FIGURE 6.

Model illustrating the H-NS-dependent Mg²⁺ response of the PhoP/PhoQ system. In wild-type, the PhoP protein is phosphorylated during growth in low Mg²⁺. The type I PhoP-activated genes (left) are transcriptionally activated via competition of the phosphorylated PhoP, which binds with high affinity (probably facilitated by other factors) to the PhoP box (the red box) over H-NS near the UP motif (the green box), whereas type II PhoP-activated genes (green circles) are activated during growth in low Mg²⁺ when PhoP-activated SlyA, along with the phosphorylated PhoP, bind to the promoters replacing H-NS in the SlyA box and PhoP box, and stimulating their transcription in S. typhimurium. Competition over the unphosphorylated PhoP allows the H-NS protein to occupy these chromosomal regions and repress transcription of the type I and II genes when wild-type bacteria are grown in high Mg²⁺ conditions. In the absence of H-NS (Hns^- strain), the unphosphorylated PhoP, regardless of its low DNA affinity, is able to bind to type I promoters, which facilitates transcriptional activation of these genes even in high Mg²⁺ conditions. Meanwhile, type II promoters become independent of SlyA and PhoP in Hns^- bacteria.