SEQUENCE AND ANALYSIS OF THE O-ANTIGEN GENE (RFB) CLUSTER OF ESCHERICHIA-COLI-O111

The O antigens found in Salmonella enterica (Se) and Escherichia coli (Ec) show a great deal of diversity, and only three structures are kno wn to be common to both genera. Two of them contain the 3,6-dideoxyhex ose colitose, not found in other serogroups of the two species. The fi rst of these is common to Ec O111 and Se O:35 (sv Adelaide); the other is found in both Ec O55 and Se O:50 (sv Greenside). The genes specifi c for the synthesis of O antigen are generally located in the rfb gene cluster at map position 45 min in Ec and 42 min in Se. The rfb (O ant igen) gene cluster of an Ec O111 strain M92 had been cloned earlier an d hybridisation analysis suggested that the rfb clusters of Ec M92 and a Se sv Adelaide strain had been acquired separately by the two speci es since their divergence. We have now sequenced part of the rfb clust er from Ec M92. We identify two genes of the GDP-colitose pathway, rfb M and rfbK, and show that several other ORFs have similarity to the rf b and cps (capsular polysaccharide) genes. Downstream of this block of genes is an ORF which encodes a protein with predicted transmembrane segments which is presumed to correspond to the rfbX gene. The % G+C v alues of the Ec M99 rfb sequence are extremely low, indicating that th e rfb evolved in a low % G+C species of bacteria before transfer into Ec.