Identification of an Escherichia coli operon required for formation of the O-antigen capsule

Escherichia coli produces polysaccharide capsules that, based on their mechanisms of synthesis and assembly, have been classified into four groups. The group 4 capsule (G4C) polysaccharide is frequently identical to that of the cognate lipopolysaccharide O side chain and has, therefore, also been termed the O-antigen capsule. The genes involved in the assembly of the group 1, 2, and 3 capsules have been described, but those required for G4C assembly remained obscure. We found that enteropathogenic E. coli (EPEC) produces G4C, and we identified an operon containing seven genes, ymcD, ymcC, ymcB, ymcA, yccZ, etp, and etk, which are required for formation of the capsule. The encoded proteins appear to constitute a polysaccharide secretion system. The G4C operon is absent from the genomes of enteroaggregative E. coli and uropathogenic E. coli. E. coli K-12 contains the G4C operon but does not express it, because of the presence of IS1 at its promoter region. In contrast, EPEC, enterohemorrhagic E. coli, and Shigella species possess an intact G4C operon. Escherichia coli strains exhibit variations in their cell surface polysaccharides. The primary surface polysaccharides, lipopolysaccharide (LPS) and capsule, are serotype specific (28, 33). Based on their mechanisms of synthesis and assembly, the E. coli capsules have been classified into four groups (33). Since in some cases the polysaccharide of the group 4 capsule (G4C) is identical to that of the cognate LPS O side chain, G4C was also termed O-antigen capsule (13). E. coli O111 and O127, serotypes which are typical also of enteropathogenic E. coli (EPEC) and enterohemorrhagic E. coli, were reported to form G4C (13, 27). The specific genes required for the assembly of group 1, 2, and 3 capsules have been identified and characterized (33). However, until now, the genes necessary for the assembly of the G4C were not known. LPS is an amphipathic glycoconjugate whose hydrophobic domain, lipid A, forms the outer leaflet of the outer membrane. The core oligosaccharide links lipid A to the oligosaccharide side chain, known as the O antigen. The structure of lipid A and of the core oligosaccharide is conserved, but the structure of the O antigen is variable (28). The E. coli O antigen may be synthesized by several alternative mechanisms, including the Wzy-dependent pathway. The O antigen, which is synthesized via the Wzy-dependent pathway, is made up of repeat units typically consisting of three to five sugar residues (28). The structural diversity of the O antigens stems mainly from variations in sugar composition, sequence, and linkage. Synthesis and assembly of LPS require nu