Copyright X) 1991, American Society for Microbiology Genetic Analysis of Homology between the Virulence Plasmids of

Two segments within the virulence region of Salmonella dublin plasmid pSDL2 that were homologous to regions on Yersinia pseudotuberculosis plasmid pIBl were located with regard to the four known genes (vsdA, vsdB, vsdC, and vsdD) of pSDL2. One segment mapped upstream of vsdA within an insertion element related to IS630 of ShigeUa sonnei; the second was confined to a 45-bp sequence containing an inverted repeat between vsdC and vsdD. On pIBl, both areas were located in an intergenic region upstream of yopH. These results indicate that the homology between Yersinia and SalmoneUa virulence plasmids is not located within structural genes. The common pathogenetic features of nontyphoid Salmo-nella and Yersinia infections prompted us to search for common plasmid-mediated mechanisms for virulence (7). In both genera, progressive systemic infection is clearly depen-dent on the presence of virulence plasmids which are be-lieved to enable the microorganisms to survive and grow in the phagocytic cells of the reticuloendothelial organs (5, 18). We found three DNA segments on virulence plasmid pSDL