The RstB sensor acts on the PhoQ sensor to control expression of PhoP-regulated genes

The PhoP response regulator and PhoQ sensor, which are encoded by the phoPQ operon, constitute the PhoP/PhoQ two-component system. Genome-wide transcription analysis revealed that heterologous expression of the RstB protein, a sensor of the RstA/RstB two-component system, leads to enhanced transcription of PhoP-activated genes in wild-type Salmonella. We determined that RstB-induction increases the levels of phoP mRNA as well as PhoP protein, while lack of the phoPQ genes abolishes RstB-promoted transcription of the PhoP-regulated genes. This regulatory function of RstB did not require its enzymatic activities, and thus the truncated RstB protein containing only periplasmic and transmembrane regions was able to promote PhoP-activated transcription. The RstB protein appeared to target the PhoQ sensor rather than the PhoP response regulator because RstB-induction failed to enhance transcription of the PhoP-regulated genes in a strain maintaining the normal PhoP function, even without PhoQ.