IDENTIFICATION OF A VIRULENCE LOCUS ENCODING A 2ND TYPE-III SECRETIONSYSTEM IN SALMONELLA-TYPHIMURIUM

Mapping the insertion points of 16 signature-tagged transposon mutants on the Salmonella typhimurium chromosome led to the identification of a 40-kb virulence gene cluster at minute 30.7. This locus is conserve d among all other Salmonella species examined but is not present in a variety of other pathogenic bacteria or in Escherichia coli K-12. Nucl eotide sequencing of a portion of this locus revealed 11 open reading frames whose predicted proteins encode components of a type III secret ion system. To distinguish between this and the type III secretion sys tem encoded by the inv/spa invasion locus known to reside on a pathoge nicity island, we refer to the inv/spa locus as Salmonella pathogenici ty island (SPI) 1 and the new locus as SPI2. SPI2 has a lower G+C cont ent than that of the remainder of the Salmonella genome and is flanked by genes whose products share greater than 90% identity with those of the E. coli ydhE and pykF genes. Thus SPI2 was probably acquired hori zontally by insertion into a region corresponding to that between the ydhE and pykF genes of E. coli. Virulence studies of SPI2 mutants have shown them to be attenuated by at least five orders of magnitude comp ared with the wild-type strain after oral or intraperitoneal inoculati on of mice.