EXPRESSION PROFILE AND SUBCELLULAR LOCATION OF THE PLASMID-ENCODED VIRULENCE (SPV) PROTEINS IN WILD-TYPE SALMONELLA-DUBLIN

The plasmid-encoded virulence genes (spvABCD) in nontyphoid Salmonella strains mediate lethal infections in a variety of animals. Previous s tudies have shown that these genes are transcriptionally regulated by stationary-phase growth. We studied the expression profile and the sub cellular locations of the SpvABCD proteins in wild-type S. dublin by u sing polyclonal antibodies against SpvA, SpvB, SpvC, and SpvD. The cel lular levels of the individual proteins were determined during growth by quantitative immunoblotting, As expected, SpvA, SpvB, SpvC, and Spv D were not detectable before the late logarithmic growth phase and app eared in the sequence SpvA, SpvB, SpvC, and SpvD. In contrast to the t ranscriptional regulation, however, SpvA and SpvB reached their maxima l expression shortly after induction and declined during further growt h whereas SpvC and SpvD expression remained high throughout the statio nary phase, indicating that the Spy proteins are individually regulate d at a posttranscriptional level. To localize SpvABCD within the bacte ria, the cells were fractionated into the periplasmic, cytoplasmic, in ner membrane, and outer membrane components. The cell fractions and th e culture supernatant were analyzed by immunoblotting. SpvA was presen t in the outer membrane, SpvB was present in the cytoplasm and the inn er membrane, and SpvC was present in the cytoplasm. SpvD was secreted into the supernatant; however, a substantial portion of this protein w as also detected in the cytoplasm and membranes. The molecular weights of SpvD in the supernatant and in the cytoplasm appeared to be equal, suggesting that SpvD is not cleaved upon secretion.