The 0 antigen of Salmonella group B strains contains the sugar abequose, whereas those from group A and D strains contain paratose or tyvelose in its place. This is the essential difference between these Salmonella groups. Only the final step in the biosynthesis of abequose differs from that of paratose, and the abequose confers on group B strains their specific 04 antigen. The gene, rJbJ, encoding the enzyme abequose synthase for this last specific step has been cloned, identified, and sequenced and has been shown to function in group A and D strains to make them 04+. This one gene thus differentiates group B from group A or group D salmonellae. The enzyme abequose synthase appears to be related to galactose epimerase, and the significance of this is discussed. The rjbJ gene and adjacent DNA is of much lower G+C content than is usual for salmonellae, indicating that the region did not originate in a salmonella but was transferred from outside.We are studying the genetics of a major polymorphism in bacteria and present here the genetic base for the 04 antigenic epitope, which characterizes one form of the 0 antigen polymorphism.The 0 antigen is a polysaccharide which covers much of the surface of gram-negative bacteria, although it may be masked by a capsule external to the cell proper. The 0 antigen is typically a polymer with a repeating oligosaccharide of 3 to 6 sugar residues, which is linked through an oligosaccharide core to lipid A, the whole comprising the lipopolysaccharide (LPS), which is the major lipid of the outer leaflet of the outer membrane characteristic of gramnegative bacteria. The lipid A and core do not vary greatly within a genus and are thought to be invariable within salmonellae. The 0 antigen, however, is extremely polymorphic and in salmonellae, about 40 major forms are currently recognized. It should be noted that the genus Salmonella comprises a single species, S. enterica; unfortunately, the many serovars are generally given full species rank, although DNA hybridization and other factors show that the general level of variation between these serovars is that expected for a single bacterial species (4,22

A cosmid bank of the DNA (including cryptic plasmid DNA) of a virulent strain of Salmonella typhimurium was prepared in Escherichia coli K12, and clones that contained cryptic plasmid DNA were detected by probing. Two such clones expressed a new outer membrane protein of 11 kilodaltons (kDa) and were serum resistant (E. coli K12 is serum sensitive). The gene encoding the 11-kDa protein was subcloned in a 2.1-kilobase fragment and shown to mediate serum resistance in both E. coli K12 and a cryptic plasmid-free (serum-sensitive) strain of S. typhimurium. The cryptic plasmid-free S. typhimurium strain did not express normal lipopolysaccharide, but introduction of the 11-kDa protein gene into the strain rendered the strain serum resistant without restoration of normal lipopolysaccharide synthesis. The 11-kDa protein gene was not sufficient to restore either macrophage resistance or virulence to a cryptic plasmid-free strain of S. typhimurium.

Biosynthesis of the Salmonella typhimurium LT2 O antigen is encoded by genes which map in the rfb cluster. The cloning and restriction enzyme analysis of part of this cluster have been described previously (H. N. Brahmbhatt, N. B. Quigley, and P. R. Reeves, Mol. Gen. Genet. 203:172-176, 1986). The entire rfb gene cluster has now been cloned, and a detailed restriction enzyme map has been constructed which has enabled us to map the approximate positions of individual rfb genes.

A cosmid bank of the DNA of Salmonella typhimurium LT2 was prepared in Escherichia coli K12, and probed with radiolabelled cryptic plasmid from the same Salmonella strain. 2 of the 7 clones thus detected expressed an obvious new outer membrane protein of 23 kDa. The gene encoding the polypeptide was subcloned, in a 6.6‐kb ClaI fragment, from one of the cosmids into pBR322. The clone was mapped with restriction enzymes. The gene location and the direction of transcription were defined with the aid of transposon Tn1725 insertions. The gene product was detected in minicells. The protein was partially peptidoglycan‐associated in E. coli.