spp, lack the structural information necessary to influe nce host specificity, whereas the variable It antigens may affect strain-cu ltivar interactions.