fleN, a gene that regulates flagellar number in Pseudomonas aeruginosa

The single polar flagellum of Pseudomonas aeruginosa plays an important rol e in the pathogenesis of infection by this organism. However, regulation of the assembly of this organelle has not been delineated. In analyzing the s equence available at the Pseudomonas genome database, an open reading frame (ORF), flanked by flagellar genes flhF and fliA, that coded for a protein (280 amino acids) with an ATP-binding motif at its N terminus was found, Th e ORF was inactivated by inserting a gentamicin cassette in P. aeruginosa P AK and PAO1, The resulting mutants were nonmotile on motility agar plates, but under a light microscope they exhibited random movement and tumbling be havior. Electron microscopic studies of the wild-type and mutant strains re vealed that the mutants were multiflagellate, with three to six polar flage lla per bacterium as rather than one as in the wild type, indicating that t his ORF was involved in regulating the number of flagella and chemotactic m otility in P. aeruginosa. The ORF was named fleN. An intact copy of fleN on a plasmid complemented the mutant by restoring motility and monoflagellate status. The beta-galactosidase activities of eight flagellar operon or gen e promoters in the wild-type and fleN mutant strains revealed a direct corr elation between six promoters that were upregulated in the fleN mutant (fli LMNOPQ, flgBCDE, fliEFG, fliDS orf126, fleSR, and fliC) and positive regula tion by FleQ, an NtrC-like transcriptional regulator for flagellar genes, B ased on these results, we propose a model where FleN influences FleQ activi ty (directly or indirectly) in regulating flagellar number in P. aeruginosa .