The (O antigen) gene cluster of a group C1 strain was sequenced; it comprised seven open reading frames which precisely replaced the 16 open reading frames of a group B strain. Two genes of the mannose biosynthetic pathway were present: (phosphomannomutase) had a G + C content of 0.61 and had only 40% identity to of group B but was very similar to of the capsular polysaccharide pathway with 96% identity, whereas [guanosine diphosphomannose (GDP-Man) pyrophosphorylase] had a G + C content of 0.39. Other genes had G + C contents ranging from 0.24 to 0.28. (C1) and (B) had 60% identity, which is much less than expected within a species, but nonetheless indicates a much more recent common ancestor than for . The other genes showed much lower or no similarity to genes of other strains. It appears that the gene cluster evolved outside of Salmonella in a species with low G + C content: the gene presumably derives from that period whereas the gene appears to have arisen after transfer of the cluster to by duplication of the gene, presumably replacing an gene of low G + C content.


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