FptA, the Fe(III)-pyochelin receptor of Pseudomonas aeruginosa: a phenolate siderophore receptor homologous to hydroxamate siderophore receptors.

The Pseudomonas aeruginosa siderophore pyochelin is structurally unique among siderophores and possesses neither hydroxamate- nor catecholate-chelating groups. The structural gene encoding the 75-kDa outer membrane Fe(III)-pyochelin receptor FptA has been isolated by plasmid rescue techniques and sequenced. The N-terminal amino acid sequence of the isolated FptA protein corresponded to that deduced from the nucleotide sequence of the fptA structural gene. The mature FptA protein has 682 amino acids and a molecular mass of 75,993 Da and has considerable overall homology with the hydroxamate siderophore receptors FpvA of P. aeruginosa, PupA and PupB of Pseudomonas putida, and FhuE of Escherichia coli. This observation indicates that homologies between siderophore receptors are an unreliable predictor of siderophore ligand class recognition by a given receptor. The fptA gene was strongly regulated by iron; fptA transcription was totally repressed by 30 microM FeCl3, as determined by Northern (RNA) blotting. The promoter of the fptA gene contained the sequence 5'-ATAATGATAAGCATTATC-3', which matches the consensus E. coli Fur-binding site at 17 of 18 positions. The -10 promoter region and transcriptional start site of the fptA gene reside within this Fur-binding site.