Mapping of mutations in Pseudomonas aeruginosa defective in pyoverdin production

Cell division factor ZapE regulates Pseudomonas aeruginosa biofilm formation by impacting the pqs quorum sensing system

Pseudomonas aeruginosa is one of the leading nosocomial pathogens that causes both severe acute and chronic infections. The strong capacity of P. aeruginosa to form biofilms can dramatically increase its antibiotic resistance and lead to treatment failure. The biofilm resident bacterial cells display distinct gene expression profiles and phenotypes compared to their free-living counterparts. Elucidating the genetic determinants of biofilm formation is crucial for the development of antibiofilm drugs. In this study, a high-throughput transposon-insertion site sequencing (Tn-seq) approach was employed to identify novel P. aeruginosa biofilm genetic determinants. When analyzing the novel biofilm regulatory genes, we found that the cell division factor ZapE (PA4438) controls the P. aeruginosa pqs quorum sensing system. The ∆zapE mutant lost fitness against the wild-type PAO1 strain in biofilms and its production of 2-heptyl-3-hydroxy-4(1H)-quinolone (PQS) had been reduced. Further biochemical analysis showed that ZapE interacts with PqsH, which encodes the synthase that converts 2-heptyl-4-quinolone (HHQ) to PQS. In addition, site-directed mutagenesis of the ATPase active site of ZapE (K72A) abolished the positive regulation of ZapE on PQS signaling. As ZapE is highly conserved among the Pseudomonas group, our study suggests that it is a potential drug target for the control of Pseudomonas infections.

FvbA is required for vibriobactin utilization in Pseudomonas aeruginosa

Bacteria acquire iron through a highly specific mechanism involving iron-chelating molecules termed siderophores. The Gram-negative bacterium Pseudomonas aeruginosa can utilize siderophores produced by other micro-organisms to facilitate iron uptake. Here we show that a P. aeruginosa strain deficient in siderophore production can use the Vibrio cholerae siderophore vibriobactin as an iron source. In addition, we identified a P. aeruginosa gene, PA4156 (fvbA), encoding a protein highly homologous to the V. cholerae vibriobactin receptor (ViuA). A P. aeruginosa mutant in the two endogenous siderophores (pyoverdine and pyochelin) and in fvbA was unable to utilize vibriobactin as an iron source. Additionally, preliminary analyses revealed the involvement of vibriobactin, Fur protein and an IclR-type regulator, FvbR (PA4157), in fvbA regulation.

Role of TonB1 in pyoverdine-mediated signaling in Pseudomonas aeruginosa

Pyoverdines are siderophores secreted by Pseudomonas aeruginosa. Uptake of ferripyoverdine in P. aeruginosa PAO1 occurs via the FpvA receptor protein and requires the energy-transducing protein TonB1. Interaction of (ferri)pyoverdine with FpvA activates pyoverdine gene expression in a signaling process involving the cytoplasmic-membrane-spanning anti-sigma factor FpvR and the sigma factor PvdS. Here, we show that mutation of a region of FpvA that interacts with TonB1 (the TonB box) prevents this signaling process, as well as inhibiting bacterial growth in the presence of the iron-chelating compound ethylenediamine-di(o-hydroxy-phenylacetic acid). Signaling via wild-type FpvA was also eliminated in strains lacking TonB1 but was unaffected in strains lacking either (or both) of two other TonB proteins in P. aeruginosa, TonB2 and TonB3. An absence of pyoverdine-mediated signaling corresponded with proteolysis of PvdS. These data show that interactions between FpvA and TonB1 are required for (ferri)pyoverdine signal transduction, as well as for ferripyoverdine transport, consistent with a mechanistic link between the signaling and transport functions of FpvA.

FpvA bound to non-cognate pyoverdines: molecular basis of siderophore recognition by an iron transporter

The first step in the specific uptake of iron via siderophores in Gram-negative bacteria is the recognition and binding of a ferric siderophore by its cognate receptor. We investigated the molecular basis of this event through structural and biochemical approaches. FpvA, the pyoverdine-Fe transporter from Pseudomonas aeruginosa ATCC 15692 (PAO1 strain), is able to transport ferric-pyoverdines originating from other species, whereas most fluorescent pseudomonads are only able to use the one they produce among the more than 100 known different pyoverdines. We solved the structure of FpvA bound to non-cognate pyoverdines of high- or low-affinity and found a close correlation between receptor-ligand structure and the measured affinities. The structure of the first amino acid residues of the pyoverdine chain distinguished the high- and low-affinity binders while the C-terminal portion of the pyoverdines, often cyclic, does not appear to contribute extensively to the interaction between the siderophore and its transporter. The specificity of the ferric-pyoverdine binding site of FpvA is conferred by the structural elements common to all ferric-pyoverdines, i.e. the chromophore, iron, and its chelating groups.

The Pseudomonas aeruginosa pyochelin-iron uptake pathway and its metal specificity

Pyochelin (Pch) is one of the two major siderophores produced and secreted by Pseudomonas aeruginosa PAO1 to assimilate iron. It chelates iron in the extracellular medium and transports it into the cell via a specific outer membrane transporter, FptA. We used the fluorescent properties of Pch to show that this siderophore chelates, in addition to Fe(3+) albeit with substantially lower affinities, Ag(+), Al(3+), Cd(2+), Co(2+), Cr(2+), Cu(2+), Eu(3+), Ga(3+), Hg(2+), Mn(2+), Ni(2+), Pb(2+), Sn(2+), Tb(3+), Tl(+), and Zn(2+). Surprisingly, the Pch complexes with all these metals bound to FptA with affinities in the range of 10 nM to 4.8 microM (the affinity of Pch-Fe is 10 nM) and were able to inhibit, with various efficiencies, Pch-(55)Fe uptake in vivo. We used inductively coupled plasma atomic emission spectrometry to follow metal uptake by P. aeruginosa. Energy-dependent metal uptake, in the presence of Pch, was efficient only for Fe(3+). Co(2+), Ga(3+), and Ni(2+) were also transported, but the uptake rates were 23- to 35-fold lower than that for Fe(3+). No uptake was seen for all the other metals. Thus, cell surface FptA has broad metal specificity at the binding stage but is much more selective for the metal uptake process. This uptake pathway does not appear to efficiently assimilate any metal other than Fe(3+).

The metal dependence of pyoverdine interactions with its outer membrane receptor FpvA

To acquire iron, Pseudomonas aeruginosa secretes the fluorescent siderophore pyoverdine (Pvd), which chelates iron and shuttles it into the cells via the specific outer membrane transporter FpvA. We studied the role of iron and other metals in the binding and transport of Pvd by FpvA and conclude that there is no significant affinity between FpvA and metal-free Pvd. We found that the fluorescent in vivo complex of iron-free FpvA-Pvd is in fact a complex with aluminum (FpvA-Pvd-Al) formed from trace aluminum in the growth medium. When Pseudomonas aeruginosa was cultured in a medium that had been treated with a metal affinity resin, the in vivo formation of the FpvA-Pvd complex and the recycling of Pvd on FpvA were nearly abolished. The accumulation of Pvd in the periplasm of Pseudomonas aeruginosa was also reduced in the treated growth medium, while the addition of 1 microM AlCl(3) to the treated medium restored the effects of trace metals observed in standard growth medium. Using fluorescent resonance energy transfer and surface plasmon resonance techniques, the in vitro interactions between Pvd and detergent-solubilized FpvA were also shown to be metal dependent. We demonstrated that FpvA binds Pvd-Fe but not Pvd and that Pvd did not compete with Pvd-Fe for FpvA binding. In light of our finding that the Pvd-Al complex is transported across the outer membrane of Pseudomonas aeruginosa, a model for siderophore recognition based on a metal-induced conformation followed by redox selectivity for iron is discussed.

Real Time Fluorescent Resonance Energy Transfer Visualization of Ferric Pyoverdine Uptake in Pseudomonas aeruginosa

To acquire iron, Pseudomonas aeruginosa secretes a major fluorescent siderophore, pyoverdine (PvdI), that chelates iron and shuttles it into the cells via the specific outer membrane transporter, FpvAI. We took advantage of the fluorescence properties of PvdI and its metal chelates as well as the efficient FRET between donor tryptophans in FpvAI and PvdI to follow the fate of the siderophore during iron uptake. Our findings with PvdI-Ga and PvdI-Cr uptake indicate that iron reduction is required for the dissociation of PvdI-Fe, that a ligand exchange for iron occurs, and that this dissociation occurs in the periplasm. We also observed a delay between PvdI-Fe dissociation and the rebinding of PvdI to FpvAI, underlining the kinetic independence of metal release and siderophore recycling. Meanwhile, PvdI is not modified but recycled to the medium, still competent for iron chelation and transport. Finally, in vivo fluorescence microscopy revealed patches of PvdI, suggesting that uptake occurs via macromolecular assemblies on the cell surface.

Identification and characterization of an iron-regulated gene, chtA, required for the utilization of the xenosiderophores aerobactin, rhizobactin 1021 and schizokinen by Pseudomonas aeruginosa

Pseudomonas aeruginosa utilizes several xenosiderophores under conditions of iron limitation, including the citrate hydroxamate siderophore aerobactin. Analysis of the P. aeruginosa genome sequence revealed the presence of two genes, chtA (PA4675) and PA1365, encoding proteins displaying significant similarity to the aerobactin outer-membrane receptor, IutA, of Escherichia coli. The chtA and PA1365 genes were mutated by insertional inactivation and it was demonstrated that ChtA is the outer-membrane receptor for aerobactin. ChtA also mediated the utilization of rhizobactin 1021 and schizokinen, which are structurally similar to aerobactin. In contrast to the utilization of other xenosiderophores by P. aeruginosa, there was no apparent redundancy in the utilization of aerobactin, rhizobactin 1021 and schizokinen. The utilization of citrate hydroxamate siderophores by P. aeruginosa was demonstrated to be TonB1 dependent. A Fur box was identified in the region directly upstream of chtA and it was demonstrated by the in vivo Fur titration assay that this region is capable of binding Fur and accordingly that expression of chtA is iron regulated. The PA1365 mutant was unaffected in the utilization of citrate hydroxamate siderophores.

Binding of iron-free siderophore, a common feature of siderophore outer membrane transporters of Escherichia coli and Pseudomonas aeruginosa

TonB-dependent iron transporters present in the outer membranes of Gram-negative bacteria transport ferric-siderophore complexes into the periplasm. This requires proton motive force and an integral inner membrane complex, TonB-ExbB-ExbD. Recognition of iron-free siderophores by TonB-dependent outer membrane transporters (OMT) has only been described for a subfamily called OMT(N). These OMT(N)s have an additional domain at the N terminus, which interacts with an inner membrane regulatory protein to activate a cytoplasmic sigma factor. This induces transcription of iron transport genes. Here we showed that the ability to bind aposiderophores is not specific to the OMT(N) subfamily but may be a more general feature of OMTs. FhuA, the ferrichrome OMT in Escherichia coli, and FptA, the pyochelin (Pch) OMT in Pseudomonas aeruginosa, were both able to bind in vitro and in vivo the apo-forms and the ferric forms of their corresponding siderophore at a common binding site. FptA produced in P. aeruginosa cells grown in an iron-deficient medium copurifies with a ligand that, as characterized by fluorescence, is iron-free Pch. As described previously for the FpvA transporter (pyoverdine OMT in P. aeruginosa), it appears that in conditions of iron limitation all the FptA receptors at the cell surface are loaded with apoPch. This FptA-Pch complex is less stable in vitro than the previously described copurified FpvA-Pvd complex and can be loaded with iron in vitro in the presence of Pch-Fe, citrate-Fe, or ferrichrome-Fe. These findings improved our understanding of the iron uptake mechanism via siderophores in Gram-negative bacteria.

Identification of rhtX and fptX, novel genes encoding proteins that show homology and function in the utilization of the siderophores rhizobactin 1021 by Sinorhizobium meliloti and pyochelin by Pseudomonas aeruginosa, respectively

Rhizobactin 1021 is a hydroxymate siderophore produced by the soil bacterium Sinorhizobium meliloti 2011. A regulon comprising rhtA, encoding the outer membrane receptor protein for the ferrisiderophore; the biosynthesis operon rhbABCDEF; and rhrA, the Ara-C-like regulator of the receptor and biosynthesis genes has been previously described. We report the discovery of a gene, located upstream of rhbA and named rhtX (for "rhizobactin transport"), which is required, in addition to rhtA, to confer the ability to utilize rhizobactin 1021 on a strain of S. meliloti that does not naturally utilize the siderophore. Rhizobactin 1021 is structurally similar to aerobactin, which is transported in Escherichia coli via the IutA outer membrane receptor and the FhuCDB inner membrane transport system. E. coli expressing iutA and fhuCDB was found to also transport rhizobactin 1021. We demonstrated that RhtX alone could substitute for FhuCDB to transport rhizobactin 1021 in E. coli. RhtX shows similarity to a number of uncharacterized proteins which are encoded proximal to genes that are either known to be or predicted to be involved in iron acquisition. Among these is PA4218 of Pseudomonas aeruginosa, which is located close to the gene cluster that functions in pyochelin biosynthesis and outer membrane transport. PA4218 was mutated by allelic replacement, and the mutant was found to have a pyochelin utilization-defective phenotype. It is proposed that PA4218 be named fptX (for "ferripyochelin transport"). RhtX and FptX appear to be members of a novel family of permeases that function as single-subunit transporters of siderophores.

The Binding Mechanism of Pyoverdin with the Outer Membrane Receptor FpvA in Pseudomonas aeruginosa Is Dependent on Its Iron-Loaded Status

In iron-deficient conditions, Pseudomonas aeruginosa secretes a major fluorescent siderophore named pyoverdin (Pvd), which after chelating iron(III) is transported back into the cell via its outer membrane receptor FpvA. FpvA is a TonB-dependent transport protein and has the ability to bind Pvd in its apo- or iron-loaded form. The fluorescence properties of Pvd were used to determine the binding kinetics of metal-free and metal-loaded Pvd to FpvA and showed two major features. First, the kinetics of formation of the FpvA-Pvd complex, in vivo and in vitro, are markedly slower compared to those observed for FpvA-Pvd-metal. Second, apo-Pvd and Pvd-metal absorbed with biphasic kinetics to FpvA: the bimolecular step (association of the ligand with the receptor) is followed by a slower step (t(1/2) values of 5 and 34 min for Pvd-metal and Pvd, respectively) that presumably leads to a more stable complex. The most likely explanation for this second step is that the binding of the ligand to the receptor induces a conformational change on FpvA, which may be different, depending on the loading status of Pvd. Analysis of the dissociation of metal-free Pvd from FpvA revealed an energy and a TonB dependency. The dissociation of iron-free Pvd from FpvA in the absence of the TonB protein occurs with slow kinetics in the range of hours, but it can be highly activated by the protonmotive force and TonB to reach a kinetic with a t(1/2) of 1 min. Apparently, under iron-limited conditions, TonB activates the FpvA receptor, resulting in a fast release of iron-free Pvd and generating an unloaded FpvA receptor, competent for binding extracellular Pvd-Fe.

Identification and characterization of novel pyoverdine synthesis genes in Pseudomonas aeruginosa

Fluorescent pseudomonads secrete yellow-green siderophores named pyoverdines or pseudobactins. These comprise a dihydroxyquinoline derivative joined to a type-specific peptide and, usually, a carboxylic acid or amide. In Pseudomonas aeruginosa strain PAO1, six genes that encode proteins required for pyoverdine synthesis (pvd genes) have been identified previously. Expression of all of these genes requires an alternative sigma factor PvdS. The purpose of this research was to identify other genes that are required for pyoverdine synthesis in P. aeruginosa PAO1. Fourteen candidate genes were identified from the PAO1 genome sequence on the basis of their location in the genome, the functions of homologues in other bacteria, and whether their expression was likely to be PvdS-dependent. The candidate genes were mutated and the effects of the mutations on pyoverdine production were determined. Eight new pvd genes were identified. The presence of homologues of pvd genes in other strains of P. aeruginosa was determined by Southern blotting and in other fluorescent pseudomonads by interrogation of genome sequences. Five pvd genes were restricted to strains of P. aeruginosa that make the same pyoverdine as strain PAO1, suggesting that they direct synthesis of the type-specific peptide. The remaining genes were present in all strains of P. aeruginosa that were examined and homologues were present in other Pseudomonas species. These genes are likely to direct synthesis of the dihydroxyquinoline moiety and the attached carboxylic acid/amide group. It is likely that most if not all of the genes required for pyoverdine synthesis in P. aeruginosa PAO1 have now been identified and this will form the basis for a biochemical description of the pathway of pyoverdine synthesis.

The interaction between pyoverdin and its outer membrane receptor in Pseudomonas aeruginosa leads to different conformers: a time-resolved fluorescence study

In iron limitation conditions, Pseudomonas aeruginosa secretes a major fluorescent siderophore named pyoverdin (PaA). PaA has an extremely high affinity for Fe(3+) but also chelates other ions such as Al(3+) and Ga(3+) with a lower affinity. The transfer of PaA-Fe(3+) across the outer membrane of the bacteria is mediated by the receptor FpvA, a TonB-dependent outer membrane transport protein. FpvA binds the iron-free and iron-loaded forms of pyoverdin with similar affinities, but only PaA-Fe(3+) is taken up by the cell, suggesting that FpvA adopts different conformations depending on its loading status. We used time-resolved fluorescence spectroscopy to characterize the different forms of FpvA-PaA in vitro. We showed that the FpvA-PaA complex adopts two different conformations depending on how it was prepared (formed in vitro or in vivo prior to purification). The dihydroquinoline moiety of both conformers is fully protonated, or coordinated by protein charged groups, but the polarity of its environment, its solvent accessibility, and its rotational dynamics are much slower when the FpvA-PaA complex is formed in vivo than in vitro. In the presence of Ga(3+) or Al(3+) ions, the solvent accessibility and mobility of the dihydroquinoline moiety in the two FpvA-PaA complexes are intermediate between those observed for the metal-free ones. In addition, the Förster resonance energy transfer kinetics from FpvA tryptophan residues to the PaA chromophore differs from one complex to the other, revealing differences in one or more of the donor-acceptor topologies.

Recycling of pyoverdin on the FpvA receptor after ferric pyoverdin uptake and dissociation in Pseudomonas aeruginosa

Under iron-limiting conditions, Pseudomonas aeruginosa secretes a fluorescent siderophore called pyoverdin (PaA), which, after complexing iron, is transported back into the cells via its outer membrane receptor FpvA. The recent finding that all FpvA receptors on the bacterial cell surface are loaded with iron-free PaA under iron limiting conditions has raised questions about the mechanism by which P. aeruginosa transports efficiently iron. We used [(3)H]PaA' [(55)Fe]PaA-Fe, and a kinetically stable chromium-PaA complex to show that iron loading of the receptor occurs through a siderophore displacement mechanism in vivo. Moreover, the fluorescence properties of iron-free PaA revealed that, after PaA-Fe uptake and dissociation, the PaA molecule is recycled into the extracellular medium. We used fluorescence resonance energy transfer (FRET) between the PaA chromophore and the FpvA tryptophans in vivo to monitor the kinetics of PaA displacement by PaA-Fe at the cell surface, the dissociation of iron from the siderophore, and the recycling of PaA back to the receptor on the outer membrane of the bacteria in real time. The loading status of FpvA (PaA versus PaA-Fe) was shown to depend on the relative concentration of the two forms of pyoverdin in the growth medium.

Synthesis and activities of pyoverdin-quinolone adducts: a prospective approach to a specific Therapy against Pseudomonas aeruginosa

Pseudomonas aeruginosa is particularly resistant to most all the antibiotics presently available, essentially because of the very low permeability of its outer membrane. To overcome this, we synthesized four siderophore-based antibiotics formed by two quinolones - norfloxacin and benzonaphthyridone - bound to the pyoverdin of P. aeruginosa ATCC 15692 via two types of spacer arms: one stable and the other readily hydrolyzable. From the comparison of their antibacterial properties with those of the two unbound quinolones, we reached the following conclusions: (a) The adducts inhibit Escherichia coli's gyrase showing that the dissociation of the compounds is not necessary for their activity. However, the presence of the pyoverdin moiety on the molecule decreases the inhibition activity compared to the antibiotic alone. (b) They facilitate the uptake of (55)Fe using the specific pyoverdin-mediated iron-transport system of the bacterium. No uptake was observed either with P. aeruginosa ATCC 27853, which produces a structurally different pyoverdin, or with P. aeruginosa K690, which is a mutant of P. aeruginosa ATCC 15692 lacking FpvA, the outer-membrane pyoverdin receptor. (c) MIC determinations have shown that only strains P. aeruginosa ATCC 15692 and the derived outer-membrane receptor-producing but pyoverdin-deficient P. aeruginosa IA1 mutant present higher susceptibility to the pyoverdin-quinolone adducts, whereas P. aeruginosa ATCC 27853 and K690 are much more resistant. (d) Growth inhibition by these adducts confirmed these results and showed that the adducts with the hydrolyzable spacer arm have better activity than those with the stable one and that the labile spacer arm adducts present much higher activity than the quinolones alone. These results show clearly that the penetration of the antibiotic into the cells is favored when this latter is coupled with pyoverdin: Only the strains possessing the appropriate outer-membrane receptor present higher susceptibility to the adduct. In this case the antibiotic uses the pyoverdin-mediated iron-transport system. Furthermore, better efficiency is obtained when the spacer arm is labile and favors the antibiotic release inside the cell, allowing better inhibition of gyrase.

Iron-free pyoverdin binds to its outer membrane receptor FpvA in Pseudomonas aeruginosa: a new mechanism for membrane iron transport

Under iron limitation, Pseudomonas aeruginosa secretes a fluorescent siderophore called pyoverdin, which, after complexing iron, is transported back into the cell via its outer membrane receptor FpvA. Previous studies demonstrated co-purification of FpvA with iron-free PaA and reported similar binding affinities of iron-free pyoverdin and ferric-pyoverdin to purified FpvA. The fluorescence resonance energy transfer between iron-free PaA and the FpvA receptor here reveals the existence of an FpvA-pyoverdin complex in P. aeruginosa in vivo, suggesting that the pyoverdin-loaded FpvA is the normal state of the receptor in the absence of iron. Using tritiated ferric-pyoverdin, it is shown that iron-free PaA binds to the outer membrane but is not taken up into the cell, and that in vitro and, presumably, in vivo ferric-pyoverdin displaces the bound iron-free pyoverdin on FpvA-PaA to form FpvA-PaA-Fe complexes. In vivo, the kinetics of formation of this FpvA-PaA-Fe complex are more than two orders of magnitude faster than in vitro and depend on the presence of TonB. In P. aeruginosa, two tonB genes have been identified (tonB1 and tonB2). TonB1 is directly involved in ferric-pyoverdin uptake, and TonB2 seems to be able partially to replace TonB1 in its role in iron acquisition. However, no effect of TonB1 or TonB2 on the apparent affinity of free pyoverdin to FpvA was observed, and a 17-fold difference was measured between the affinities of the two forms of pyoverdin (PaA and PaA-Fe) to FpvA in the absence of TonB1 or TonB2. The mechanism of iron uptake in P. aeruginosa via the pyoverdin pathway is discussed in view of these new findings.

Impact of siderophore production on Pseudomonas aeruginosa infections in immunosuppressed mice

Pseudomonas aeruginosa produces siderophores, pyoverdin and pyochelin, for high-affinity iron uptake. To investigate their contribution to P. aeruginosa infections, we constructed allelic exchange mutants from strain PAO1 which were deficient in producing one or both of the siderophores. When inoculated into the calf muscles of immunosuppressed mice, pyochelin-deficient and pyoverdin-deficient mutants grew and killed the animals as efficiently as PAO1. In contrast, the pyochelin- and pyoverdin-deficient (double) mutant did not show lethal virulence, although it did infect the muscles. On the other hand, when inoculated intranasally, all mutants grew in the lungs and killed immunosuppressed mice. Compared with PAO1, however, the pyoverdin-deficient mutant and the double mutant grew poorly in the lungs, and the latter was significantly attenuated for virulence. Irrespective of the inoculation route, the pyoverdin-deficient and doubly deficient mutants detected in the blood were significantly less numerous than PAO1. Additionally, in vitro examination demonstrated that the growth of the double mutant was extremely reduced under a free-iron-restricted condition with apotransferrin but that the growth reduction was completely canceled by supplementation with hemoglobin as a heme source. These results suggest that both pyoverdin and pyochelin are required for efficient bacterial growth and full expression of virulence in P. aeruginosa infection, although pyoverdin may be comparatively more important for bacterial growth and dissemination. However, the siderophores were not always required for infection. It is possible that non-siderophore-mediated iron acquisition, such as via heme uptake, might also play an important role in P. aeruginosa infections.

Role of ornibactin biosynthesis in the virulence of Burkholderia cepacia: characterization of pvdA, the gene encoding L-ornithine N(5)-oxygenase

Burkholderia cepacia is a frequent cause of respiratory infections in cystic fibrosis patients. B. cepacia has been shown to produce at least four siderophores which may play a role in the virulence of this organism. To characterize genes involved in the synthesis of siderophores, Tn5-OT182 mutants were isolated in strain K56-2, which produces two siderophores, salicylic acid (SA) and ornibactins. Two mutants were characterized that did not produce zones on Chrome Azurol S agar in a commonly used assay to detect siderophore activity. These mutants were determined to produce sevenfold more SA than K56-2 yet did not produce detectable amounts of ornibactins. These mutants, designated I117 and T10, had a transposon insertion in genes with significant homology to pyoverdine biosynthesis genes of Pseudomonas aeruginosa. I117 contained an insertion in a pvdA homolog, the gene for the enzyme L-ornithine N(5)-oxygenase, which catalyzes the hydroxylation of L-ornithine. Ornibactin synthesis in this mutant was partially restored when the precursor L-N(5)-OH-Orn was added to the culture medium. T10 contained an insertion in a pvdD homolog, which is a peptide synthetase involved in pyoverdine synthesis. beta-Galactosidase activity was iron regulated in both I117 and T10, suggesting that the transposon was inserted downstream of an iron-regulated promoter. Tn5-OT182 contains a lacZ gene that is expressed when inserted downstream of an active promoter. Both I117 and T10 were deficient in uptake of iron complexed to either ornibactins or SA, suggesting that transposon insertions in ornibactin biosynthesis genes also affected other components of the iron transport mechanism. The B. cepacia pvdA homolog was approximately 47% identical and 59% similar to L-ornithine N(5)-oxygenase from P. aeruginosa. Three clones were identified from a K56-2 cosmid library that partially restored ornibactin production, SA production, and SA uptake to parental levels but did not affect the rate of (59)Fe-ornibactin uptake in I117. A chromosomal pvdA deletion mutant was constructed that had a phenotype similar to that of I117 except that it did not hyperproduce SA. The pvdA mutants were less virulent than the parent strain in chronic and acute models of respiratory infection. A functional pvdA gene appears to be required for effective colonization and persistence in B. cepacia lung infections.

Copurification of the FpvA ferric pyoverdin receptor of Pseudomonas aeruginosa with its iron-free ligand: implications for siderophore-mediated iron transport

The Pseudomonas aeruginosa FpvA receptor is a TonB-dependent outer membrane transport protein that catalyzes uptake of ferric pyoverdin across the outer membrane. Surprisingly, FpvA expressed in P. aeruginosa grown in an iron-deficient medium copurifies with a ligand X that we have characterized by UV, fluorescence, and mass spectrometry as being iron-free pyoverdin (apo-PaA). PaA was absent from FpvA purified from a PaA-deficient P. aeruginosa strain. The properties of ligand binding in vitro revealed very similar affinities of apo-PaA and ferric-PaA to FpvA. Fluorescence resonance energy transfer was used to study in vitro the formation of the FpvA-PaA-Fe complex in the presence of PaA-Fe or citrate-Fe. The circular dichroism spectrum of FpvA indicated a 57% beta-structure content typical of porins and in agreement with the 3D structures of the siderophore receptors FhuA and FepA. In the absence of the protease's inhibitors, a truncated form of FpvA lacking 87 amino acids at its N-terminus was purified. This truncated form still bound PaA, and its beta-sheet content was conserved. This N-terminal region displays significant homology to the N-terminal periplasmic extensions of FecA from Escherichia coli and PupB from Pseudomonas putida, which were previously shown to be involved in signal transduction. This suggests a similar function for FpvA. The mechanism of iron transport in P. aeruginosa via the pyoverdin pathway is discussed in the light of all these new findings.

Use of siderophores to type pseudomonads: the three Pseudomonas aeruginosa pyoverdine systems

Eighty-eight Pseudomonas aeruginosa isolates, most of them from the Collection of Bacterial Strains of the Institut Pasteur, Paris, were analysed for their pyoverdine-mediated iron incorporation system by different methods, including pyoverdine isoelectrofocusing analysis, pyoverdine-mediated growth stimulation, immunoblot detection of (ferri)pyoverdine outer-membrane receptor and pyoverdine-facilitated iron uptake. The same grouping of the strains was reached by each of these methods, resulting in the classification of the P. aeruginosa isolates, even those which were devoid of pyoverdine production, into three different siderophore types. Forty-two percent of the strains were identified with the type-strain P. aeruginosa ATCC 15,692 (group I), 42% were identical with the second type-strain P. aeruginosa ATCC 27,853 (group II) and 16% reacted identically with the clinical isolate P. aeruginosa Pa6, whose pyoverdine was recognized in this study to be identical in structure to the pyoverdine produced by a natural isolate, P. aeruginosa strain R. No new pyoverdine species was detected among these strains.

PchR, a regulator of ferripyochelin receptor gene (fptA) expression in Pseudomonas aeruginosa, functions both as an activator and as a repressor

The product of the pchR gene, an AraC-like regulatory protein, is required for production of the FptA ferric pyochelin receptor in response to iron limitation and pyochelin (D. E. Heinrichs and K. Poole, J. Bacteriol. 175:5882-5889, 1993). The influence of iron, pyochelin, PchR, and FptA on fptA and pchR gene expression was assessed with fptA-lacZ and pchR-lacZ transcriptional fusions. As was expected, the expression of fptA decreased dramatically following the inactivation of pchR by the insertion of an OmegaHg cartridge, although the effect (> 10-fold) was not as dramatic as that of pyochelin deficiency, which obviated fptA gene expression. Insertional inactivation of pchR in a pyochelin-deficient (Pch-) background restored fptA expression to levels observed in the pyochelin-producing (Pch+) PchR- strain, suggesting that PchR represses fptA expression in the absence of pyochelin. Consistent with this, the cloned gene caused a five-fold decrease in the expression of the fptA-lacZ fusion in Escherichia coli. pchR gene expression was inducible by iron limitation, a result in agreement with the previous identification of a Fur box upstream of the gene, although the magnitude of the induction was less than that observed for fptA in response to iron limitation. Expression of pchR was effectively absent in a pyochelin-deficient strain, and insertional inactivation of pchR in a Pch+ or Pch- background caused an increase in pchR gene expression. PchR, thus, negatively regulates its own expression. Two related heptameric sequences, CGAGGAA and CGTGGAT, were identified upstream of the putative -35 region of both fptA and pchR and may function as a binding site for PchR. Insertional inactivation of fptA caused a marked decrease in fptA expression in a Pch+ background and obviated the apparent repression of fptA expression in a Pch- background, reminiscent of the effect of a pchR mutation. The fptA mutant did not, however, exhibit a defect in pchR expression. Interestingly, fptA mutants were unable to grow in the presence of pyochelin, suggesting that FptA is the sole outer membrane receptor for ferric pyochelin. These data indicate that PchR functions as both an activator and a repressor in controlling the expression of fptA and pchR. The involvement of FptA in this control is unclear, although it may be important in mediating the pyochelin effect on fptA expression, possibly by modulating PchR activity.

Novel pyoverdine biosynthesis gene(s) of Pseudomonas aeruginosa PAO

Conjugational mobilization of a Pseudomonas aeruginosa PAO1 cosmid bank (in pMMB33) into a pyoverdine-deficient (pvd) mutant harbouring a mutation in the 47 min region of the chromosome yielded one clone which restored yellow-green pigmentation and fluorescence when grown on iron-deficient medium. The relevant pMMB33-derivative cosmid, pPYP17, contained a 15.1 kb insert which was subcloned into pKT240 as a 10.8 Sacl-CIal fragment conferring the same phenotype. This derivative, pPYP180, like pPYP17, also conferred an apparent wild-type phenotype on pvd mutants previously shown to map genetically in the 23 min region of the P. aeruginosa PAO chromosomes. Physical mapping indicated that the cloned DNA fragment is located at the 66-70 min region of the PAO chromosome, demonstrating that the restored apparent wild-type phenotype observed for the transconjugants was not the result of a true gene complementation. A gene interruption was obtained by replacing a 0.6 kb BgIll-BgIll region of pPYP180 necessary for the expression of the pigmentation/fluorescence phenotype, by a Hgr interposon (omega Hg). After conjugational transfer and introduction of the mutagenized fragment into the PAO1 chromosome by gene replacement, pyoverdine-deficient mutants were recovered, indicating that the fragment indeed contained at least one gene involved in pyoverdine synthesis. The yellow-green fluorescent compound produced by such cells harbouring plasmids pPYP17 or pPYP180 differed from pyoverdine in several aspects and was consequently named pseudoverdine. Although pseudoverdine was able to complex iron, it was unable to restore growth to pvd mutants in the presence of the iron chelator ethylenediamine di(o-hydroxyphenylacetic acid), or to mediate iron uptake into PAO1. Pseudoverdine lacked a peptide chain but possessed spectral properties similar to pyoverdine, suggesting that it was structurally related to the chromophore of the pyoverdine molecule. The recent structural determination of pseudoverdine as a coumarin derivative confirmed this view and sheds some light on the biosynthetic pathway of the pyoverdine chromophore.

Iron-regulated transcription of the pvdA gene in Pseudomonas aeruginosa: effect of Fur and PvdS on promoter activity

The pvdA gene, encoding the enzyme L-ornithine N5-oxygenase, catalyzes a key step of the pyoverdin biosynthetic pathway in Pseudomonas aeruginosa. Expression studies with a promoter probe vector made it possible to identify three tightly iron-regulated promoter regions in the 5.9-kb DNA fragment upstream of pvdA. The promoter governing pvdA expression was located within the 154-bp sequence upstream of the pvdA translation start site. RNA analysis showed that expression of PvdA is iron regulated at the transcriptional level. Primer extension and S1 mapping experiments revealed two 5'termini of the pvdA transcript, 68 bp (T1) and 43 bp (T2) 5' of the PvdA initiation. The pvdA transcripts were monocystronic, with T1 accounting for 90% of the pvdA mRNA. Fur box-like sequences were apparently absent in the regions 5' of pvdA transcription start sites. A sequence motif resembling the -10 hexamer of AlgU-dependent promoters and the iron starvation box of pyoverdin genes controlled by the sigmaE -like factor PvdS were identified 5' of the T1 start site. The minimum DNA region required for iron-regulated promoter activity was mapped from bp -41 to -154 relative to the ATG translation start site of pvdA. We used pvdA'::lacZ transcriptional fusions and Northern (RNA) analyses to study the involvement of Fur and PvdS in the iron-regulated expression of pvdA. Two fur mutants of P. aeruginosa were much less responsive than wild-type PAO1 to the iron-dependent regulation of pvdA expression. Transcription from the pvdA promoter did not occur in a heterologous host unless in the presence of the pvdS gene in trans and was abrogated in a pvdS mutant of P. aeruginosa. Interaction of the Fur repressor with a 150-bp fragment encompassing the pvdS promoter was demonstrated in vivo by the Fur titration assay and confirmed in vitro by gel retardation experiments with a partially purified Fur preparation. Conversely, the promoter region of pvdA did not interact with Fur. Our results support the hypothesis that the P. aeruginosa Fur repressor indirectly controls pvdA transcription through the intermediary sigma factor PvdS; in the presence of sufficient iron, Fur blocks the pvdS promoter, thus preventing PvdS expression and consequently transcription of pvdA and other pyoverdin biosynthesis genes.

A positive regulatory gene, pvdS, for expression of pyoverdin biosynthetic genes in Pseudomonas aeruginosa PAO

In response to iron limitation Pseudomonas aeruginosa PAO induces production of pyoverdin, a low-molecular-weight siderophore able to capture ferric ion with a very high affinity. The pvd genes involved in the pyoverdin biosynthesis are organized in a chromosomal region termed the pvd region, and expression of some pvd genes is regulated at the transcriptional level. Two sets of promoter regions for the pvd genes were defined that were transcriptionally derepressed under iron-limiting conditions. Analysis of transcription from such promoters in Escherichia coli led to isolation and identification of a positive regulatory gene, pvdS, for expression of the pvd genes, and pvdS was localized in the pvd region. A genomic pvdS mutant of PAO, constructed by allelic exchange mutagenesis, produced no pyoverdin and did not allow transcription from the pvd promoters. Nucleotide sequence analysis revealed that PvdS shows considerable similarity to FecI of E. coli, a positive regulator for transcription of the fec (ferric citrate transport system) operon. The promoter region of pvdS has the sequence that matches well the consensus binding site for the E. coli Fur protein, a global negative regulatory protein that represses the transcription of the iron-repressible genes. Consistent with the presence of such a consensus sequence, addition of iron repressed transcription of the pvdS gene in P. aeruginosa.

Genetic and physical mapping of genes involved in pyoverdin production in Pseudomonas aeruginosa PAO

Pseudomonas aeruginosa PAO was mutagenized with Tn1737KH, a type I transcription probe transposon containing a promoterless lacZ (beta-galactosidase) gene, and 24 insertion mutants that did not grow under iron-deficient conditions were isolated. None of the culture supernatants from any mutants contained pyoverdin, a low-molecular-weight siderophore able to sequester ferric iron at very high affinity, and the growth defects of the mutants were all phenotypically recovered by the addition of the culture supernatant from the wild-type strain. These phenotypes led to the inference that all the mutants had defects in the genes (pvd genes) for production of pyoverdin. In some pvd::Tn1737KH mutants, high levels of beta-galactosidase activities were observed, and such activities were drastically reduced by the addition of ferric ion in the culture media, indicating that the expression of at least some pvd genes is regulated at the transcriptional level. Molecular cloning and physical analysis of the chromosomal fragments with Tn1737KH insertions allowed us to allocate all the mutations within a 103-kb region, referred to as the pvd region, that was found to locate at 47 min on the genetic map of PAO. Further physical mapping and Southern analysis showed that there is a 10-kb overlap between the pvd region and the 125-kb catA region described by Zhang and Holloway (C. Zhang and B. W. Holloway, J. Gen. Microbiol. 138:1097-1107, 1992). We could hence illustrate the physical map of the P. aeruginosa chromosome with a size of 218 kb.

The sss gene product, which affects pyoverdin production in Pseudomonas aeruginosa 7NSK2, is a site-specific recombinase

Pyoverdin production by Pseudomonas aeruginosa strain 7NSK2 was induced by Zn(II) in the presence of iron. A mutant was isolated in which Zn(II) no longer induced pyoverdin production. The sss gene which was inactivated in this mutant was cloned and sequenced. Its protein sequence showed 50% identity to the XerC protein of Escherichia coli, which is a member of the lambda integrase family of site-specific recombinases. An open reading frame was found upstream of sss whose protein sequence showed strong identity to DapF, the diaminopimelate epimerase. In E. coli, xerC is part of a multicistronic unit that also contains dapF. The sss gene of P. aeruginosa could restore site-specific recombination at cer in an E. coli xerC mutant and the E. coli xerC gene could complement a genomic sss mutation in P. aeruginosa.

Cloning and nucleotide sequence of the pvdA gene encoding the pyoverdin biosynthetic enzyme L-ornithine N5-oxygenase in Pseudomonas aeruginosa

The enzyme L-ornithine N5-oxygenase catalyzes the hydroxylation of L-ornithine (L-Orn), which represents an early step in the biosynthesis of the peptidic moiety of the fluorescent siderophore pyoverdin in Pseudomonas aeruginosa. A gene bank of DNA from P. aeruginosa PAO1 (ATCC 15692) was constructed in the broad-host-range cosmid pLAFR3 and mobilized into the L-Orn N5-oxygenase-defective (pvdA) P. aeruginosa mutant PALS124. Screening for fluorescent transconjugants made it possible to identify the trans-complementing cosmid pPV4, which was able to restore pyoverdin synthesis and L-Orn N5-oxygenase activity in the pvdA mutant PALS124. The 17-kb PAO1 DNA insert of pPV4 contained at least two genetic determinants involved in pyoverdin synthesis, i.e., pvdA and pvdC4, as shown by complementation analysis of a set of mutants blocked in different steps of the pyoverdin biosynthetic pathway. Deletion analysis, subcloning, and transposon mutagenesis enabled us to locate the pvdA gene in a minimum DNA fragment of 1.7 kb flanked by two SphI restriction sites. Complementation of the pvdA mutation was under stringent iron control; both pyoverdin synthesis and L-Orn N5-oxygenase activity were undetectable in cells of the trans-complemented mutant which had been grown in the presence of 100 microM FeCl3. The entire nucleotide sequence of the pvdA gene, from which the primary structure of the encoded polypeptide was deduced, was determined. The pvdA structural gene is 1,278 bp; the cloned DNA fragment contains at the 5' end of the gene a putative ribosome-binding site but apparently lacks known promoterlike sequences. The P. aeruginosa L-Orn N5-oxygenase gene codes for a 426-amino-acid peptide with a predicted molecular mass of 47.7 kDa and an isoelectric point of 8.1. The enzyme shows approximately 50% homology with functional analogs, i.e., L-lysine N6-hydroxylase of aerobactin-producing Escherichia coli and L-Orn N5-oxygenase of ferrichrome-producing Ustilago maydis. The pvdA gene was expressed in P. aeruginosa under the control of the T7 promoter. Induction of the T7 RNA polymerase system resulted in parallel increases of the L-Orn N5-oxygenase activity and of the amount of a 47.7-kDa polypeptide. We also constructed a site-specific pvdA mutant by insertion of a tetracycline-resistance cassette in the chromosomal pvdA gene of P. aeruginosa PAO1. Similarly to strain PALS124, the pvdA mutant obtained by gene disruption also disclosed no pyoverdin synthesis, lacked L-Orn N5-oxygenase activity, was complemented by the cloned pvdA gene, and produced pyoverdin at wild-type levels when fed with the biosynthetic precursor L-N5-OH-Orn. Southern blot analysis indicated that genes homologous to pvdA could be located within a 1.7-kb DNA fragment from SphI-digested genomic DNA of different hydroxamate-producing Pseudomonas spp. Our results suggest that omega-amino acid oxygenases have been conserved over a wide evolutionary range and probably evolved from a common ancestor.

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.

Multiple antibiotic resistance in Pseudomonas aeruginosa: evidence for involvement of an efflux operon

An outer membrane protein of 50 kDa (OprK) was overproduced in a siderophore-deficient mutant of Pseudomonas aeruginosa capable of growth on iron-deficient minimal medium containing 2,2'-dipyridyl (0.5 mM). The expression of OprK in the mutant (strain K385) was associated with enhanced resistance to a number of antimicrobial agents, including ciprofloxacin, nalidixic acid, tetracycline, chloramphenicol, and streptonigrin. OprK was inducible in the parent strain by growth under severe iron limitation, as provided, for example, by the addition of dipyridyl or ZnSO4 to the growth medium. The gene encoding OprK (previously identified as ORFC) forms part of an operon composed of three genes (ORFABC) implicated in the secretion of the siderophore pyoverdine. Mutants defective in ORFA, ORFB, or ORFC exhibited enhanced susceptibility to tetracycline, chloramphenicol, ciprofloxacin, streptonigrin, and dipyridyl, consistent with a role for the ORFABC operon in multiple antibiotic resistance in P. aeruginosa. Sequence analysis of ORFC (oprK) revealed that its product is homologous to a class of outer membrane proteins involved in export. Similarly, the products of ORFA and ORFB exhibit homology to previously described bacterial export proteins located in the cytoplasmic membrane. These data suggest that ORFA-ORFB-oprK (ORFC)-dependent drug efflux contributes to multiple antibiotic resistance in P. aeruginosa. We propose, therefore, the designation mexAB (multiple efflux) for ORFAB.

Cloning and sequence analysis of a gene (pchR) encoding an AraC family activator of pyochelin and ferripyochelin receptor synthesis in Pseudomonas aeruginosa

Pseudomonas aeruginosa K372 is deficient in the production of both the 75-kDa ferripyochelin receptor protein and pyochelin. A 1.8-kb EcoRI-SalI fragment which restored production of both the receptor protein and pyochelin was cloned. Nucleotide sequencing of the fragment revealed an open reading frame of 888 bp, designated pchR (pyochelin), capable of encoding a 296-amino-acid protein of a 32,339-Da molecular mass. By using a phage T7-based expression system, a protein of ca. 32 kDa was produced off the 1.8-kb fragment, confirming that this open reading frame was indeed expressed. A region exhibiting homology to the consensus Fur-binding site of Escherichia coli was identified upstream of the pchR coding region overlapping a putative promoter. In addition, the C-terminal 80 amino acid residues of PchR showed approximately 50% homology (identity, 31%; conserved changes, 19%) to the carboxy terminus of AraC, a known transcriptional activator of gene expression in E. coli, Salmonella typhimurium, Citrobacter freundii, and Erwinia chrysanthemi. Within the C-terminal region of PchR, AraC, and a number of other members of the AraC family of transcriptional activators, there exists a highly conserved 17-residue domain where, in fact, two residues are strictly maintained and two others exhibit only conserved changes, suggesting a common functional significance to this region in all of these proteins. These data are consistent with a role for PchR as a transcriptional activator of pyochelin and ferripyochelin receptor synthesis in P. aeruginosa. In agreement with this, a PchR mutant obtained by in vitro mutagenesis and gene replacement was deficient in production of the ferripyochelin receptor and pyochelin.

Construction and use of a self-cloning promoter probe vector for gram-negative bacteria

Transposon Tn5 has been used extensively for the genetic analysis of Gram- bacteria. We describe here the construction and use of a Tn5 derivative which contains the ColE1 origin of DNA replication, thereby allowing the cloning of DNA adjacent to the Tn without the need for construction of genomic libraries. The Tn is derived from Tn5-B21 [Simon et al., Gene 80 (1989) 161-169] and contains a promoter-probe lacZ gene and genes encoding resistance to tetracycline and beta-lactams. It is housed within a mobilisable suicide plasmid which can be transferred to a wide range of Gram- bacteria. The Tn was tested using pyoverdine siderophore-synthesis genes (pvd) from Pseudomonas aeruginosa. The simple cloning procedure allowed 15.9 kb of pvd-associated DNA to be cloned; in addition, the lacZ reporter gene allowed the transcription of pvd genes to be studied. The bacteria were resistant to carbenicillin only if the Tn (and hence the beta-lactamase-encoding gene) was downstream from an active promoter.

Isolation and characterization of Pseudomonas aeruginosa mutants blocked in the synthesis of pyoverdin

We have isolated and characterized by chemical and enzymatic analyses three distinct types of pyoverdin-defective (pvd) mutants of Pseudomonas aeruginosa PAO1. The pvd-1 mutant is an L-N5-hydroxyornithine (L-N5-OH-Orn) auxotroph unable to hydroxylate L-ornithine (L-Orn) in a cell-free system and requiring L-N5-OH-Orn for pyoverdin production. The other two types of mutants appear to be blocked in further steps of the biosynthetic pathway leading to pyoverdin, namely, the acylation of L-N5-OH-Orn (pvd-2) and chromophore synthesis (pvd-3). The different pvd mutations were all found to be located in the catA1 region at 47 min of the genetic map of P. aeruginosa PAO1.

Enterobactin-mediated iron transport in Pseudomonas aeruginosa

A pyoverdine-deficient strain of Pseudomonas aeruginosa was unable to grow in an iron-deficient minimal medium in the presence of the nonmetabolizable iron chelator ethylene diamine-di(omega-hydroxyphenol acetic acid) (EDDHA), although addition of enterobactin to EDDHA-containing minimal media did restore growth of the pyoverdine-deficient P. aeruginosa. Consistent with the apparent ability of enterobactin to provide iron to P. aeruginosa, enterobactin-dependent 55Fe3+ uptake was observed in cells of P. aeruginosa previously grown in an iron-deficient medium containing enterobactin (or enterobactin-containing Escherichia coli culture supernatant). This uptake was energy dependent, was observable at low concentrations (60 nM) of FeCl3, and was absent in cells cultured without enterobactin. A novel protein with a molecular weight of approximately 80,000 was identified in the outer membranes of cells grown in iron-deficient minimal medium containing enterobactin, concomitant with the induction of enterobactin-dependent iron uptake. A Tn501 insertion mutant lacking this protein was isolated and shown to be deficient in enterobactin-mediated iron transport at 60 nM FeCl3, although it still exhibited enterobactin-dependent growth in iron-deficient medium containing EDDHA. It was subsequently observed that the mutant was, however, capable of enterobactin-mediated iron transport at much higher concentrations (600 nM) of FeCl3. Indeed, enterobactin-dependent iron uptake at this concentration of iron was observed in both the mutant and parent strains irrespective of whether they had been cultured in the presence of enterobactin. Apparently, at least two uptake systems for ferrienterobactin exist in P. aeruginosa: one of higher affinity which is specifically inducible by enterobactin under iron-limiting conditions and the second, of lower affinity, which is also inducible under iron-limiting conditions but is independent of enterobactin for induction.

Precursor activation in a pyoverdine biosynthesis

The siderophore produced by Azotobacter vinelandii strain UW belongs to a large family of peptidic siderophores collectively called pyoverdines. The biosynthesis of the peptidyl moiety of this siderophore was shown to involve activation of the constituent amino acids as their adenylates, as demonstrated by amino acid-dependent ATP-[32P]pyrophosphate exchange. The enzyme system responsible for this activation was partially purified by chromatographic techniques.

Ferrioxamine transport mutants and the identification of the ferrioxamine receptor protein (FoxA) in Erwinia herbicola (Enterobacter agglomerans)

Iron deprivation of Erwinia herbicola (Enterobacter agglomerans) induces the biosynthesis of six high-Mr outer-membrane proteins and large amounts of ferrioxamine E. Mutagenesis with N-methyl-N'-nitro-N-nitrosoguanidine and selection with ferrimycin A yielded mutants of E. herbicola K4 (wild type), defective in the expression of a 76-kDa outer-membrane protein, as determined by SDS/polyacrylamide gel electrophoresis. While in bioassays wild-type cells showed growth promotion in the presence of ferrioxamines (B, D1, D2, E, G), enterobactin, citrate, ferrichrome and coprogen, these mutants failed to respond to ferrioxamines. Moreover, experiments with 55Fe-labelled siderophores confirmed that iron transport mediated by ferrioxamine E and B in the mutants was completely inhibited, whereas iron transport by other hydroxamate siderophores, such as ferrichrome and coprogen was unaffected. The results are evidence that the 76-kDa protein in the outer membrane represents the receptor protein (FoxA) for ferrioxamines in E. herbicola.

Isolation and characterization of Pseudomonas aeruginosa mutants requiring salicylic acid for pyochelin biosynthesis

Pseudomonas aeruginosa mutants requiring salicylic acid for pyochelin biosynthesis were isolated after chemical mutagenesis by plating on a siderophore detection medium. Like the wild type, these mutants incorporated 7-[14C]salicylic acid into pyochelin, demonstrating that salicylic acid is an intermediate in the biosynthesis pathway of pyochelin.

Characterization of a pyoverdine-deficient mutant of Pseudomonas fluorescens impaired in the secretion of extracellular lipase

A mutant of Pseudomonas fluorescens strain B52 deficient in the synthesis of the fluorescent pigment, pyoverdine, was isolated. Absence of pyoverdine and other siderophores was confirmed by gel filtration, a specific siderophore assay, and inhibition studies with the iron chelator EDDA. Both parent and mutant synthesized additional outer membrane proteins in response to iron-limitation. Mutant cells cultured in the absence of iron(III) accumulated 55Fe-labeled pyoverdine. The mutant produced extracellular proteinase normally on various media, but was deficient in lipase secretion. Growth of the mutant with partially-purified pyoverdine resulted in a 2.5-fold stimulation of lipase secretion. The mutant grew poorly in deferrated medium; however, the addition of iron(III) stimulated growth. Proteinase secretion in deferrated medium was stimulated over a narrow range of iron(III) concentration, while lipase secretion was only slightly affected. The data suggest that separate regulatory mechanisms exist for the control of proteinase and lipase secretion by iron(III).

Genetic organization and transcriptional analysis of a major gene cluster involved in siderophore biosynthesis in Pseudomonas putida WCS358

In iron-limited environments, the plant-growth-stimulating Pseudomonas putida WCS358 produces a yellow-green fluorescent siderophore called pseudobactin 358. The transcriptional organization and the iron-regulated expression of a major gene cluster involved in the biosynthesis and transport of pseudobactin 358 were analyzed in detail. The cluster comprises a region with a minimum length of 33.5 kilobases and contains at least five transcriptional units, of which some are relatively large. The directions of transcription of four transcriptional units were determined by RNA-RNA hybridization and by analysis in Escherichia coli minicells. The latter also demonstrated that large polypeptides were encoded by these transcriptional units. The results allowed us to localize several promoter regions on the DNA. The iron-dependent expression of at least two genes within this cluster appears to be regulated at the transcriptional level.

Characterization of Pyoverdin pss , the Fluorescent Siderophore Produced by Pseudomonas syringae pv. syringae

Pseudomonas syringae pv. syringae B301D produces a yellow-green, fluorescent siderophore, pyoverdin pss , in large quantities under iron-limited growth conditions. Maximum yields of pyoverdin pss of approximately 50 μg/ml occurred after 24 h of incubation in a deferrated synthetic medium. Increasing increments of Fe(III) coordinately repressed siderophore production until repression was complete at concentrations of ≥ 10 μM. Pyoverdin pss was isolated, chemically characterized, and found to resemble previously characterized pyoverdins in spectral traits (absorbance maxima of 365 and 410 nm for pyoverdin pss and its ferric chelate, respectively), size (1,175 molecular weight), and amino acid composition. Nevertheless, pyoverdin pss was structurally unique since amino acid analysis of reductive hydrolysates yielded β-hydroxyaspartic acid, serine, threonine, and lysine in a 2:2:2:1 ratio. Pyoverdin pss exhibited a relatively high affinity constant for Fe(III), with values of 10 25 at pH 7.0 and 10 32 at pH 10.0. Iron uptake assays with [ 55 Fe]pyoverdin pss demonstrated rapid active uptake of 55 Fe(III) by P. syringae pv. syringae B301D, while no uptake was observed for a mutant strain unable to acquire Fe(III) from ferric pyoverdin pss . The chemical and biological properties of pyoverdin pss are discussed in relation to virulence and iron uptake during plant pathogenesis.