The Pseudomonas aeruginosa phosphate transport protein PstS plays a phosphate-independent role in biofilm formation

Pseudomonas aeruginosa (PA) is a primary cause of nosocomial infections. A key element in PA pathogenicity is its ability to form biofilms that with-stand eradication by antibiotics and the immune system. Biofilm formation is controlled by phosphate signaling and here we provide evidence that PstS, a subunit of the PA Pst phosphate transporter, has a surprising role in this process. Using X-ray crystallography, we characterized the unique underpinnings of PstS phosphate binding and identified an unusual 15-residue N′ loop extension. Structure-based experiments showed that PstS-mediated phosphate uptake and biofilm formation are in fact two distinct functions. Specifically, a point mutation that abrogated phosphate binding did not eliminate biofilm formation; conversely, truncation of the N′ loop diminished the ability of PA to form biofilms but had no effect on phosphate binding and uptake. This places PstS at a junction that separately controls phosphate sensing and uptake and the ultrastructure organization of bacteria.