Characterization of a soluble ferric reductase from Neisseria gonorrhoeae

Free flavins accelerate release of ferrous iron from iron storage proteins by both free flavin-dependent and -independent ferric reductases in Escherichia coli

Ferredoxin NADP⁺ oxidoreductase (Fpr) and oxygen-insensitive NAD(P)H nitroreductase (NfnB) are purified from Escherichia coli JM109 (E. coli JM109) as a predominant free flavin-independent ferric reductase. In the present study, we prepared natural iron storage proteins, E. coli ferritin A (FtnA) and bacterioferritin (Bfr), to show the effective ferrous iron release from these proteins by Fpr and NfnB in the presence of free flavins. Fpr and NfnB showed flavin reductase activity for flavin adenine dinucleotide (FAD), flavin mononucleotide (FMN) and riboflavin, and their ferrous iron release activities were positively associated with the catalytic efficiencies (k_cat/K_m) for individual flavins. The ferrous iron release activity of E. coli cell-free extracts was affected by flavin reductase activity of the extracts. The Butyl TOYOPEARL column chromatography of the extracts, on the basis of NAD(P)H-dependent flavin reductase activity, resulted in the separation of six active fractions containing Fpr, NfnB, NAD(P)H-quinone oxidoreductase (QOR), flavin reductase (Fre) or alkyl hydroperoxide reductase subunit F (AhpF) as major components. Like Fpr and NfnB, recombinant QOR, Fre, and AhpF showed flavin reductase activity and ferrous iron release activity in the presence of free flavins, indicating an association of flavin reductase activity with ferrous iron releasing activity. Taken together, both free flavin-dependent and free flavin-independent ferric reductases in E. coli require free flavins to mediate an electron transfer from NAD(P)H to ferric iron in the iron storage proteins for the effective ferrous iron release.

Pyrophosphate-mediated iron acquisition from transferrin in Neisseria meningitidis does not require TonB activity

The ability to acquire iron from various sources has been demonstrated to be a major determinant in the pathogenesis of Neisseria meningitidis. Outside the cells, iron is bound to transferrin in serum, or to lactoferrin in mucosal secretions. Meningococci can extract iron from iron-loaded human transferrin by the TbpA/TbpB outer membrane complex. Moreover, N. meningitidis expresses the LbpA/LbpB outer membrane complex, which can extract iron from iron-loaded human lactoferrin. Iron transport through the outer membrane requires energy provided by the ExbB-ExbD-TonB complex. After transportation through the outer membrane, iron is bound by periplasmic protein FbpA and is addressed to the FbpBC inner membrane transporter. Iron-complexing compounds like citrate and pyrophosphate have been shown to support meningococcal growth ex vivo. The use of iron pyrophosphate as an iron source by N. meningitidis was previously described, but has not been investigated. Pyrophosphate was shown to participate in iron transfer from transferrin to ferritin. In this report, we investigated the use of ferric pyrophosphate as an iron source by N. meningitidis both ex vivo and in a mouse model. We showed that pyrophosphate was able to sustain N. meningitidis growth when desferal was used as an iron chelator. Addition of a pyrophosphate analogue to bacterial suspension at millimolar concentrations supported N. meningitidis survival in the mouse model. Finally, we show that pyrophosphate enabled TonB-independent ex vivo use of iron-loaded human or bovine transferrin as an iron source by N. meningitidis. Our data suggest that, in addition to acquiring iron through sophisticated systems, N. meningitidis is able to use simple strategies to acquire iron from a wide range of sources so as to sustain bacterial survival.

Adaptative biochemical pathways and regulatory networks in Klebsiella oxytoca BAS-10 producing a biotechnologically relevant exopolysaccharide during Fe(III)-citrate fermentation

BACKGROUND

A bacterial strain previously isolated from pyrite mine drainage and named BAS-10 was tentatively identified as Klebsiella oxytoca. Unlikely other enterobacteria, BAS-10 is able to grow on Fe(III)-citrate as sole carbon and energy source, yielding acetic acid and CO2 coupled with Fe(III) reduction to Fe(II) and showing unusual physiological characteristics. In fact, under this growth condition, BAS-10 produces an exopolysaccharide (EPS) having a high rhamnose content and metal-binding properties, whose biotechnological applications were proven as very relevant.

RESULTS

Further phylogenetic analysis, based on 16S rDNA sequence, definitively confirmed that BAS-10 belongs to K. oxytoca species. In order to rationalize the biochemical peculiarities of this unusual enterobacteriun, combined 2D-Differential Gel Electrophoresis (2D-DIGE) analysis and mass spectrometry procedures were used to investigate its proteomic changes: i) under aerobic or anaerobic cultivation with Fe(III)-citrate as sole carbon source; ii) under anaerobic cultivations using Na(I)-citrate or Fe(III)-citrate as sole carbon source. Combining data from these differential studies peculiar levels of outer membrane proteins, key regulatory factors of carbon and nitrogen metabolism and enzymes involved in TCA cycle and sugar biosynthesis or required for citrate fermentation and stress response during anaerobic growth on Fe(III)-citrate were revealed. The protein differential regulation seems to ensure efficient cell growth coupled with EPS production by adapting metabolic and biochemical processes in order to face iron toxicity and to optimize energy production.

CONCLUSION

Differential proteomics provided insights on the molecular mechanisms necessary for anaeorobic utilization of Fe(III)-citrate in a biotechnologically promising enterobacteriun, also revealing genes that can be targeted for the rational design of high-yielding EPS producer strains.

Type IV secretion machinery promotes ton-independent intracellular survival of Neisseria gonorrhoeae within cervical epithelial cells

Survival of Neisseria gonorrhoeae within host epithelial cells is expected to be important in the pathogenesis of gonococcal disease. We previously demonstrated that strain FA1090 derives iron from a host cell in a process that requires the Ton complex and a putative TonB-dependent transporter, TdfF. FA1090, however, lacks the gonococcal genetic island (GGI) that is present in the majority of strains. The GGI in strain MS11 has been partially characterized, and it encodes a type IV secretion system (T4SS) involved in DNA release. In this study we investigated the role of iron acquisition and GGI-encoded gene products in gonococcal survival within cervical epithelial cells. We demonstrated that intracellular survival of MS11 was dependent on acquisition of iron from the host cell, but unlike the findings for FA1090, expression of the Ton complex was not required. Survival was not dependent on a putative TonB-like protein encoded in the GGI but instead was directly linked to T4SS structural components in a manner independent of the ability to release or internalize DNA. These data suggest that expression of selected GGI-encoded open reading frames confers an advantage during cervical cell infection. This study provides the first link between expression of the T4SS apparatus and intracellular survival of gonococci.

Microbial ferric iron reductases

Almost all organisms require iron for enzymes involved in essential cellular reactions. Aerobic microbes living at neutral or alkaline pH encounter poor iron availability due to the insolubility of ferric iron. Assimilatory ferric reductases are essential components of the iron assimilatory pathway that generate the more soluble ferrous iron, which is then incorporated into cellular proteins. Dissimilatory ferric reductases are essential terminal reductases of the iron respiratory pathway in iron-reducing bacteria. While our understanding of dissimilatory ferric reductases is still limited, it is clear that these enzymes are distinct from the assimilatory-type ferric reductases. Research over the last 10 years has revealed that most bacterial assimilatory ferric reductases are flavin reductases, which can serve several physiological roles. This article reviews the physiological function and structure of assimilatory and dissimilatory ferric reductases present in the Bacteria, Archaea and Yeast. Ferric reductases do not form a single family, but appear to be distinct enzymes suggesting that several independent strategies for iron reduction may have evolved.

Identification and characterization of a novel ferric reductase from the hyperthermophilic Archaeon Archaeoglobus fulgidus

Archaeoglobus fulgidus, a hyperthermophilic sulfate-reducing Archaeon, contains high Fe(3+)-EDTA reductase activity in its soluble protein fraction. The corresponding enzyme, which constitutes about 0.75% of the soluble protein, was purified 175-fold to homogeneity. Based on SDS-polyacrylamide gel electrophoresis, the ferric reductase consists of a single subunit with a M(r) of 18,000. The M(r) of the native enzyme was determined by size exclusion chromatography to be 40,000 suggesting that the native ferric reductase is a homodimer. The enzyme uses both NADH and NADPH as electron donors to reduce Fe(3+)-EDTA. Other Fe(3+) complexes and dichlorophenolindophenol serve as alternative electron acceptors, but uncomplexed Fe(3+) is not utilized. The purified enzyme strictly requires FMN or FAD as a catalytic intermediate for Fe(3+) reduction. Ferric reductase also reduces FMN and FAD, but not riboflavin, with NAD(P)H which classifies the enzyme as a NAD(P)H:flavin oxidoreductase. The enzyme exhibits a temperature optimum of 88 degrees C. When incubated at 85 degrees C, the enzyme activity half-life was 2 h. N-terminal sequence analysis of the purified ferric reductase resulted in the identification of the hypothetical gene, AF0830, of the A. fulgidus genomic sequence. The A. fulgidus ferric reductase shares amino acid sequence similarity with a family of NAD(P)H:FMN oxidoreductases but not with any ferric reductases suggesting that the A. fulgidus ferric reductase is a novel enzyme.

Extracellular iron reductases: identification of a new class of enzymes by siderophore-producing microorganisms

This study identifies extracellular iron reductases in culture supernatant fluids of the siderophore-producing microorganisms Escherichia coli and Pseudomonas aeruginosa. These enzymes were constitutively produced and reduced and released iron from a variety of ferric chelators. Dialyzable cofactors, necessary for the transfer of electrons in the enzymatic reduction of iron, were identified. The reductases were sensitive to treatment with proteinase K and guanidine-HCl, were not associated with siderophore activity, and were apparently released from the cell as extracellular enzymes. The acquisition of 59Fe2+ by cell suspensions of E. coli and P. aeruginosa was saturable, suggesting that the ferrous iron generated by these reductases can be bound and transported. Salmonella typhimurium mutants feoB, tonB, entB, and entBfeoB, deficient in numerous known iron uptake pathways, were found to exhibit substantial extracellular iron-reducing activities over that of the parent. A hypothesis is proposed in which the extracellular iron reductases excreted by siderophore-producing microorganisms may be responsible for the mobilization of iron during conditions of iron repletion when siderophores are repressed and may also function in concert with siderophores during periods of iron starvation.

Identification and characterization of a novel extracellular ferric reductase from Mycobacterium paratuberculosis

A novel extracellular mycobacterial enzyme was identified in the ruminant pathogen Mycobacterium paratuberculosis. The enzyme was capable of mobilizing iron from different sources such as ferric ammonium citrate, ferritin, and transferrin by reduction of the metal. The purified reductase had a calculated Mr of 17,000, was sensitive to proteinase K treatment, and had an isoelectric point of pH 9. Analysis of the amino acid composition revealed glycine, serine, asparagine (or aspartic acid), and glutamine (or glutamic acid) as the most frequently occurring residues. Enzymatic activity was highest at 37 degrees C and between pH 5 and 10. The calculated Km and Vmax for ferric ammonium citrate were 0.213 mM and 0.345 mM min(-1) mg(-1), respectively. Using a specific antireductase antibody in immunoelectron microscopy, we were able to detect the enzyme associated with intracellular mycobacteria in naturally M. paratuberculosis-infected bovine tissue. We prepose that the reductase of M. paratuberculosis represents an alternative strategy of mycobacteria to mobilize ferric iron and discuss its potential role in bacterial evasion of intracellular defense mechanisms.

Binding and surface exposure characteristics of the gonococcal transferrin receptor are dependent on both transferrin-binding proteins

Neisseria gonorrhoeae is capable of iron utilization from human transferrin in a receptor-mediated event. Transferrin-binding protein 1 (Tbp1) and Tbp2 have been implicated in transferrin receptor function, but their specific roles in transferrin binding and transferrin iron utilization have not yet been defined. We utilized specific gonococcal mutants lacking Tbp1 or Tbp2 to assess the relative transferrin-binding properties of each protein independently of the other. The apparent affinities of the wild-type transferrin receptor and of Tbp1 and Tbp2 individually were much higher than previously estimated for the gonococcal receptor and similar to the estimates for the mammalian transferrin receptor. The binding parameters of both of the mutants were distinct from those of the parent, which expressed two transferrin-binding sites. Tbp2 discriminated between ferrated transferrin and apotransferrin, while Tbp1 did not. Results of transferrin-binding affinity purification, and protease accessibility experiments were consistent with the hypothesis that Tbp1 and Tbp2 interact in the wild-type strain, although both proteins were capable of binding to transferrin independently when separated in the mutants. The presence of Tbp1 partially protected Tbp2 from trypsin proteolysis, and Tbp2 also protected Tbp1 from trypsin exposure. Addition of transferrin to wild-type but not mutant cells protected Tbp1 from trypsin but increased the trypsin susceptibility of Tbp2. These observations indicate that Tbp1 and Tbp2 function together in the wild-type strain to evoke binding conformations that are distinct from those expressed by the mutants lacking either protein.

Iron piracy: acquisition of transferrin-bound iron by bacterial pathogens

The mechanism of iron utilization from transferrin has been most extensively characterized in the pathogenic Neisseria species and Haemophilus species. Two transferrin-binding proteins, Tbp1 and Tbp2, have been identified in these pathogens and are thought to be components of the transferrin receptor. Tbp1 appears to be an integral, TonB-dependent outer membrane protein while Tbp2, a lipoprotein, may be peripherally associated with the outer membrane. The relative contribution of each of these proteins to transferrin binding and utilization is discussed and a model of iron uptake from transferrin is presented. Sequence comparisons of the genes encoding neisserial transferrin-binding proteins suggest that they are probably under positive selection for variation and may have resulted from inter-species genetic exchange.

Reduction and mobilization of iron by a NAD(P)H:flavin oxidoreductase from Escherichia coli

Iron is an essential element in all living cells. Solubilization, uptake and transport of iron by microorganisms is controlled by highly efficient and specific Fe(3+)-chelating agents named siderophores. However, mechanisms of mobilization of iron from ferrisiderophores are still enigmatic. Here, we demonstrate that Escherichia coli contains a powerful enzymatic system for the reduction of ferrisiderophores. Siderophores have a much lower affinity for ferrous iron, which then can be liberated. This system has been previously purified and characterized as a NAD(P)H:flavin oxidoreductase [Fontecave, M., Eliasson, R. and Reichard, P. (1987) J. Biol. Chem. 262, 12,325-12,331)]. It catalyzes the reduction of free flavins, FMN, FAD or riboflavin by NADH or NADPH. Reduced flavins, in turn, transfer their electrons to physiological ferric complexes: ferrisiderophores, ferric citrate and ferritins. The reaction is inhibited by molecular oxygen and greatly stimulated by Fe(2+)-acceptors such as ferrozine or the iron-free form of ribonucleotide reductase subunit R2. We suggest that the reduction and the mobilization of iron from ferrisiderophores in the cell might be regulated by the presence of physiological ferrous traps such as apoproteins.

Ferrisiderophore reductases of Pseudomonas. Purification, properties and cellular location of the Pseudomonas aeruginosa ferripyoverdine reductase

Purification of the ferripyoverdine reductase from Pseudomonas aeruginosa, strain PAO1, lead to the isolation of a soluble protein of M(r) 27,000-28,000, as determined by HPLC sieving filtration and by denaturating gel electrophoresis. In the presence of NADH as the reductant, ferripyoverdine as the iron substrate, ferrozine as an iron(II)-trapping agent and FMN, this protein displayed an iron-reductase activity which resulted in the formation of ferrozine-iron(II) complex, providing that the enzymic assay was run under strict anaerobiosis. FMN was absolutely required for the activity to occur, but the lack of a visible spectrum and the lack of fluorescence for the protein in solution suggested that ferripyoverdine reductase is not a flavin-containing protein and that covalently bound FMN is not a prerequisite for the enzymatic reaction. A search of ferripyoverdine reductase by immunological detection amongst the different cellular compartments of P. aeruginosa lead to the conclusion that the soluble enzyme, which represented more than 95% of the total cellular enzyme, is not located in the periplasm but specifically in the cytoplasm. A strongly immunoreacting material, corresponding to a protein with identical M(r) as the ferripyoverdine reductase of P. aeruginosa PAO1, was detected in all the eighteen fluorescent pseudomonad strains belonging to the P. aeruginosa, P. fluorescens, P. putida and P. chlororaphis species, as well as in P. stutzeri, a non-fluorescent species, suggesting that the enzyme acting as a ferripyoverdine reductase in P. aeruginosa PAO1 is ubiquitous among the Pseudomonas.