Genetic and DNA sequence analysis of the Salmonella typhimurium virulence plasmid gene encoding the 28,000-molecular-weight protein.

We have confirmed that the 28,000-molecular-weight (28K) protein encoded by the virA gene of the 90-kilobase Salmonella typhimurium virulence plasmid is a virulence factor. It was previously shown that a Tn5 insertion, vir-22::Tn5, located in the virulence plasmid greatly attenuated virulence for mice and inhibited the production of a 28K protein (P.A. Gulig and R. Curtiss III, Infect. Immun. 56:3262-3271, 1988). Plasmid pYA426 fully complemented vir-22::Tn5 to virulence by increasing splenic infection after oral inoculation and encoded the 28K protein. To identify the virulence gene(s) of pYA426 mutated by vir-22::Tn5, we constructed nested deletions in pYA426 and examined deletion derivatives for their abilities to complement vir-22::Tn5. Only derivatives still producing the 28K protein complemented vir-22::Tn5. Furthermore, the smallest complementing derivative encoded only the 28K protein, as determined by DNA sequence analysis. Therefore, the 28K protein is sufficient for complementation of the attenuating mutation vir-22::Tn5 and must be the virulence factor inhibited by the insertion. We determined the nucleotide sequence of the 1.2-kilobase BamHI-EcoRI fragment encoding the 28K protein and identified the structural gene, virA. A 723-base-pair open reading frame which encodes a peptide with a molecular weight of 27,572 was found.