STRUCTURE OF THE O-ANTIGEN OF ESCHERICHIA-COLI-K-12 AND THE SEQUENCE OF ITS RFB GENE-CLUSTER

Escherichia coli K-12 has long been known not to produce an 0 antigen. We recently identified two independent mutations in different lineage s of K-12 which had led to loss of 0 antigen synthesis (D. Liu and P. R. Reeves, Microbiology 140:49-57, 1994) and constructed a strain with all rfb (0 antigen) genes intact which synthesized a variant of 0 ant igen 016, giving cross-reaction with anti-017 antibody. We determined the structure of this 0 antigen to be 1-->3)-alpha-L-Rhap-(1-->3)-alph a-D-GlcpNAc-(1-->, with an 0-acetyl group on C-2 of the rhamnose and a side chain alpha-D-Glcp on C-6 of GlcNAc. 0 antigen synthesis is rfe dependent, and D-GlcpNAc is the first sugar of the biological repeat u nit. We sequenced the rfb (0 antigen) gene cluster and found 11 open r eading frames. Four rhamnose pathway genes are identified by similarit y to those of other strains, the rhamnose transferase gene is identifi ed by assay of its product, and the identities of other genes are pred icted with various degrees of confidence. We interpret earlier observa tions on interaction between the rfb region of Escherichia coli K-12 a nd those of E. coli 04 and E. coli Flexneri. All K-12 db genes were of low G+C content for E. coli. The rhamnose pathway genes were similar in sequence to those of (Shigella) Dysenteriae 1 and Flexneri, but the other genes showed distant or no similarity. We suggest that the K-12 gene cluster is a member of a family of rfb gene clusters, including those of Dysenteriae 1 and Flexneri, which evolved outside E. coli and was acquired by lateral gene transfer.