Genetic and structural analyses ofEscherichia coliO107 and O117 O-antigens

The O-antigen, consisting of many repeats of an oligosaccharide, is an essential component of the lipopolysaccharide on the surface of Gram-negative bacteria. The O-antigen is one of the most variable cell constituents, and different O-antigen forms are almost entirely due to genetic variations in O-antigen gene clusters. In this paper, we present structural and genetic evidence for a close relationship between Escherichia coli O107 and E. coli O117 O antigens. The O-antigen of E. coli O107 has a pentasaccharide repeating unit with the following structure: -->4)-beta-D-GalpNAc-(1-->3)-alpha-L-Rhap-(1-->4)-alpha-D-GlcpNAc-(1-->4)-beta-D-Galp-(1-->3)-alpha-D-GalpNAc-(1-->, which differs from the known repeating unit of E. coli O117 only in the substitution of D-GlcNAc for D-Glc. The O-antigen gene clusters of E. coli O107 and O117 share 98.6% overall DNA identity and contain the same set of genes in the same organization. It is proposed that one cluster was evolved from another via mutations, and the substitution of a few amino acids residues in predicted glycosyltransferases resulted in the functional change of one such protein for transferring different sugars in O107 (D-GlcNAc) and O117 (D-Glc), leading to different O-antigen structures. This is an example of the O-antigen alteration caused by nucleotide mutations, which is less commonly reported for O-antigen variations.