O-antigen variation in Salmonella spp.: rfb gene clusters of three strains.

The O antigens of Salmonella serogroups A, B, and D differ structurally in their side-chain sugar residue. These genes encoding O-antigen biosynthesis are clustered in the rfb operon. We report here the molecular cloning and analysis of the rfb operons of Salmonella paratyphi A (serogroup A) and S. typhi (serogroup D). The regions of DNA nonhomology between the rfb operons of these serogroup A, B, and D representatives are identified, and the evolutionary derivation of serogroup A from a serogroup D progenitor is discussed.