The Pseudomonas aeruginosa PAO1 Two-Component Regulator CarSR Regulates Calcium Homeostasis and Calcium-Induced Virulence Factor Production through Its Regulatory Targets CarO and CarP

Abstract

Pseudomonas aeruginosa is an opportunistic human pathogen that causes severe, life threatening infections in patients with cystic fibrosis (CF), endocarditis, wounds, or with artificial implants. During CF pulmonary infections, P. aeruginosa often encounters environments where the levels of calcium (Ca2+) are elevated. Previously, we showed that P. aeruginosa responds to externally added Ca2+ through enhanced biofilm formation, increased production of several secreted virulence factors, and by developing a transient increase in the intracellular Ca2+ followed by its removal to the basal sub-micromolar level. However, the molecular mechanisms responsible for regulating Ca2+-induced virulence factor production and Ca2+ homeostasis are not known. Here, we characterized the genome-wide transcriptional response of P. aeruginosa to elevated [Ca2+] in both planktonic cultures and in biofilms. Among the genes induced by CaCl2 in strain PAO1 was an operon containing the two-component regulator PA2656-PA2657 (here called carS and carR), while the closely related two-component regulators, phoPQ and pmrAB, were repressed by CaCl2 addition. To identify the regulatory targets of CarSR, we constructed a deletion mutant of carR, and performed transcriptome analysis of the mutant strain at low and high [Ca2+]. Among the genes regulated by CarSR in response to CaCl2 are the predicted periplasmic OB-fold protein, PA0320 (here called carO) and the inner membrane-anchored five-bladed β-propeller protein, PA0327 (here called carP). Mutations in both carO and carP affected Ca2+ homeostasis, reducing the ability of P. aeruginosa to export excess Ca2+. In addition, a mutation in carP had a pleotropic effect in a Ca2+-dependent manner, altering swarming motility, pyocyanin production, and tobramycin sensitivity. Overall, the results indicate that the two-component system CarSR is responsible for sensing high levels of external Ca2+, and responding through its regulatory targets that modulate Ca2+ homeostasis, surface-associated motility, and production of the virulence factor, pyocyanin.

IMPORTANCE During infectious disease, Pseudomonas aeruginosa encounters environments with high calcium (Ca2+) concentration, yet the cells maintain intracellular Ca2+ at levels that are orders of magnitude less than the external environment. In addition, Ca2+ signals P. aeruginosa to induce production of several virulence factors. Compared to eukaryotes, little is known about how bacteria maintain Ca2+ homeostasis, or how Ca2+ acts as a signal. In this study, we identified a two-component regulatory system in P. aeruginosa PAO1, termed CarRS, that is induced at elevated Ca2+. CarRS modulates Ca2+ signaling and Ca2+ homeostasis through its regulatory targets, CarO and CarP. The results demonstrate that P. aeruginosa uses a two-component regulatory system to sense external Ca2+, and relays that information for Ca2+-dependent cellular processes.