Salmonella enterica can cause intestinal or systemic
infections in humans and animals mainly by the presence of pathogenicity islands
SPI-1 and SPI-2, containing 39 and 44 genes, respectively. The AraC-like regulator HilD positively controls the expression of the
SPI-1 genes, as well as many other Salmonella virulence genes including those located in SPI-2. A previous report indicates that the two-component system CpxR/A regulates the
SPI-1 genes: the absence of the sensor
kinase CpxA, but not the absence of its cognate response regulator CpxR, reduces their expression. The presence and absence of cell envelope stress activates
kinase and
phosphatase activities of CpxA, respectively, which in turn controls the level of phosphorylated CpxR (CpxR-P). In this work, we further define the mechanism for the CpxR/A-mediated regulation of
SPI-1 genes. The negative effect exerted by the absence of CpxA on the expression of
SPI-1 genes was counteracted by the absence of CpxR or by the absence of the two
enzymes, AckA and Pta, which render
acetyl-phosphate that phosphorylates CpxR. Furthermore, overexpression of the
lipoprotein NlpE, which activates CpxA
kinase activity on CpxR, or overexpression of CpxR, repressed the expression of
SPI-1 genes. Thus, our results provide several lines of evidence strongly supporting that the absence of CpxA leads to the phosphorylation of CpxR via the AckA/Pta
enzymes, which represses both the
SPI-1 and SPI-2 genes. Additionally, we show that in the absence of the
Lon protease, which degrades HilD, the CpxR-P-mediated repression of the
SPI-1 genes is mostly lost; moreover, we demonstrate that CpxR-P negatively affects the stability of HilD and thus decreases the expression of HilD-target genes, such as hilD itself and hilA, located in
SPI-1. Our data further expand the insight on the different regulatory pathways for gene expression involving CpxR/A and on the complex regulatory network governing virulence in Salmonella.