The response regulator BqsR/CarR controls ferrous iron (Fe²⁺) 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 regulates biofilm formation and dispersal by sensing extracytoplasmic Fe²⁺, but the mechanistic details of this process are poorly understood. In this work, we report the crystal and solution structures of the PaBqsR response regulator receiver domain, comprising a (βα)5 response regulator assembly, and the DNA-binding domain, comprising a helix-turn-helix motif. Consistent with its cognate stimulus being Fe²⁺, we show that PaBqsR binds directly to the promoter region of the feo operon that encodes the bacterial Fe²⁺ transport system FeoABC. Corroborating these in vitro results, transcriptional studies show that PaBqsR is a global regulator controlling many important genes in PAO1, including the feo operon. Intriguingly, promoter-based assays reveal that PaBqsR is a dynamic regulator that responds to bioavailable Fe²⁺, likely through the ability of PaBqsR to bind Fe²⁺ directly via a His-rich motif, independent of the PaBqsS membrane His kinase. This mode of regulation is unprecedented among OmpR-like response regulators but represents an important level of control over Fe²⁺ acquisition in P. aeruginosa that could be an attractive therapeutic target to treat nosocomial infections.