The PhoP/PhoQ two-component system is a master regulator of Salmonella pathogenicity. Here we report that induction of the PhoP/PhoQ system results in an initial surge of PhoP phosphorylation; the occupancy of target promoters by the PhoP protein; and the transcription of PhoP-activated genes, which then subsides to reach new steady-state levels. This surge in PhoP activity is due to PhoP positively activating its own transcription, because a strain constitutively expressing the PhoP protein attained steady-state levels of activation asymptotically, without the surge. The strain constitutively expressing the PhoP protein was attenuated for virulence in mice, demonstrating that the surge conferred by PhoP's positive feedback loop is necessary to jump-start Salmonella's virulence program.
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