Ferripyochelin uptake genes are involved in pyochelin-mediated signalling in Pseudomonas aeruginosa

New roles for bacterial siderophores in metal transport and tolerance

Siderophores are chelators with extremely strong affinity for ferric iron and are best known for their capacity to feed microorganisms with this metal. Despite their preference for iron, they can also chelate numerous other metals with variable affinities. There is also increasing evidence that metals other than iron can activate the production of siderophores by bacteria, thereby implicating siderophores in the homeostasis of metals other than iron and especially heavy metal tolerance. This article considers this new concept that siderophores play a role in protecting bacteria against metal toxicity and discusses the possible contribution of these chelators to the transport of biological relevant metals in addition to iron.

Yersiniabactin iron uptake: mechanisms and role in Yersinia pestis pathogenesis

Yersiniabactin (Ybt) is a siderophore-dependent iron uptake system encoded on a pathogenicity island that is widespread among pathogenic bacteria including the Yersiniae. While biosynthesis of the siderophore has been elucidated, the secretion mechanism and a few components of the uptake/utilization pathway are unidentified. ybt genes are transcriptionally repressed by Fur but activated by YbtA, likely in combination with the siderophore itself. The Ybt system is essential for the ability of Yersinia pestis to cause bubonic plague and important in pneumonic plague as well. However, the ability to cause fatal septicemic plague is independent of Ybt.

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.

Iron acquisition with the natural siderophore enantiomers pyochelin and enantio-pyochelin in Pseudomonas species

The bacterial siderophore pyochelin is composed of salicylate and two cysteine-derived heterocycles, the second of which is modified by reduction and N-methylation during biosynthesis. In Pseudomonas aeruginosa, the first cysteine residue is converted to its D-isoform during thiazoline ring formation, whereas the second cysteine remains in its L-configuration. Stereochemistry is opposite in the Pseudomonas fluorescens siderophore enantio-pyochelin, in which the first ring originates from L-cysteine and the second ring from D-cysteine. Both siderophores promote growth of the producer organism during iron limitation and induce the expression of their biosynthesis genes by activating the transcriptional AraC-type regulator PchR. However, neither siderophore is functional as an iron carrier or as a transcriptional inducer in the other species, demonstrating that both processes are highly stereospecific. Stereospecificity of pyochelin/enantio-pyochelin-mediated iron uptake is ensured at two levels: (i) by the outer membrane siderophore receptors and (ii) by the cytosolic PchR regulators.

Stereospecific recognition of pyochelin and enantio-pyochelin by the PchR proteins in fluorescent pseudomonads

The siderophore pyochelin of Pseudomonas aeruginosa promotes growth under iron limitation and induces the expression of its biosynthesis genes via the transcriptional AraC/XylS-type regulator PchR. Pseudomonas fluorescens strain CHA0 makes the optical antipode of pyochelin termed enantio-pyochelin, which also promotes growth and induces the expression of its biosynthesis genes when iron is scarce. Growth promotion and signalling by pyochelin and enantio-pyochelin are highly stereospecific and are known to involve the pyochelin and enantio-pyochelin outer-membrane receptors FptA and FetA, respectively. Here we show that stereospecificity in signalling is also based on the stereospecificity of the homologous PchR proteins of P. aeruginosa and P. fluorescens towards their respective siderophore effectors. We found that PchR functioned in the heterologous species only if supplied with its native ligand and that the FptA and FetA receptors enhanced the efficiency of signalling. By constructing and expressing hybrid and truncated PchR regulators we showed that the weakly conserved N-terminal domain of PchR is responsible for siderophore specificity. Thus, both uptake and transcriptional regulation confer stereospecificity to pyochelin and enantio-pyochelin biosynthesis.

The yersiniabactin transport system is critical for the pathogenesis of bubonic and pneumonic plague

Iron acquisition from the host is an important step in the pathogenic process. While Yersinia pestis has multiple iron transporters, the yersiniabactin (Ybt) siderophore-dependent system plays a major role in iron acquisition in vitro and in vivo. In this study, we determined that the Ybt system is required for the use of iron bound by transferrin and lactoferrin and examined the importance of the Ybt system for virulence in mouse models of bubonic and pneumonic plague. Y. pestis mutants unable to either transport Ybt or synthesize the siderophore were both essentially avirulent via subcutaneous injection (bubonic plague model). Surprisingly, via intranasal instillation (pneumonic plague model), we saw a difference in the virulence of Ybt biosynthetic and transport mutants. Ybt biosynthetic mutants displayed an approximately 24-fold-higher 50% lethal dose (LD(50)) than transport mutants. In contrast, under iron-restricted conditions in vitro, a Ybt transport mutant had a more severe growth defect than the Ybt biosynthetic mutant. Finally, a Delta pgm mutant had a greater loss of virulence than the Ybt biosynthetic mutant, indicating that the 102-kb pgm locus encodes a virulence factor, in addition to Ybt, that plays a role in the pathogenesis of pneumonic plague.

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+).

Stereospecificity of the siderophore pyochelin outer membrane transporters in fluorescent pseudomonads

Pyochelin (Pch) and enantio-pyochelin (EPch) are enantiomer siderophores that are produced by Pseudomonas aeruginosa and Pseudomonas fluorescens, respectively, under iron limitation. Pch promotes growth of P. aeruginosa when iron is scarce, and EPch carries out the same biological function in P. fluorescens. However, the two siderophores are unable to promote growth in the heterologous species, indicating that siderophore-mediated iron uptake is highly stereospecific. In the present work, using binding and iron uptake assays, we found that FptA, the Fe-Pch outer membrane transporter of P. aeruginosa, recognized (K(d) = 2.5 +/- 1.1 nm) and transported Fe-Pch but did not interact with Fe-EPch. Likewise, FetA, the Fe-EPch receptor of P. fluorescens, was specific for Fe-EPch (K(d) = 3.7 +/- 2.1 nm) but did not bind and transport Fe-Pch. Growth promotion experiments performed under iron-limiting conditions confirmed that FptA and FetA are highly specific for Pch and EPch, respectively. When fptA and fetA along with adjacent transport genes involved in siderophore uptake were swapped between the two bacterial species, P. aeruginosa became able to utilize Fe-EPch as an iron source, and P. fluorescens was able to grow with Fe-Pch. Docking experiments using the FptA structure and binding assays showed that the stereospecificity of Pch recognition by FptA was mostly due to the configuration of the siderophore chiral centers C4'' and C2'' and was only weakly dependent on the configuration of the C4' carbon atom. Together, these findings increase our understanding of the stereospecific interaction between Pch and its outer membrane receptor FptA.

Citrate-mediated iron uptake in Pseudomonas aeruginosa: involvement of the citrate-inducible FecA receptor and the FeoB ferrous iron transporter

In an attempt to identify components of a ferric citrate uptake system in Pseudomonas aeruginosa, a mutant library of a siderophore-deficient strain (IA614) was constructed and screened for defects in citrate-promoted growth in an Fe-restricted medium. A mutant disrupted in gene PA3901, encoding a homologue of the outer-membrane ferric citrate receptor, FecA, of Escherichia coli (FecA(E.c.)), was recovered and shown to be deficient in citrate-promoted growth and citrate-mediated Fe uptake. A mutant disrupted in gene PA4825, encoding a homologue of the MgtA/MgtB Mg2+ transporters in Salmonella enterica, was similarly deficient in citrate-promoted growth, though this was due to a citrate sensitivity of the mutant apparently resulting from citrate-promoted acquisition of Fe2+ and resultant oxidative stress. Consistent with citrate delivering Fe to cells as Fe2+, a P. aeruginosa mutant lacking the FeoB Fe2+ transporter homologue, PA4358, was compromised for citrate-promoted growth in Fe-restricted medium and showed markedly reduced citrate-mediated Fe uptake. Subsequent elimination of two Fe3+ transporter homologues, PA5216 and PA4687, in the feoB mutant failed to further compromise citrate-promoted growth or Fe uptake, though the additional loss of pcoA, encoding a periplasmic ferroxidase implicated in Fe2+ acquisition, completely abrogated citrate-mediated Fe uptake. Fe acquisition mediated by other siderophores (e.g. pyoverdine) was, however, unaffected in the quadruple knockout strain. These data indicate that Fe delivered to P. aeruginosa by citrate is released as Fe2+, probably in the periplasm, prior to its transport into cells via Fe transport components.

A Burkholderia cenocepacia orphan LuxR homolog is involved in quorum-sensing regulation

Burkholderia cenocepacia utilizes quorum sensing to control gene expression, including the expression of genes involved in virulence. In addition to CepR and CciR, a third LuxR homolog, CepR2, was found to regulate gene expression and virulence factor production. All B. cenocepacia strains examined contained this orphan LuxR homolog, which was not associated with an adjacent N-acyl-homoserine lactone synthase gene. Expression of cepR2 was negatively autoregulated and was negatively regulated by CciR in strain K56-2. Microarray analysis and quantitative reverse transcription-PCR determined that CepR2 did not influence expression of cepIR or cciIR. However, in strain K56-2, CepR2 negatively regulated expression of several known quorum-sensing-controlled genes, including genes encoding zinc metalloproteases. CepR2 exerted positive and negative regulation on genes on three chromosomes, including strong negative regulation of a gene cluster located adjacent to cepR2. In strain H111, which lacks the CciIR quorum-sensing system, CepR2 positively regulated pyochelin production by controlling transcription of one of the operons required for the biosynthesis of the siderophore in an N-acyl-homoserine lactone-independent manner. CepR2 activation of a luxI promoter was demonstrated in a heterologous Escherichia coli host, providing further evidence that CepR2 can function in the absence of signaling molecules. This study demonstrates that the orphan LuxR homolog CepR2 contributes to the quorum-sensing regulatory network in two distinct strains of B. cenocepacia.

A novel streptococcal integrative conjugative element involved in iron acquisition

In this study, we determined the function of a novel non-ribosomal peptide synthetase (NRPS) system carried by a streptococcal integrative conjugative element (ICE), ICESe2. The NRPS shares similarity with the yersiniabactin system found in the high-pathogenicity island of Yersinia sp. and is the first of its kind to be identified in streptococci. We named the NRPS product 'equibactin' and genes of this locus eqbA-N. ICESe2, although absolutely conserved in Streptococcus equi, the causative agent of equine strangles, was absent from all strains of the closely related opportunistic pathogen Streptococcus zooepidemicus. Binding of EqbA, a DtxR-like regulator, to the eqbB promoter was increased in the presence of cations. Deletion of eqbA resulted in a small-colony phenotype. Further deletion of the irp2 homologue eqbE, or the genes eqbH, eqbI and eqbJ encoding a putative ABC transporter, or addition of the iron chelator nitrilotriacetate, reversed this phenotype, implicating iron toxicity. Quantification of (55)Fe accumulation and sensitivity to streptonigrin suggested that equibactin is secreted by S. equi and that the eqbH, eqbI and eqbJ genes are required for its associated iron import. In agreement with a structure-based model of equibactin synthesis, supplementation of chemically defined media with salicylate was required for equibactin production.

Functional characterization of the gene PA2384 in large-scale gene regulation in response to iron starvation in Pseudomonas aeruginosa

The function unknown gene PA2384 of Pseudomonas aeruginosa PAO1 has been previously shown dramatically responsive to iron limitation. In the present study, a bioinformatics analysis showed that PA2384 has a weak similarity to the N-terminus DNA-binding domain of Fur, the well-known ferric uptake regulator. To investigate the potential function of PA2384 in iron regulation a P. aeruginosa PAO1 recombinant (pUCP20::PA2384) over-expressing PA2384 and a PA2384 disrupted mutant PAO1*PA2384 were constructed. Physiological characterization showed that the knockout mutant had a longer lag phase. Genome-scale transcriptional profiles at different growth stages were compared between the wild type and the DeltaPA2384 mutant grown under iron-limiting conditions. The expression of more than 350 genes was affected by the knockout of PA2384. Among them, 71 genes involved in iron uptake were significantly down-regulated in the absence of PA2384. One hundred two quorum sensing (QS) dependent genes displayed differential transcriptions, including genes involved in the biosynthesis of some important virulence factors such as pyocyanin, rhamnolipids and hydrogen cyanide. The transcription of genes responsible for the synthesis of Pseudomonas quinolone signal (PQS) was greatly enhanced by the knockout of PA2384. Furthermore, the knockout of PA2384 also resulted in an altered expression of genes involved in electron transfer, central metabolism, phosphorus starvation and translation. It implies that PA2384 might affect more physiological processes than iron acquisition in P. aeruginosa.

Pseudomonas fluorescens CHA0 produces enantio-pyochelin, the optical antipode of the Pseudomonas aeruginosa siderophore pyochelin

The siderophore pyochelin is made by a thiotemplate mechanism from salicylate and two molecules of cysteine. In Pseudomonas aeruginosa, the first cysteine residue is converted to its D-isoform during thiazoline ring formation whereas the second cysteine remains in its L-configuration, thus determining the stereochemistry of the two interconvertible pyochelin diastereoisomers as 4'R, 2''R, 4''R (pyochelin I) and 4'R, 2''S, 4''R (pyochelin II). Pseudomonas fluorescens CHA0 was found to make a different stereoisomeric mixture, which promoted growth under iron limitation in strain CHA0 and induced the expression of its biosynthetic genes, but was not recognized as a siderophore and signaling molecule by P. aeruginosa. Reciprocally, pyochelin promoted growth and induced pyochelin gene expression in P. aeruginosa, but was not functional in P. fluorescens. The structure of the CHA0 siderophore was determined by mass spectrometry, thin-layer chromatography, NMR, polarimetry, and chiral HPLC as enantio-pyochelin, the optical antipode of the P. aeruginosa siderophore pyochelin. Enantio-pyochelin was chemically synthesized and confirmed to be active in CHA0. Its potential biosynthetic pathway in CHA0 is discussed.