The GTPase Activity of FlhF Is Dispensable for Flagellar Localization, but Not Motility, in Pseudomonas aeruginosa

The opportunistic human pathogen Pseudomonas aeruginosa uses two surface organelles, flagella and pili, for motility and adhesion in biotic and abiotic environments. Polar flagellar placement and number are influenced by FlhF, which is a signal recognition particle (SRP)-type GTPase. The FlhF proteins of Bacillus subtilis and Campylobacter jejuni were recently shown to have GTPase activity. However, the phenotypes associated with flhF deletion and/or mutation differ between these organisms and P. aeruginosa, making it difficult to generalize a role for FlhF in pseudomonads. In this study, we confirmed that FlhF of P. aeruginosa binds and hydrolyzes GTP. We mutated FlhF residues that we predicted would alter nucleotide binding and hydrolysis and determined the effects of these mutations on FlhF enzymatic activity, protein dimerization, and bacterial motility. Both hydrolytically active and inactive FlhF point mutants restored polar flagellar assembly, as seen for wild-type FlhF. However, differential effects on flagellar function were observed in single-cell assays of swimming motility and flagellar rotation. These findings indicate that FlhF function is influenced by its nucleotide binding and hydrolytic activities and demonstrate that FlhF affects P. aeruginosa flagellar function as well as assembly.