Characterization of the Salmonella typhimurium pagC/pagD chromosomal region

The PhoP/PhoQ two-component system regulates Salmonella typhimurium genes that are essential to bacterial virulence and survival within macrophages. The best characterized of these PhoP-activated genes (pag) is pagC, which encodes a 188-amino-acid envelope protein (W. S. Pulkkinen and S. I. Miller, J. Bacteriol. 173:86-93, 1991). We here report the identification of four genes (pagD, envE, msgA, and envF) located 5' to pagC. Each gene is transcribed from its own promoter, two of which (msgA and pagD) were defined by primer extension analysis. Three of these genes (pagD, envE, and envF) are predicted to encode envelope proteins. The pagD gene is transcribed in a direction opposite from that of and adjacent to pagC and is positively regulated by PhoP/PhoQ. Transposon insertions within pagD and msgA attenuate bacterial virulence and survival within macrophages; however, deletion of pagD has no effect on virulence. The product of the envF gene is predicted to be a lipoprotein on the basis of the presence of a consensus lipid attachment site. The low G + C content of these genes and the homology of msgA to Shigella plasmid DNA suggest that this region may have been acquired by horizontal transmission.