Growth regulation of a Salmonella plasmid gene essential for virulence.

The Salmonella dublin plasmid gene vsdC is essential for virulence. We have constructed a vsdC-lacZ translational fusion to demonstrate that vsdC is selectively expressed during the stationary phase of bacterial cell growth. This pattern of expression has been confirmed by mRNA hybridization studies. Carbon starvation is able to induce vsdC expression by limiting bacterial growth. The expression of vsdC is dependent upon an upstream gene, vsdA, whose gene product possesses significant amino-terminus homology with the LysR family of transcriptional activator proteins. We have further demonstrated that vsdC expression is not dependent upon the known Salmonella chromosomal virulence regulatory loci ompR, phoP, and cya-crp and that vsdC can be expressed in a range of nontyphoidal Salmonella serovars, including some serovars in which introduction of the virulence plasmid does not confer mouse virulence. The vsd system provides a model for the study of transcriptional activation, a basis for the development of new expression vectors, and a novel mechanism of virulence gene regulation. Bacterial growth limitation within the phagosomes of host phagocytic cells may be the environmental signal inducing plasmid-mediated virulence gene expression in salmonellae.