Siderophores in fluorescent pseudomonads: new tricks from an old dog

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REVIEW

Siderophores in fluorescent pseudomonads: new tricks from an old dog Dimitris Mossialos† & Grigoris D Amoutzias †Author

for correspondence University of Thessaly, Department of Biochemistry & Biotechnology, GR-41221 Larissa, Greece Tel.: +30 241 056 5283; Fax: +30 241 056 5290; [email protected]

Keywords: biodegradation, iron uptake, pseudomonas, pyridine-2, 6-bis thiocarboxylic acid (PDTC), (thio)quinolobactin, siderophores part of

Iron is an essential nutrient for almost all bacteria; however, at neutral pH its bioavailability is limited. Siderophores are iron-binding compounds of low molecular weight that enable the microorganisms that produce them to obtain the necessary iron from the environment. Fluorescent pseudomonads include those that are plant growth promoting, human and plant pathogens, as well as bacteria involved in the biodegradation of xenobiotics. Although pyoverdine is the main siderophore produced by different fluorescent pseudomonads, other siderophores produced by fluorescent pseudomonads include pyochelin, (thio)quinolobactin and pyridine-2, 6-bis thiocarboxylic acid. Research on siderophores continues to reveal new information on their regulation, biosynthesis, function and properties. In this review, we focus on recent advances in the field, particularly on newly characterized siderophores produced by fluorescent pseudomonads and their biotechnological potential.

Iron is an essential nutrient for almost all bacteria. Its main role is as a cofactor of proteins that participate in respiration (cytochromes and ferredoxines) and RNA synthesis (ribonucleotide reductases) [1]. Although iron is the fourth most abundant element in the Earth’s crust, at neutral pH it forms insoluble hydroxides, which are not easily available to bacteria [2]. The iron concentration that is required for the optimal growth of bacteria is 10-8–10-6 M, but at neutral pH and under aerobic conditions, free Fe+3 is limited to an equilibrium concentration of approximately 10-17–10-18 M [3]. Bacteria that confront a serious iron shortage must find ways to fulfil their nutritional needs. The most frequent solution is the production of low-molecular-weight, ironchelating compounds, named siderophores (‘iron carrier’, in Greek) [2]. Bacterial siderophores are classified according to their main chemical groups (which participate in iron chelation) as catecholates, hydroxamates or carboxylates, although some siderophores that do not exclusively belong to any of these classes have been described. They have high affinity for iron and their formation constants range between 1023–1025 for carboxylates, 1029–1032 for trihydroxamates, and up to 1052 for the catecholate siderophore, enterobactin [4]. Ferri-siderophore transport in Gram-negative bacteria requires specific receptors located on the outer membrane (Figure 1). These receptors are expressed under iron-limiting conditions and their molecular weight is usually 75–90 kDa [3]. Binding of the cognate ferri-siderophore to the receptor initiates active transport across the outer

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membrane. The TonB protein transduces energy from the proton-motive force to the receptor. TonB is essential for ferri-siderophore receptor function and is thought to provide a mechanical linkage between the cytoplasmic and outer membrane of Gram-negative bacteria. TonB forms a trimeric protein complex with the ExbB and ExbD proteins, located on the cytoplasmic membrane. ExbB and ExbD are important in the recycling of TonB between its active and inactive form, as well as for its structural stability [5,6]. Once in the periplasm, Fe3+ or ferrisiderophores should be transported across the cytoplasmic membrane into the cytoplasm. A periplasmic-binding protein, which delivers Fe3+ or ferri-siderophores to a specific ATP-binding cassette (ABC) transporter located on the cytoplasmic membrane, is necessary for this to occur. ABC transporters consist of one or two inner membrane proteins that act as a channel, and one or two cytoplasmic ATPases that provide energy for the transportation, by hydrolyzing ATP [5]. Siderophores, ferri-siderophore receptors, transport machinery and enzymatic systems for the release of iron from the siderophore are referred to as high-affinity iron uptake systems [3]. The bacterial genus Pseudomonas comprises several hundred species, characterized as straight or slightly curved Gram-negative rods with one or more polar flagella, and do not form spores. Pseudomonads usually grow under aerobic conditions, although anaerobic growth via nitrogen assimilation is possible. The genus is commonly divided into five groups based on rRNA homology. Group I is further subdivided into Future Microbiol. (2007) 2(4), 387–395

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Figure 1. Ferri-siderophore uptake in Gram-negative bacteria. Ferri-siderophore Siderophore receptor

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ABC: ATP-binding cassette; PBP: Periplasmic-binding protein.

fluorescent and nonfluorescent pseudomonads [7]. Pseudomonas species include those that are plant growth promoting, human and plant pathogens, as well as bacteria involved in the biodegradation of xenobiotics. The scope of this review is to highlight recent advances in the field of siderophores produced by fluorescent pseudomonads, with a focus on newly characterized siderophores and their biotechnological potential. Primary siderophores in fluorescent pseudomonads: pyoverdines

A common feature of fluorescent pseudomonads (under conditions of iron limitation) is the production of fluorescent yellow-green siderophores named pyoverdines (Figure 2) or pseudobactins in soil isolates [8]. Pyoverdines consist of three distinct structural parts: • A conserved dihydroxyquinoline chromophore, responsible for their fluorescence • A variable peptide arm, comprising six to 12 amino acids (depending on the producing strain) • A small dicarboxylic acid (or its monoamide) connected amidically to the NH2-group of the chromophore [8] Pyoverdines contain both catechol and hydroxamate groups. The chromophore contains one catechol group, and the peptide arm contains two hydroxamate groups. The chromophore and the peptide arm both participate in Fe+3 binding [8]. Moreover, some aminoacyl residues of the peptide arm are involved in the specific recognition of pyoverdines by their cognate receptor [9]. Another special feature of the peptide 388

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arm is the presence of D-amino acids, which presumably protect pyoverdines from proteolytic enzymes [10]. Pseudomonas aeruginosa is a major opportunistic human pathogen. Individuals who are highly predisposed to P. aeruginosa infections can be divided into two groups. The first group involves patients whose immune systems are weakened by a disease (AIDS or severe burns) or by deliberate immune suppression (as in cancer therapy or organ transplantation). The second group includes patients with cystic fibrosis. In the latter group, P. aeruginosa is involved in serious, and often fatal, infections of the respiratory tract [11,12]. P. aeruginosa strains produce structurally different pyoverdines (with different peptide arms) and therefore can be classified into three distinct types (siderovars) [13,14]. Historically, FpvA was the first ferri-pyoverdine receptor to be identified [15], and is responsible for type I ferri-pyoverdine uptake. The crystal structure of FpvA loaded with iron-free pyoverdine revealed the existence of a transmembrane 22-stranded β-barrel domain occluded by an N-terminal domain containing a mixed four-stranded β-sheet [16]. Recently, the crystal structure of FpvA loaded with ferri-pyoverdine was described at an even higher resolution (2.7 Å) [17]. In addition, an alternative type I (FpvB), as well as type II and III ferri-pyoverdine receptor, has recently been identified [18,19]. Intensive experimental studies on pyoverdine biosynthesis, combined with the availability of different Pseudomonas genomes, led to significant improvement in our knowledge. Most, if not all, genes required for pyoverdine biosynthesis have been identified (for a comprehensive review on pyoverdine biology including future science group

Siderophores in fluorescent pseudomonads: new tricks from an old dog – REVIEW

biosynthesis, regulation and its signaling pathway, see [20]). Four genes (pvdL, pvdI, pvdJ and pvdD) encode nonribosomal peptide synthetases (NRPS) and participate in the biosynthesis of the pyoverdine chromophore and peptide arm in Pseudomonas aeruginosa. NRPS are large multimodular enzymes that enable microorganisms to produce a broad range of secondary metabolites, such as siderophores, antibiotics, toxins and immunomodulators, according to the carrier thiotemplate mechanism [21,22]. PvdL is highly conserved and its orthologues in different Pseudomonas species have been identified. Pyoverdine biosynthesis begins with PvdL, which is predicted to incorporate L-Glu, D-Tyr and L-Dab (2,4 diaminobutyric acid) into a precursor peptide [23]. Through condensation and other modifications this peptide finally forms the dihydroxyquinolone chromophore. The combined activity of the other three NRPS results in the generation of the peptide arm [20].

The genes involved in siderophore-mediated iron uptake are expressed or repressed in response to iron availability. Ferric uptake regulator (Fur) is a dimeric protein, which acts as a transcriptional repressor of iron-regulated promoters by virtue of its Fe2+-dependent DNA binding activity and downregulates pyoverdine biosynthesis. When the concentration of the cellular ferrous iron increases, Fur binds to Fe2+ and the complex subsequently binds to a 19-bp DNA recognition element, located close to the regulated promoter (termed the iron box) [24]. Fur indirectly upregulates iron homeostasis genes (e.g., bacterioferritins and superoxide dismutase) by directly downregulating two nearly identical small regulatory RNAs (sRNAs) designated PrrF1 and PrrF2, which are expressed under iron-depleted conditions [25,26]. Once bacteria confront conditions of iron limitation, the cellular iron concentration decreases and thus, Fur-mediated repression

Figure 2. Pseudomonad siderophores.

Pyoverdine Pseudomonas aeruginosa PAO1 O

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D-Ser-Arg-D-Ser-fOHOrn-c(Lys-fOHOrn-Thr-Thr)

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PDTC: Pyridine-2, 6-bis thiocarboxylic acid.

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cannot occur, due to the incapability to bind ferrous ion as a cofactor. Furthermore, positive regulation is required. The existence of a transcriptional activator involved in pyoverdine biosynthesis was suggested, after it was shown that sequences similar to the Fur iron box were not present in the promoters of some iron-regulated genes involved in pyoverdine biosynthesis. On the other hand, a motif required for expression of these iron-regulated genes was identified, and it was thought that it could represent a binding site for a transcriptional activator. Soon after this assumption was published, a gene designated pvdS, which is required for pyoverdine biosynthesis in P. aeruginosa, was cloned and characterized. Sequencing of the pvdS gene revealed that it is a member of the extracytoplasmic function (ECF) subfamily of the RNA polymerase σ-factors. It was found that the promoter region of pvdS contained the iron box sequence, indicating that it is suppressed by Fur [27]. Adding to the complexity of pyoverdine and pyoverdine receptor regulation, a twobranch signaling pathway was described. It consists of ferripyoverdine, its own membrane receptor FpvA, the anti-σ factor FpvR (common parts of both branches) and two ECF σ-factors, PvdS and FpvI. The first branch leads to the expression of pyoverdine biosynthesis genes and some virulence factors (exotoxin A and PrpL endoprotease) through PvdS. The second branch induces the FpvA receptor through the FpvI σ-factor [20,28–30]. Recently, new signaling pathways were described in P. aeruginosa. They involve the exogenous siderophores ferrioxamine B and ferrichrome, anti-σ and σ-factors, respectively, regulating the expression of their cognate outer membrane receptors [31]. Secondary siderophores in fluorescent pseudomonads Pyochelin

Secondary siderophores demonstrate lower ironaffinity than pyoverdines (primary siderophores). In several cases, secondary siderophores are produced in smaller amounts compared with pyoverdines. Their biological role is not well understood. It is hypothesized that under certain environmental or physiological conditions, sufficient iron is provided to the cell (in the absence of pyoverdines) or functions other than iron sequestration are fulfilled. Pyochelin (Figure 2) contains a hydroxyphenyl thiazoline group linked to a thiazolidine carboxylic group, and it forms 2:1 complexes with Fe+3. Although pyochelin has a 390


low affinity for iron and the typical formation constant in ethanol is 5 × 105, it can be very active in iron transport [32]. Ferric pyochelin was found to interact with pyocyanin (phenazine metabolite of P. aeruginosa), thus producing hydroxyl free radicals. These radicals damage the pulmonary artery endothelial and airway epithelial cells in the case of P. aeruginosa infections in humans [33]. Salicylic acid and two cysteine residues are required for pyochelin biosynthesis. Two operons, pchDCBA and pchEFGHI [34–36], which are involved in pyochelin biosynthesis, form a cluster with the pyochelin receptor gene fptA [37] and the regulatory gene pchR [38] on the chromosome of P. aeruginosa. Elucidation of the crystal structure of the FptA receptor bound to ferric-pyochelin revealed structural similarities with the pyoverdine receptor FpvA, a typical TonB-dependent receptor [39]. The genes pchA and pchB are involved in the biosynthesis of salicylic acid, via a pathway leading from chorismate to isochorismate and then finally to salicylic acid with pyruvate as a side product [40,41]. PchD adenylates salicylic acid and later, two NRPS, PchE and PchF, incorporate two cysteine residues through the formation of intermediate products [35,42]. PchG is a reductase essential for pyochelin synthesis [36]. Although PchF contains a thioesterase domain necessary for the release of pyochelin from this enzyme, a second thioesterase, PchC, was described that has a ‘proofreading’ role in pyochelin biosynthesis. PchC removes incorrectly charged substrates from PchE and possibly PchF, thus regenerating peptide synthetase activity [43]. Biosynthesis of pyochelin and its cognate membrane receptor is repressed by Fur under iron-repleted conditions [35]. PchR is an AraCtype regulator, which in the presence of pyochelin, upregulates pyochelin biosynthesis genes as well as fptA [38]. PchR binds a recently identified PchR box present in the promoter region of pyochelin synthesis genes and binding relies on (ferri)pyochelin as an effector. Moreover, it was demonstrated that PchR represses its own gene in the presence of pyochelin, in contrast with some other members of the AraC-type family, whose repression is increased in the absence of the inducing molecule [44]. (Thio)quinolobactin

Although pyoverdine is the main siderophore of Pseudomonas fluorescens ATCC 17400, a pyoverdine-negative mutant has been demonstrated to grow in iron-limiting media. Furthermore, the mutant was positive for chrome azurol S future science group


assay (an assay detecting siderophores), suggesting that a second siderophore is produced by this strain [45]. This second siderophore was eventually purified and identified as 8-hydroxy-4methoxy-quinaldic acid and was named quinolobactin [46]. Ferri-quinolobactin was taken up by the pyoverdine-negative mutant (and also by the wild type, although not as efficiently), demonstrating its role as a siderophore. Moreover, quinolobactin could promote the growth of the pyoverdine-negative mutant in iron-limiting media but not in the presence of a strong iron chelator, suggesting that it is a low-affinity siderophore, compared with pyoverdine. Quinolobactin induced a 75-kDa iron-repressed protein, which was assumed to be its receptor [46]. Interestingly, the quinolobactin receptor was suppressed by the cognate pyoverdine, but not when the exogenous pyoverdine from P. aeruginosa PAO1 was supplied [46]. Quinolobactin (Figure 2) forms a complex with 2:1 stoichiometry (two ligands for one iron) through the nitrogen and oxygen quinoline atoms. The carboxyl group does not participate in iron chelation and its presence presumably favors aqueous solubilization of the ligand and its ferric complex [47]. The whole quinolobactin biosynthetic gene cluster contains two operons. The first operon consists of eight open reading frames (ORFs; designated qbsABCDEFGH) for quinolobactin synthesis, whereas the second operon contains four ORFs (qbsIJKL) and is transcribed in the opposite direction [48]. Based on the putative function of those genes, a pathway for quinolobactin biosynthesis was proposed, starting from tryptophan going through to kynurenine and xanthurenic acid. Xanthurenic acid is methylated, sulphurylated, then reduced to 8-hydroxy-4-methoxy-2quinoline thicarboxylic acid (thioquinolobactin), and finally hydrolyzed to quinolobactin [48]. Thioquinolobactin (Figure 2) has iron-binding properties, is structurally very similar to quinolobactin, and is considered to be the second major siderophore of Pseudomonas fluorescens ATCC 17400 [49]. Biochemical characterization of QbsC, QbsD and QbsE revealed an unanticipated protease activity exerted on QbsE from QbsD, which is then activated and can form thiocarboxylate with the help of QbsC [50]. Interestingly, thioquinolobactin, but not quinolobactin demonstrates in vitro antimicrobial activity against the oomycete Pythium debaryanum. Although the mechanism of anti-Pythium activity (exerted by thioquinolobactin) is not known, it is future science group


unlikely to occur through iron sequestration, because thioquinolobactin has a lower iron affinity than pyoverdine. A mutant that still produced pyoverdine, but not thioquinolobactin demonstrated much more reduced anti-Pythium activity, compared with the wild type, but some anti-Pythium activity was still retained [49]. Quinolobactin biosynthesis is probably regulated by Fur, since a putative Fur box was identified in the promoter region of the qsbABCDEFGH operon. Furthermore, quinolobactin biosynthesis is positively regulated by QsbA, which belongs to the AraC-type family of regulators. Inactivation of qsbA resulted in strongly decreased quinolobactin production, indicating that QbsA is a transcriptional activator of quinolobactin [48]. How the cognate pyoverdine downregulates the quinolobactin receptor (QbsI) is still unknown. Pyridine-2, 6-bis thiocarboxylic acid (PDTC)

Pseudomonas stutzeri KC was initially shown to dechlorinate carbon tetrachloride (CCl4) under iron-limiting conditions. This dechlorination converts CCl4 mainly to CO2 through a pathway that involves thiophosgene and phosgene, but not chloroform as an intermediate. Pyridine-2, 6-bis thiocarboxylic acid (PDTC) was identified from culture supernatant to dechlorinate CCl4 ([51] and references therein). PDTC (Figure 2) has remarkable chelating properties with Fe3+, Cu2+ and 12 other transition metals [52,53]. It was suggested that PDTC is a novel siderophore, produced by P. stutzeri. However, compelling evidence, such as receptor-mediated ferri-PDTC uptake was missing at that time [53]. Complementation of a spontaneous PDTCnegative mutant in P. stutzeri led to the identification of a 17-ORF gene cluster involved in PDTC biosynthesis [54]. The presence of a putative Fur box upstream of the gene cluster [54] and the demonstration of iron-regulated expression in a heterologous reporter system [55] further suggested that PDTC is a new siderophore. The compelling evidence that demonstrated that PDTC is indeed a secondary siderophore produced by P. stutzeri KC and Pseudomonas putida DSM 3601 was provided recently by Lewis et al. [56]. PDTC production was decreased in the presence of pyoverdine, the main siderophore produced by P. putida. In addition, an iron-regulated outer-membrane protein (66 kDa), presumably its receptor, was induced by PDTC. Ferri-PDTC uptake was 391


repressed by iron and significantly reduced in a mutant not producing the putative PDTC receptor without providing exogenous PDTC. Recently it was demonstrated that an AraC-type transcription factor regulates PDTC production [57], resembling, in that aspect, the regulation of other secondary siderophores, such as pyochelin and (thio)quinolobactin [44,48]. Interestingly, PDTC and thioquinolobactin are both thiocarboxylic acids, likely to form a new class of siderophores next to wellknown catecholates and hydroxamates [58]. This is further suggested by the high sequence similarity among genes involved in thioquinolobactin and PDTC biosynthesis, implying some common enzymatic steps between the two pathways [48]. Role of fluorescent-pseudomonad siderophores in xenobiotic biodegradation

Humans produce xenobiotics, such as CCl4 and organotins, which are responsible for many environmental problems. CCl4 is a known carcinogenic compound that pollutes soil and groundwater [59]. Organotin compounds, particularly triphenyltin (TPT), have been introduced into aquatic ecosystems in large quantities, owing to their extensive use as pesticides in agriculture and as antifouling agents in boat paints [60]. Bacteria have evolved biochemical pathways capable of degrading xenobiotics, usually through reactions catalyzed by specific enzymes. The role of secondary metabolites, such as siderophores, was overlooked initially, but recent research implicates siderophores in xenobiotic decomposition. PDTC in the presence of reducing agents is involved in CCl4 dechlorination, although the exact mechanism is still speculated upon. Although PDTC binds many transition metals, it is not clear if any of these metals are cofactors in CCl4 dechlorination [51]. Both pyoverdine and pyochelin are involved in organotin decomposition [61,62]. Surprisingly, pyochelin was produced in the presence of 100 µM FeCl3, suggesting that Fur suppression was somehow bypassed in the presence of TPT, although pyoverdine production was still suppressed [62]. Pyochelin, in the presence of Fe3+, generates hydroxyl radicals (HO•) and it was shown that ferri-pyochelin has a greater capacity to degrade organotin compounds than pyochelin alone. The addition of hydroxyl radical scavengers and catalase reduced TPT decomposition [63]. By contrast, addition of hydrogen peroxide enhanced it. Moreover, it 392


was shown that ferri-pyochelin forms ternary complexes with TPT, suggesting that HO• generation takes place in close proximity to TPT, therefore increasing the opportunity to decompose it [63]. TPT decomposition, mediated by pyoverdine, involves a different mechanism since it is significantly decreased in the presence of iron [61]. Various pyoverdine types decompose TPT, suggesting that a specific peptide structure is not required. This is further supported by the fact that the catechol group of the chromophore interacts with TPT [61]. Xenobiotic degradation by pseudomonad siderophores might be exploited biotechnologically. Large-scale siderophore production is not yet common practise but some research suggests that immobilized pseudomonad cells can be successfully used for that purpose [64]. Future perspective

Pyoverdine is the best studied siderophore produced by fluorescent pseudomonads, followed by pyochelin. The importance of pyoverdine production by plant growth promoting pseudomonads in the in vivo antagonism against plantroot pathogenic fungi is controversial. Research has suggested that pyoverdine production contributes to the biocontrol properties of fluorescent pseudomonads. However, other studies failed to unequivocally demonstrate a link between pyoverdine and antagonism against phytopathogenic fungi. Possible explanations for such contradictory data might be: • The use of different strains • The production of other secondary metabolites that interact with pyoverdine • The production of other siderophores that might contribute to antagonism • Ill-defined conditions that contribute to regulation of pyoverdine and its stability outside the laboratory Future research will clarify if and under which exact conditions pyoverdine production contributes to plant protection, thus allowing rationalization of the application of pseudomonads in agriculture. The siderophore repertoire produced by fluorescent pseudomonads is growing with new molecules such as (thio)quinolobactin and PDTC being recently identified and characterized. It is likely that more siderophores with interesting properties will be discovered in the future. Future research might focus on how pseudomonads future science group


coordinate the production of different siderophores, why there is siderophore redundancy and the biological significance of secondary siderophores. Recently, we got some hints, since it was shown that thioquinolobactin is the major contributing factor of in vitro anti-Pythium activity, demonstrated by P. fluorescens ATCC 17400 under iron-limiting conditions [49]. It is not far fetched to speculate that secondary siderophores play a broader role in bacterial physiology, beyond iron uptake. The induction of some virulence factors by pyoverdine-mediated signaling is well established in P. aeruginosa. It is tempting to suggest that (thio)quinolobactin is involved in cell signaling or communication. It is a rather simple molecule that chemically belongs to quinolones, such as the well-known Pseudomonas quinolone signaling molecule (PQS).

Xenobiotic-degrading properties of siderophores could be exploited, once the mechanisms responsible are fully studied. Further research is necessary to improve siderophore production by genetically manipulated Pseudomonas strains and pilot experiments in the field should be performed in order to evaluate their performance in environmental problems. Acknowledgements We would like to thank the three anonymous reviewers. Their comments helped us to improve the present review. Financial disclosure The authors have no relevant financial interests including employment, consultancies, honoraria, stock ownership or options, expert testimony, grants or patents received or pending, or royalties related to this manuscript.

Executive summary Iron uptake in Gram-negative bacteria • Siderophores are low-molecular-weight, iron-binding metabolites that enable the microorganisms that produce them to acquire iron from the environment. • Ferri-siderophore transport requires an outer membrane receptor, a functional TonB system and an ATP-binding cassette transporter located on the inner membrane. Primary siderophores in fluorescent pseudomonads: pyoverdines • Pyoverdines consist of three distinct structural parts: a conserved dihydroxyquinoline chromophore, a variable peptide arm, comprising six to 12 amino acids and a small dicarboxylic acid connected amidically to the chromophore. • Four nonribosomal peptide synthetases are key pyoverdine biosynthetic enzymes. Auxiliary enzymes modify the pyoverdine molecule or its precursor. • Pyoverdine biosynthesis is negatively regulated by the Fur dimeric transcriptional repressor and positively regulated by the extracytoplasmic function σ-factor, PvdS. • Pyoverdine autoregulates the biosynthesis of its own receptor FpvA through a signaling pathway. Secondary siderophores in fluorescent pseudomonads: pyochelin, (thio)quinolobactin & pyridine-2, 6-bis thiocarboxylic acid • Pyochelin biosynthesis requires salicylate and two cysteine residues. • Quinolobactin biosynthesis has been proposed to require a biochemical pathway, starting from tryptophan through to kynurenine and xanthurenic acid. • Pyridine-2, 6-bis thiocarboxylic acid (PDTC) biosynthesis involves a gene cluster of 17 open reading frames. The complete biochemical pathway is not fully elucidated but there are probably enzymatic steps in common with quinolobactin biosynthesis. • Secondary siderophores are negatively regulated by Fur and positively regulated by AraC-type regulators. Role of siderophores produced by fluorescent pseudomonads in xenobiotic biodegradation • PDTC dechlorinates carbon tetrachloride in the presence of reducing agents, but the exact mechanism is still elusive. • Pyoverdine and pyochelin decompose organotins. Iron enhances pyochelin-mediated decomposition of organotins, but has the opposite effect on pyoverdine-mediated decomposition of organotins. • Pyochelin decomposes organotins through hydroxyl radical generation in the presence of iron. Future perspective • The importance of pyoverdine production in in vivo antagonism against plant-root pathogenic fungi is controversial. Further research is required to clarify if and under which exact conditions pyoverdine production contributes to plant protection. • Some outstanding questions remain: how do pseudomonads coordinate the production of different siderophores? Why is there siderophore redundancy and what is the biological significance of secondary siderophores? • The application of siderophores in bioremediation is a potential biotechnological application that requires further research.



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Affiliations • Dimitris Mossialos University of Thessaly, Department of Biochemistry & Biotechnology, GR-41221 Larissa, Greece Tel.: +30 241 056 5283; Fax: +30 241 056 5290; [email protected] • Grigoris D Amoutzias University of Lausanne, Department of Ecology & Evolution, Biophore, CH-1015, Switzerland and, Institute of Agrobiotechnology, Center For Research & Technology – Hellas, 6th Km Charilaou–Thermi Rd, PO Box 361, 570 01 Thermi, Thessaloniki, Greece [email protected]

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