The Effects of the H-NS Protein on PhoP-dependent Transcriptional Regulation of the mgtCBRU-cigR Operon in Salmonella enterica serovar Typhimurium

PhoQP is a two-component system that regulates the transcription of ~5% of the genes of Salmonella enterica. The membrane-bound PhoQ protein is phosphorylated in response to low extracellular Mg²⁺ concentration, acid pH, and a number of antimicrobial peptides. The inorganic phosphate bound to PhoQ is transferred to PhoP, which according to the classical model, acts as a typical transcriptional activator of its target genes. However, Will et al. (doi.org/10.1038/ncomms6270) proposed an alternate “counter-silencing” model, according to which genes in the PhoP regulon that were acquired by Salmonella via horizontal transfer are repressed by the generalized DNA-binding protein H-NS at high [Mg²⁺] and are induced at low [Mg²⁺] because the phosphorylated PhoP displaces the H-NS from the promoters and lifts repression. We evaluated this model by examining the transcriptional regulation of the mgtCBRU-cigR operon, which encodes the virulence protein MgtC and the Mg²⁺ transport protein MgtB and is in the SPI-3 pathogenesis island that has been acquired by Salmonella via horizontal transfer. Our main finding was that in the non-pathogenic strain of S. Typhimurium (LT2), induction of the mgtCBRU-cigR operon by Mg²⁺ limitation requires a functional PhoP protein, regardless of the presence or absence of H-NS. Interestingly, the pathogenic strain of S. Typhimurium (ATCC 14028s) revealed PhoP-independent transcription in the absence of H-NS, but only under inducing conditions. Thus, our results do not support the counter-silencing model and are consistent with the canonical view that PhoP is needed as a transcriptional activator of genes in the PhoP regulon.