Molecular genetic basis for complex flagellar antigen expression in a triphasic serovar of Salmonella

Strains of most Salmonella serovars produce either one (monophasic) or two (diphasic) antigenic forms of flagellin protein, but strains capable of expressing three or more serologically distinct flagellins ("complex" serovars) have occasionally been reported. A molecular genetic analysis of a triphasic strain of the normally diphasic serovar Salmonella rubislaw revealed that it has three flagellin genes, including the normal fliC (phase 1) and fljB (phase 2) chromosomal genes encoding type r and type e,n,x flagellins, respectively, and a third locus (herein designated as flpA) that is located on a large plasmid (pRKS01) and codes for a type d flagellin. The coding sequence of the plasmid-borne gene is similar to that of a phase 1 chromosomal gene, but the sequence of its promoter region is homologous to that of a phase 2 chromosomal gene. The irreversible loss of the ability to express a type d flagellin that occurs when the triphasic strain is grown in the presence of d antiserum is caused by deletion of part or all of the flpA gene. Thus, the molecular basis for the unusual serological reactions of the triphasic strain of S. rubislaw and, by inference, other complex serovars of Salmonella is explained. Plasmids of the type carried by the triphasic strain of S. rubislaw provide a mechanism for the generation of new serovars through the lateral transfer and recombination of flagellin genes.