MOLECULAR ANALYSIS OF THE RFB GENE-CLUSTER OF A GROUP D2 SALMONELLA-ENTERICA STRAIN - EVIDENCE FOR ITS ORIGIN FROM AN INSERTION SEQUENCE-MEDIATED RECOMBINATION EVENT BETWEEN GROUP-E AND GROUP-D1 STRAINS

The Salmonella enterica O antigen is a highly variable surface polysac charide composed of a repeated oligosaccharide (the O unit). The O uni t produced by serogroup D2 has structural features in common with thos e of groups D1 and E1, and hybridization studies had previously sugges ted that the D2 rfb gene cluster responsible for O-unit biosynthesis i s indeed a hybrid of the two. In this study, the rfb gene fluster was cloned from a group D2 strain of S. enterica sv. Strasbourg. Mapping, hybridization, and DNA sequencing showed that the organization of the D2 rfb genes is similar to that of group D1, with the alpha-mannosyl t ransferase gene rfbU replaced by rfbO, the E1-specific P-mannosyl tran sferase gene. The E1-specific polymerase gene (rfc) has also been acqu ired. Interestingly, the D1-like and El-like rfb regions are separated by an additional sequence closely related to an element (Hinc repeat [H-rpt]) associated with the Rhs loci of Escherichia coli. The H-rpt r esembles an insertion sequence and possibly mediated the intraspecific recombination events which produced the group D2 rfb gene organizatio n.