Environmental regulation of Salmonella pathogenicity island 2 gene expression. The enteric pathogen Salmonella typhimurium co-ordinates the expression of virulence determinants in response to environmental cues from the host organism. S. typhimurium possesses Salmonella pathogenicity island 2 (SPI2), a large virulence locus encoding a type III secretion system for virulence determinants required for systemic infections and accumulation inside host cells. We have generated transcriptional fusions to SPI2 genes to analyse expression and used antibodies against recombinant SPI2 proteins to monitor levels of SPI2 proteins under various conditions. Here, we demonstrate that SPI2 gene expression is induced by Mg2+ deprivation and phosphate starvation. These conditions are likely to represent the environmental cues encountered by S. typhimurium inside the phagosome of infected host cells. The induction of SPI2 gene expression is modulated by the global regulatory system PhoPQ and is dependent on SsrAB, a two-component regulatory system encoded by SPI2.