Alignment of the Pseudomonas aeruginosa ferric pyoverdine receptor, FpvA, with similar ferric-siderophore receptors revealed that the mature protein carries an extension of ca. 70 amino acids at its N terminus, an extension shared by the ferric pseudobactin receptors of P. putida. Deletion of fpvA from the chromosome of P. aeruginosa reduced pyoverdine production in this organism, as a result of a decline in expression of genes (e.g., pvdD) associated with the biosynthesis of the pyoverdine peptide moiety. Wild-type fpvA restored pvd expression in the mutant, thereby complementing its pyoverdine deficiency, although a deletion derivative of fpvA encoding a receptor lacking the N terminus of the mature protein did not. The truncated receptor was, however, functional in pyoverdine-mediated iron uptake, as evidenced by its ability to promote pyoverdine-dependent growth in an iron-restricted medium. These data are consistent with the idea that the N-terminal extension plays a role in FpvA-mediated pyoverdine biosynthesis in P. aeruginosa.

A number of aromatic residues were seen to cluster in the upper portion of the three-dimensional structure of the FpvA ferric pyoverdine receptor of Pseudomonas aeruginosa, reminiscent of the aromatic binding pocket for ferrichrome in the FhuA receptor of Escherichia coli. Alanine substitutions in three of these, W362, W391, and F795, markedly compromised ferric pyoverdine binding and transport, consistent with a role of FpvA in ferric pyoverdine recognition.Iron acquisition by Pseudomonas aeruginosa is often facilitated by high-affinity iron chelating molecules, termed siderophores, that, together with cell surface receptors specific for the iron-siderophore complexes, serve to provide the organism with iron under nutritionally dilute conditions (20). A major siderophore produced by P. aeruginosa and, indeed, all fluorescent pseudomonads is pyoverdine, a mixed catecholate-hydroxamate siderophore characterized by a conserved dihydroxyquinoline chromophore to which is attached a peptide chain of variable length and composition (3,18). This variation likely explains the noted specificity vis-à-vis pyoverdine utilization by Pseudomonas spp., where, for example, a given strain will often use only its own pyoverdine but not that of other Pseudomonas strains (4, 13), and suggests that the peptide moiety is involved in receptor recognition and binding. Some P. aeruginosa strains can also use so-called heterologous pyoverdines (i.e., those produced by other pseudomonads) of different chemical structure, though these often exhibit some peptide feature or partial amino acid sequence in common with the endogenous siderophore (1,19,28), again highlighting the importance of the peptide for receptor recognition. Three major, structurally distinct pyoverdines have been described for P. aeruginosa, dubbed types I, II, and III (17). Outer membrane receptors for all three have been described (FpvA [or FpvAI], FpvAII, and FpvAIII), and their genes have been cloned (7,21). A second receptor for type I pyoverdine, FpvB, has also recently been reported for P. aeruginosa (10). The FpvA receptor, like other ferric siderophore receptors (12), has been shown to bind both iron-free and iron-bound siderophores (25-27), although there appear to be differences in the ways that iron-free and iron-bound pyoverdines interact with FpvA (5, 9). Still, both compete with a common or at least overlapping site on FpvA (5), and iron-bound pyoverdine effectively displaces iron-free pyoverdine on the receptor during transport (24,25). Recently, the FpvA crystal structure with bound pyoverdine was solved at 3.6 Å (6), revealing a cluster of aromatic residues reminiscent of the FhuA ferrichrome receptor of Escherichia coli, where such residues were implicated in ferrichrome binding (8). We report here a study that confirms the importance of three residues in this cluster (W362, W391, and F795) in ferric pyoverdine binding and transport by FpvA.Bacterial strains and plasmids used in this study are listed in Table 1. A pyoverdine-deficient ⌬pvdD derivative ...