Programmed Delay of a Virulence Circuit Promotes Salmonella Pathogenicity

Signal transduction systems dictate various cellular behaviors in response to environmental changes. To operate cellular programs appropriately, organisms have sophisticated regulatory factors to optimize the signal response. The PhoP/PhoQ master virulence regulatory system of the intracellular pathogen Salmonella enterica is activated inside acidic macrophage phagosomes. Here we report that Salmonella delays the activation of this system inside macrophages using an inhibitory protein, EIIA(Ntr) (a component of the nitrogen-metabolic phosphotransferase system). We establish that EIIA(Ntr) directly restrains PhoP binding to its target promoter, thereby negatively controlling the expression of PhoP-activated genes. PhoP furthers its activation by promoting Lon-mediated degradation of EIIA(Ntr) at acidic pH. These results suggest that Salmonella ensures robust activation of its virulence system by suspending the activation of PhoP until a sufficient level of active PhoP is present to overcome the inhibitory effect of EIIA(Ntr). Our findings reveal how a pathogen precisely and efficiently operates its virulence program during infection. IMPORTANCE To accomplish successful infection, pathogens must operate their virulence programs in a precise, time-sensitive, and coordinated manner. A major question is how pathogens control the timing of virulence gene expression during infection. Here we report that the intracellular pathogen Salmonella controls the timing and level of virulence gene expression by using an inhibitory protein, EIIA(Ntr). A DNA binding master virulence regulator, PhoP, controls various virulence genes inside acidic phagosomes. Salmonella decreases EIIA(Ntr) amounts at acidic pH in a Lon- and PhoP-dependent manner. This, in turn, promotes expression of the PhoP-activated virulence program because EIIA(Ntr) hampers activation of PhoP-regulated genes by interfering with PhoP binding to DNA. EIIA(Ntr) enables Salmonella to impede the activation of PhoP-regulated gene expression inside macrophages. Our findings suggest that Salmonella achieves programmed delay of virulence gene activation by adjusting levels of an inhibitory factor.