SUMMARY: The group C1 O antigen structure has a Man-Man-Man-Man-GlcNAc backbone with a glucose branch, which differs from the group B O antigen structure which has a Man-Rha-Gal backbone with abequose as side-chain. We have cloned the group C1 (O antigen) gene cluster from serovar strain M40, using a low-copy-number cosmid vector. The restriction map of the group C1 (M40) 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 regions. We localized the mannose pathway genes and and one of the genes, , shows considerably similarity to of strain LT2, suggesting that part of the mannose pathway in the group C1 cluster is derived from a gene of the M antigen () 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 cluster has isogenes of and , presumably as part of a fucose pathway. We discuss the structure and possible evolution of the group C1 gene cluster.


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