<i>Yersinia pestis</i> plasminogen activator protease is regulated by the PhoP/PhoQ two-component system

<i>Yersinia pestis</i> is a gram-negative bacterium and the causative agent of plague. The <i>Y. pestis</i> virulence factor plasminogen activator protease (Pla) is an outer membrane aspartic protease that facilitates the dissemination of bacteria from the site of inoculation to deeper tissue during bubonic plague. During pneumonic plague, Pla acts as an adhesin, which contributes to the suppression of early innate immune responses in the lungs, and as a protease that aids in resisting bacterial killing by neutrophils. Two-component regulatory systems (TCSs) are involved in bacterial adaptation to environmental stressors such as changes in pH, changes in ion concentrations, and the presence of cationic antimicrobial peptides. TCSs consist of a membrane-bound sensor kinase that detects environmental stressors and activates a response regulator to coordinately alter gene expression. The PhoP/PhoQ TCS regulates virulence factors and known Pla homologs in a variety of gram-negative pathogenic bacteria including <i>Escherichia coli</i> and <i>Salmonella</i> species. In the work described here, we evaluate whether <i>pla</i> is regulated by PhoP/PhoQ in <i>Y. pestis</i>. We identify a putative PhoP-binding site within the -10 box and the +1 transcription start site of <i>pla</i> that is bound by recombinant PhoP. Surprisingly, we show that the expression of <i>pla</i> is suppressed by PhoP/PhoQ under a variety of physiologically relevant PhoP/PhoQ-inducing conditions that are expected to be encountered during infection. This work demonstrates the regulation of an essential <i>Y. pestis</i> virulence factor by the PhoP/PhoQ TCS for the first time and highlights the importance of tightly regulating virulence factors that function as proteases.IMPORTANCE<i>Yersinia pestis</i> causes plague, a highly lethal infection that results from inoculation via an infected flea (bubonic plague) or inhalation of contaminated respiratory droplets via person-to-person transmission (pneumonic plague). The plasminogen activator protease (Pla) is a critical <i>Y. pestis</i> virulence factor that is essential to the progression of infection via either route of inoculation. In this work, we show for the first time that the well-established two-component regulatory system PhoP/PhoQ regulates the expression of <i>pla</i>. Under conditions found during mammalian infection, PhoP/PhoQ suppresses <i>pla</i> expression, presumably to limit aberrant cleavage of Pla substrates during the critical early stages of infection. These results show interaction between two key virulence loci for the first time, and shed light on the regulation of a critical <i>Y. pestis</i> virulence determinant.