Genetic evidence for a periplasmic protein as a third component for a subset of NtrYX family two-component systems

<i>Bordetella</i> species, including <i>B. pertussis</i>, cause respiratory infections in humans and other animals. Their PlrSR two-component regulatory systems are required for survival in the lower respiratory tract. We characterized the six-gene cluster that includes <i>plrS</i> and <i>plrR</i>, identifying two internal promoters that drive expression of <i>plrP</i> and <i>plrS</i> separately from <i>plrR</i>. Our data indicate that the <i>plrP</i> gene product is an essential third component of the PlrSR two-component regulatory system (TCS), functioning to prevent PlrS from acting as a strong phosphatase <i>in vitro</i>. PlrP homologs are present, and may function similarly, in NtrYX-family TCSs in other β- and γ-proteobacteria. Our results are important because they provide insight into how bacteria, including pathogens, sense and respond to their environment.