The Response Regulator BqsR/CarR Controls Ferrous Iron (Fe ²⁺ ) Acquisition in Pseudomonas aeruginosa

Pseudomonas aeruginosa is a ubiquitous, Gram-negative bacterium that forms biofilms and is responsible for antibiotic-resistant nosocomial infections in humans. The P. aeruginosa BqsRS two-component system (TCS) regulates biofilm formation and dispersal by sensing extracytoplasmic Fe ²⁺ , but the mechanistic details of this process are poorly understood. In this work, we report the crystal and solution structures of the Pa BqsR response regulator (RR) receiver domain (RD), comprising a (βα) ₅ response regulator assembly, and DNA-binding domain (DBD), comprising a winged helix-turn-helix motif. Consistent with its cognate stimulus being Fe ²⁺ , we show that Pa BqsR binds directly to the promoter region of the feo operon that encodes the bacterial Fe ²⁺ transport system Feo. Transcriptional studies show that Pa BqsR is a global regulator that negatively controls the feo operon, and deletion of bqsR provides a growth advantage of PAO1 in the presence of Fe ²⁺ as its only iron source. Surprisingly, X-ray absorption spectroscopy (XAS) shows that recombinant Pa BqsR binds a single Fe ²⁺ ion in a His-rich site, divergent from other members of the OmpR-like family. These exciting results reveal that the BqsRS system controls Fe ²⁺ acquisition in P. aeruginosa and suggest that this system may be an attractive future therapeutic target to treat nosocomial infections.