Cloning and structure of group C1 O antigen (rfb gene cluster) from Salmonella enterica serovar montevideo.

The Salmonella enterica group C1 O antigen structure has a Man-Man-Man-Man-GlcNAc backbone with a glucose branch, which differs from the S. enterica group B O antigen structure which has a Man-Rha-Gal backbone with abequose as side-chain. We have cloned the group C1 rfb (O antigen) gene cluster from serovar montevideo strain M40, using a low-copy-number cosmid vector. The restriction map of the group C1 (M40) rfb gene cluster was compared with that of group B strain LT2 by Southern hybridization and restriction enzyme analysis. The results indicate that the flanking genes are very similar in the two strains, but there is no detectable similarity in the rfb regions. We localized the mannose pathway genes rfbM and rfbK and one of the genes, rfbK, shows considerably similarity to cpsG of strain LT2, suggesting that part of the mannose pathway in the group C1 rfb cluster is derived from a gene of the M antigen (cps) cluster. The M antigen, which forms a capsule, is comprised of four sugars, including fucose. The biosynthetic pathway of GDP-fucose has steps in common with the GDP-mannose pathway, and the cps cluster has isogenes of rfbK and rfbM, presumably as part of a fucose pathway. We discuss the structure and possible evolution of the group C1 rfb gene cluster.