Transcription of the SsrAB regulon is repressed by alkaline pH and is independent of PhoPQ and magnesium concentration

The Salmonella pathogenicity island 2 (SPI-2) type III secretion system is expressed by intracellular bacteria and translocates effector proteins across the vacuolar membrane. Signals sensed by Salmonella enterica serovar Typhimurium in the intracellular compartment activate SPI-2 gene expression through the two-component regulatory system SsrAB. The effects of environmental and genetic signals on expression of the SsrAB-regulated gene sspH2 were examined. SsrAB-dependent activation of sspH2 was detected in the presence of both low and moderate concentrations of magnesium or calcium and at acidic and neutral pHs. The levels of expression were comparable to those detected in bacteria recovered from cultured macrophages. The induction in media at alkaline pHs (pH 7.5 and 8.0) was greatly reduced compared to the induction observed at pH 7.0 or at a lower pH, suggesting that alkaline pH represses SsrAB activation. In addition, the PhoPQ two-component system, which is also activated intracellularly, was not required for activation of SsrAB.