The O-antigen is a repetitive glycan polymer with a repeat unit of two to six sugar residues, and comprises the outermost and immunogenic domain of the lipopolysaccharide (
map00540). Most of the sugar residues are derived from nucleotide sugars and the genes for their biosynthesis seem to be well conserved among Gram-negative bacterial species. They include UDP-sugars synthesized from Glc-1P via UDP-Glc or from GlcNAc-1P via UDP-GlcNAc, GDP-sugars synthesized from Fru-6P via GDP-Man, and dTDP-sugars and CDP-sugars synthesized from Glc-1P via dTDP-Glc and CDP-Glc, respectively. CMP-sugars are also synthesized from UDP-GlcNAc. The O-antigen nucleotide sugar biosynthesis genes often form gene clusters with glycosyltransferase genes and other processing genes such as for translocation and polymerization.