A Single Amino Acid Change in the Response Regulator PhoP, Acquired during Yersinia pestis Evolution, Affects PhoP Target Gene Transcription and Polymyxin B Susceptibility

<p><em>Yersinia pestis</em>, the causative agent of plague, evolved from the closely related pathogen <em>Yersinia pseudotuberculosis</em>. During its emergence, <em>Y. pestis</em> is believed to have acquired its unique pathogenic characteristics through numerous gene gains/losses, genomic rearrangements, and single nucleotide polymorphism (SNP) changes. One such SNP creates a single amino acid variation in the DNA binding domain of PhoP, the response regulator in the PhoP/PhoQ two-component system. <em>Y. pseudotuberculosis</em> and the basal human-avirulent strains of <em>Y. pestis</em> harbor glycines at position 215 of PhoP, whereas the modern human-virulent strains (e.g., KIM and CO92) harbor serines at this residue. Since PhoP plays multiple roles in the adaptation of <em>Y. pestis</em> to stressful host conditions, we tested whether this amino acid substitution affects PhoP activity or the ability of <em>Y. pestis</em> to survive in host environments. Compared to the parental KIM6+ strain carrying the modern allele of <em>phoP</em> (<em>phoP-S215</em>), a derivative carrying the basal allele (<em>phoP-G215</em>) exhibited slightly defective growth under a low-Mg2+ condition and decreased transcription of a PhoP target gene, <em>ugd</em>, as well as an ∼8-fold increase in the susceptibility to the antimicrobial peptide polymyxin B. The <em>phoP-G215</em> strain showed no apparent defect in flea colonization, although a <em>phoP</em>-null mutant showed decreased flea infectivity in competition experiments. Our results suggest that the amino acid variation at position 215 of PhoP causes subtle changes in the PhoP activity and raise the possibility that the change in this residue have contributed to the evolution of increased virulence in <em>Y. pestis</em>.</p>