Ferric reductase is associated with the membranes of anaerobically grown Shewanella putrefaciens MR-1

Isolation and characterization of a soluble NADPH-dependent Fe(III) reductase from Geobacter sulfurreducens

NADPH is an intermediate in the oxidation of organic compounds coupled to Fe(III) reduction in Geobacter species, but Fe(III) reduction with NADPH as the electron donor has not been studied in these organisms. Crude extracts of Geobacter sulfurreducens catalyzed the NADPH-dependent reduction of Fe(III)-nitrilotriacetic acid (NTA). The responsible enzyme, which was recovered in the soluble protein fraction, was purified to apparent homogeneity in a four-step procedure. Its specific activity for Fe(III) reduction was 65 micromol. min(-1). mg(-1). The soluble Fe(III) reductase was specific for NADPH and did not utilize NADH as an electron donor. Although the enzyme reduced several forms of Fe(III), Fe(III)-NTA was the preferred electron acceptor. The protein possessed methyl viologen:NADP(+) oxidoreductase activity and catalyzed the reduction of NADP(+) with reduced methyl viologen as electron donor at a rate of 385 U/mg. The enzyme consisted of two subunits with molecular masses of 87 and 78 kDa and had a native molecular mass of 320 kDa, as determined by gel filtration. The purified enzyme contained 28.9 mol of Fe, 17.4 mol of acid-labile sulfur, and 0.7 mol of flavin adenine dinucleotide per mol of protein. The genes encoding the two subunits were identified in the complete sequence of the G. sulfurreducens genome from the N-terminal amino acid sequences derived from the subunits of the purified protein. The sequences of the two subunits had about 30% amino acid identity to the respective subunits of the formate dehydrogenase from Moorella thermoacetica, but the soluble Fe(III) reductase did not possess formate dehydrogenase activity. This soluble Fe(III) reductase differs significantly from previously characterized dissimilatory and assimilatory Fe(III) reductases in its molecular composition and cofactor content.

Bacterial recognition of mineral surfaces: nanoscale interactions between Shewanella and alpha-FeOOH

Force microscopy has been used to quantitatively measure the infinitesimal forces that characterize interactions between Shewanella oneidensis (a dissimilatory metal-reducing bacterium) and goethite (alpha-FeOOH), both commonly found in Earth near-surface environments. Force measurements with subnanonewton resolution were made in real time with living cells under aerobic and anaerobic solutions as a function of the distance, in nanometers, between a cell and the mineral surface. Energy values [in attojoules (10(-18) joules)] derived from these measurements show that the affinity between S. oneidensis and goethite rapidly increases by two to five times under anaerobic conditions in which electron transfer from bacterium to mineral is expected. Specific signatures in the force curves suggest that a 150-kilodalton putative iron reductase is mobilized within the outer membrane of S. oneidensis and specifically interacts with the goethite surface to facilitate the electron transfer process.

Differences in Fe(III) reduction in the hyperthermophilic archaeon, Pyrobaculum islandicum, versus mesophilic Fe(III)-reducing bacteria

The discovery that all hyperthermophiles that have been evaluated have the capacity to reduce Fe(III) has raised the question of whether mechanisms for dissimilatory Fe(III) reduction have been conserved throughout microbial evolution. Many studies have suggested that c-type cytochromes are integral components in electron transport to Fe(III) in mesophilic dissimilatory Fe(III)-reducing microorganisms. However, Pyrobaculum islandicum, the hyperthermophile in which Fe(III) reduction has been most intensively studied, did not contain c-type cytochromes. NADPH was a better electron donor for the Fe(III) reductase activity in P. islandicum than NADH. This is the opposite of what has been observed with mesophiles. Thus, if previous models for dissimilatory Fe(III) reduction by mesophilic bacteria are correct, then it is unlikely that a single strategy for electron transport to Fe(III) is present in all dissimilatory Fe(III)-reducing microorganisms.

Role for outer membrane cytochromes OmcA and OmcB of Shewanella putrefaciens MR-1 in reduction of manganese dioxide

Shewanella putrefaciens MR-1 can use a wide variety of terminal electron acceptors for anaerobic respiration, including certain insoluble manganese and iron oxides. To examine whether the outer membrane (OM) cytochromes of MR-1 play a role in Mn(IV) and Fe(III) reduction, mutants lacking the OM cytochrome OmcA or OmcB were isolated by gene replacement. Southern blotting and PCR confirmed replacement of the omcA and omcB genes, respectively, and reverse transcription-PCR analysis demonstrated loss of the respective mRNAs, whereas mRNAs for upstream and downstream genes were retained. The omcA mutant (OMCA1) resembled MR-1 in its growth on trimethylamine N-oxide (TMAO), dimethyl sulfoxide, nitrate, fumarate, thiosulfate, and tetrathionate and its reduction of nitrate, nitrite, ferric citrate, FeOOH, and anthraquinone-2,6-disulfonic acid. Similarly, the omcB mutant (OMCB1) grew on fumarate, nitrate, TMAO, and thiosulfate and reduced ferric citrate and FeOOH. However, OMCA1 and OMCB1 were 45 and 75% slower than MR-1, respectively, at reducing MnO(2). OMCA1 lacked only OmcA. While OMCB1 lacked OmcB, other OM cytochromes were also missing or markedly depressed. The total cytochrome content of the OM of OMCB1 was less than 15% of that of MR-1. Western blots demonstrated that OMCB1 still synthesized OmcA, but most of it was localized in the cytoplasmic membrane and soluble fractions rather than in the OM. OMCB1 had therefore lost the ability to properly localize multiple OM cytochromes to the OM. Together, the results suggest that the OM cytochromes of MR-1 participate in the reduction of Mn(IV) but are not required for the reduction of Fe(III) or other electron acceptors.

Iron metabolism in anoxic environments at near neutral pH

Anaerobic dissimilatory ferric iron-reducing and ferrous iron-oxidizing bacteria gain energy through reduction or oxidation of iron minerals and presumably play an important role in catalyzing iron transformations in anoxic environments. Numerous ferric iron-reducing bacteria have been isolated from a great diversity of anoxic environments, including sediments, soils, deep terrestrial subsurfaces, and hot springs. In contrast, only few ferrous iron-oxidizing bacteria are known so far. At neutral pH, iron minerals are barely soluble, and the mechanisms of electron transfer to or from iron minerals are still only poorly understood. In natural habitats, humic substances may act as electron carriers for ferric iron-reducing bacteria. Also fermenting bacteria were shown to channel electrons to ferric iron via humic acids. Whether quinones or cytochromes released from cells act as electron transfer components in ferric iron reduction is still a matter of debate. Anaerobic ferrous iron-oxidizing phototrophic bacteria, on the other hand, appear to excrete complexing agents to prevent precipitation of ferric iron oxides at their cell surfaces. The present review evaluates recent findings on the physiology of ferric iron-reducing and ferrous iron-oxidizing bacteria with respect to their relevance to microbial iron transformations in nature.

Effect of electron donor and solution chemistry on products of dissimilatory reduction of technetium by Shewanella putrefaciens

To help provide a fundamental basis for use of microbial dissimilatory reduction processes in separating or immobilizing (99)Tc in waste or groundwaters, the effects of electron donor and the presence of the bicarbonate ion on the rate and extent of pertechnetate ion [Tc(VII)O(4)(-)] enzymatic reduction by the subsurface metal-reducing bacterium Shewanella putrefaciens CN32 were determined, and the forms of aqueous and solid-phase reduction products were evaluated through a combination of high-resolution transmission electron microscopy, X-ray absorption spectroscopy, and thermodynamic calculations. When H(2) served as the electron donor, dissolved Tc(VII) was rapidly reduced to amorphous Tc(IV) hydrous oxide, which was largely associated with the cell in unbuffered 0. 85% NaCl and with extracellular particulates (0.2 to 0.001 microm) in bicarbonate buffer. Cell-associated Tc was present principally in the periplasm and outside the outer membrane. The reduction rate was much lower when lactate was the electron donor, with extracellular Tc(IV) hydrous oxide the dominant solid-phase reduction product, but in bicarbonate systems much less Tc(IV) was associated directly with the cell and solid-phase Tc(IV) carbonate may have been present. In the presence of carbonate, soluble (<0.001 microm) electronegative, Tc(IV) carbonate complexes were also formed that exceeded Tc(VII)O(4)(-) in electrophoretic mobility. Thermodynamic calculations indicate that the dominant reduced Tc species identified in the experiments would be stable over a range of E(h) and pH conditions typical of natural waters. Thus, carbonate complexes may represent an important pathway for Tc transport in anaerobic subsurface environments, where it has generally been assumed that Tc mobility is controlled by low-solubility Tc(IV) hydrous oxide and adsorptive, aqueous Tc(IV) hydrolysis products.

Role of the tetraheme cytochrome CymA in anaerobic electron transport in cells of Shewanella putrefaciens MR-1 with normal levels of menaquinone

Shewanella putrefaciens MR-1 possesses a complex electron transport system which facilitates its ability to use a diverse array of compounds as terminal electron acceptors for anaerobic respiration. A previous report described a mutant strain (CMTn-1) deficient in CymA, a tetraheme cytochrome c. However, the interpretation of the electron transport role of CymA was complicated by the fact that CMTn-1 was also markedly deficient in menaquinones. This report demonstrates that the depressed menaquinone levels were the result of the rifampin resistance phenotype of the parent of CMTn-1 and not the interruption of the cymA gene. This is the first report of rifampin resistance leading to decreased menaquinone levels, indicating that rifampin-resistant strains should be used with caution when analyzing electron transport processes. A site-directed gene replacement approach was used to isolate a cymA knockout strain (MR1-CYMA) directly from MR-1. While MR1-CYMA retained menaquinone levels comparable to those of MR-1, it lost the ability to reduce iron(III), manganese(IV), and nitrate and to grow by using fumarate as an electron acceptor. All of these functions were restored to wild-type efficacy, and the presence of the cymA transcript and CymA protein was also restored, by complementation of MR1-CYMA with the cymA gene. The requirement for CymA in anaerobic electron transport to iron(III), fumarate, nitrate, and manganese(IV) is therefore not dependent on the levels of menaquinone in these cells. This represents the first successful use of a suicide vector for directed gene replacement in MR-1.

Purification and characterization of triheme cytochrome c7 from the metal-reducing bacterium, Geobacter metallireducens

A soluble c-type cytochrome was first purified from Geobacter metallireducens to an electrophoretically homogeneous state. The purified cytochrome c showed absorption peaks at 530 and 409 nm in the oxidized form and 552, 522, and 418 nm in the reduced form. Polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate allowed us to calculate the molecular mass at 9.5 kDa. It contained 3 mol of heme c per molecule of the protein on the basis of heme c and protein concentration. The mid-point redox potential at pH 7.0 was determined to be -190 mV. Although the N-terminal amino acid sequence of the first 17 residues was similar to that of Desulfuromonas acetoxidans cytochrome c7, G. metallireducens cytochrome c did not show Fe(III)-reducing activity.

Dissimilatory reduction of Fe(III) and other electron acceptors by a Thermus isolate

A thermophilic bacterium that can use O2, NO3-, Fe(III), and S0 as terminal electron acceptors for growth was isolated from groundwater sampled at a 3.2-km depth in a South African gold mine. This organism, designated SA-01, clustered most closely with members of the genus Thermus, as determined by 16S rRNA gene (rDNA) sequence analysis. The 16S rDNA sequence of SA-01 was >98% similar to that of Thermus strain NMX2 A.1, which was previously isolated by other investigators from a thermal spring in New Mexico. Strain NMX2 A.1 was also able to reduce Fe(III) and other electron acceptors. Neither SA-01 nor NMX2 A.1 grew fermentatively, i.e., addition of an external electron acceptor was required for anaerobic growth. Thermus strain SA-01 reduced soluble Fe(III) complexed with citrate or nitrilotriacetic acid (NTA); however, it could reduce only relatively small quantities (0.5 mM) of hydrous ferric oxide except when the humic acid analog 2,6-anthraquinone disulfonate was added as an electron shuttle, in which case 10 mM Fe(III) was reduced. Fe(III)-NTA was reduced quantitatively to Fe(II); reduction of Fe(III)-NTA was coupled to the oxidation of lactate and supported growth through three consecutive transfers. Suspensions of Thermus strain SA-01 cells also reduced Mn(IV), Co(III)-EDTA, Cr(VI), and U(VI). Mn(IV)-oxide was reduced in the presence of either lactate or H2. Both strains were also able to mineralize NTA to CO2 and to couple its oxidation to Fe(III) reduction and growth. The optimum temperature for growth and Fe(III) reduction by Thermus strains SA-01 and NMX2 A.1 is approximately 65 degrees C; their optimum pH is 6.5 to 7.0. This is the first report of a Thermus sp. being able to couple the oxidation of organic compounds to the reduction of Fe, Mn, or S.

Shewanella putrefaciens mtrB encodes an outer membrane protein required for Fe(III) and Mn(IV) reduction

Iron and manganese oxides or oxyhydroxides are abundant transition metals, and in aquatic environments they serve as terminal electron acceptors for a large number of bacterial species. The molecular mechanisms of anaerobic metal reduction, however, are not understood. Shewanella putrefaciens is a facultative anaerobe that uses Fe(III) and Mn(IV) as terminal electron acceptors during anaerobic respiration. Transposon mutagenesis was used to generate mutants of S. putrefaciens, and one such mutant, SR-21, was analyzed in detail. Growth and enzyme assays indicated that the mutation in SR-21 resulted in loss of Fe(III) and Mn(IV) reduction but did not affect its ability to reduce other electron acceptors used by the wild type. This deficiency was due to Tn5 inactivation of an open reading frame (ORF) designated mtrB. mtrB encodes a protein of 679 amino acids and contains a signal sequence characteristic of secreted proteins. Analysis of membrane fractions of the mutant, SR-21, and wild-type cells indicated that MtrB is located on the outer membrane of S. putrefaciens. A 5.2-kb DNA fragment that contains mtrB was isolated and completely sequenced. A second ORF, designated mtrA, was found directly upstream of mtrB. The two ORFs appear to be arranged in an operon. mtrA encodes a putative 10-heme c-type cytochrome of 333 amino acids. The N-terminal sequence of MtrA contains a potential signal sequence for secretion across the cell membrane. The amino acid sequence of MtrA exhibited 34% identity to NrfB from Escherichia coli, which is involved in formate-dependent nitrite reduction. To our knowledge, this is the first report of genes encoding proteins involved in metal reduction.

Localization and solubilization of the Iron(III) reductase of geobacter sulfurreducens

The iron(III) reductase activity of Geobacter sulfurreducens was determined with the electron donor NADH and the artificial electron donor horse heart cytochrome c. The highest reduction rates were obtained with Fe(III) complexed by nitrilotriacetic acid as an electron acceptor. Fractionation experiments indicated that no iron(III) reductase activity was present in the cytoplasm, that approximately one-third was found in the periplasmic fraction, and that two-thirds were associated with the membrane fraction. Sucrose gradient separation of the outer and cytoplasmic membranes showed that about 80% of the iron(III) reductase was present in the outer membrane. The iron(III) reductase could be solubilized from the membrane fraction with 0.5 M KCl showing that the iron(III) reductase was weakly bound to the membranes. In addition, solubilization of the iron(III) reductase from whole cells with 0.5 M KCl, without disruption of cells, indicated that the iron(III) reductase is a peripheral protein on the outside of the outer membrane. Redox difference spectra of KCl extracts showed the presence of c-type cytochromes which could be oxidized by ferrihydrite. Only one activity band was observed in native polyacrylamide gels stained for the iron(III) reductase activity. Excision of the active band from a preparative gel followed by extraction of the proteins and sodium dodecyl sulfate-polyacrylamide gel electrophoresis revealed the presence of high-molecular-mass, cytochrome-containing proteins in this iron(III) reductase activity band. From these experimental data it can be hypothesized that the iron(III) reductase of G. sulfurreducens is a peripheral outer membrane protein that might contain a c-type cytochrome.

Isolation and sequence of omcA, a gene encoding a decaheme outer membrane cytochrome c of Shewanella putrefaciens MR-1, and detection of omcA homologs in other strains of S. putrefaciens

The sequence of the omcA gene, which encodes a decaheme cytochrome c that is localized to the outer membrane (OM) of Shewanella putrefaciens MR-1, was determined. The 2202 bp nucleotide sequence of omcA encodes for 734 amino acids with a predicted molecular protein mass of 78.6 kDa. Comparison with the amino-terminal sequence of the mature protein suggests the presence of a hydrophobic leader sequence which is cleaved during translocation of the protein to the OM. This leader sequence has a lipoprotein consensus sequence for signal peptidase II at the cleavage site. The predicted mature protein is comprised of 708 amino acids with a predicted molecular mass of 75.8 kDa, but the addition of ten covalently attached heme c groups and covalent lipid modification to the amino-terminal cysteine increases the predicted mass to 82.7 kDa. This is consistent with its apparent mass of 83 kDa in SDS-PAGE gels. The predicted amino acid sequence for the OmcA protein shows no significant homology to known proteins. A RNA of approx. 2300 bases that hybridizes to the omcA gene was detected in anaerobically grown MR-1 cells. The size of this transcript is similar to the coding region of the omcA gene, suggesting that it is not part of a multicistronic operon. Similar to MR-1, four other strains of S. putrefaciens were all found to localize a majority of their membrane-bound cytochromes to the OM when grown under anaerobic conditions, and all contained an OM cytochrome of similar size to OmcA. In two of these strains, MR-4 and MR-8, a homolog of omcA was identified by RT-PCR and Southern blotting using primers and probes specific for omcA of MR-1. Western blot analysis using a polyclonal antibody to OmcA was similarly positive in strains MR-4 and MR-8. Partial nucleotide sequence analysis of these homologs demonstrated 74-77% predicted amino acid homology with OmcA of MR-1. In contrast, strains MR-30 and MR-42 tested negative for omcA homologs by Southern and Northern blots, RT-PCR, and Western blots.

Outer membrane cytochromes of Shewanella putrefaciens MR-1: spectral analysis, and purification of the 83-kDa c-type cytochrome

The metal-reducing bacterium Shewanella putrefaciens MR-1 is known to localize a majority of its membrane-bound cytochromes to its outer membrane when grown under anaerobic conditions. In this study, pyridine hemochrome spectra confirmed that these outer membrane cytochromes are c-type, and electrophoretic data demonstrated the presence of four distinct outer membrane cytochromes, with apparent molecular masses of 150, 83, 65, and 53 kDa. Fourth-order derivative analysis of 77 K spectra of the outer membrane revealed four spectrally distinct c-type hemes, with peaks at 545.4, 548.0, 550.6, and 552.6 nm. Outer membrane cytochromes in the reduced state were rapidly re-oxidized by oxidized iron and manganese, which have previously been shown to serve as electron acceptors for anaerobic respiration in this bacterium. The 83-kDa outer membrane cytochrome was purified and a specific polyclonal antibody was generated against this protein. Western blot analysis demonstrated that the vast majority of this protein was localized to the outer membrane and an intermediate density membrane fraction of similar composition. Its levels, but not its subcellular distribution, were somewhat influenced by the electron acceptor used to support anaerobic growth, with levels higher in fumarate-grown cells relative to iron(III)- or trimethylamine N-oxide-grown cells. Its specific content in cells grown under aerobic conditions was only 14% of that of fumarate-grown cells, suggesting that a switch to anaerobic conditions significantly increases the de novo synthesis of this outer membrane cytochrome.

Cloning and sequence of cymA, a gene encoding a tetraheme cytochrome c required for reduction of iron(III), fumarate, and nitrate by Shewanella putrefaciens MR-1

The cymA gene, which encodes a tetraheme cytochrome c, was cloned from Shewanella putrefaciens MR-1. This gene complemented a mutant which had a TnphoA insertion in cymA and which was deficient in the respiratory reduction of iron(III), nitrate, fumarate, and manganese(IV). The 561-bp nucleotide sequence of cymA encodes a protein of 187 amino acids with a predicted molecular mass of 20.8 kDa. No N-terminal signal sequence was readily apparent; consistent with this, a cytochrome with a size of 21 kDa was detected in the wild type but was absent in the insertional mutant. The cymA gene is transcribed into an mRNA; the major transcript was approximately 790 bases, suggesting that it is not part of a multicistronic operon. This RNA transcript was not detected in the cymA mutant. The CymA protein was found in the cytoplasmic membrane and soluble fraction of MR-1, and it shares partial amino acid sequence homology with multiheme c-type cytochromes from other bacteria. These cytochromes are ostensibly involved in the transfer of electrons from the cytoplasmic membrane to acceptors in the periplasm. The localization of the fumarate and iron(III) reductases to the periplasm and outer membrane of MR-1, respectively, suggests the possibility of a similar electron transfer role for CymA.

Dissimilatory iron(III) reduction by Rhodobacter capsulatus

The photosynthetic proteobacterium Rhodobacter capsulatus was shown to be capable of dissimilatory Fe(III) reduction. Activity was expressed during anaerobic phototrophic and microaerobic growth with malate as the carbon source, but not during equivalent aerobic growth. A variety of Fe(III) complexes were demonstrated to act as substrates for intact cells and membrane fractions of strain N22DNAR+ using a ferrozine assay for Fe(II) formation. Rates of reduction appeared to be influenced by the reduction potentials of the Fe(III) complexes. However, Fe(III) complexed by citrate, which is readily reduced by Shewanella putrefaciens, was a poor substrate for dissimilation by R. capsulatus. The Fe(III)-reducing activity of R. capsulatus was located solely in the membrane fraction. The reduction of Fe(III) complexes by intact cells was inhibited by 2-heptyl-4-hydroxyquinoline-N-oxide (HQNO), suggesting the involvement of ubiquinol: cytochrome c oxidoreductases in the electron transport chain. Lack of sensitivity to myxothiazol plus data from mutant strains implies that the cytochrome bc 1 complex and cytochrome c 2 are not obligatory for dissimilation of Fe(III)(maltol)3. Alternative pathways of electron transfer to Fe(III) must hence operate in R. capsulatus. Using strain N22DNAR+, the reduction rate of Fe(III) complexed by nitrilotriacetic acid (NTA) was elevated compared to that of Fe(III)(maltol)3, and moreover was sensitive to myxothiazol. However, these differences were not observed in the absence of the electron donor malate. The governing factor for the reduction rate of Fe(III)(maltol)3 thus appears to be the limited Fe(III)-reducing activity, whilst the reduction rate of Fe(III) complexed by NTA is controlled by the flux of electrons through the respiratory chain. The use of mutant strains confirmed that the role of the cytochrome bc 1 complex in Fe(III) reduction becomes apparent only with the superior substrate. The energy-conserving nature of Fe(III) reduction by R. capsulatus was demonstrated by electrochromic measurements, with the endogenous carotenoid pigments being employed as indicators of membrane potential generation in intact cells. Using Fe(III)EDTA as electron acceptor, periods of membrane potential generation were directly proportional to the quantity of complex added, and were extended in the presence of HQNO. Fe(III)-dependent carotenoid bandshifts were abolished by addition of the protonophoric uncoupler carbonyl cyanide p-trifluoromethoxy-phenylhydrazone.

A biochemical study of the intermediary carbon metabolism of Shewanella putrefaciens

Cell extracts were used to determine the enzymes involved in the intermediary carbon metabolism of several strains of Shewanella putrefaciens. Enzymes of the Entner-Doudoroff pathway (6-phosphogluconate dehydratase and 2-keto-3-deoxy-6-phosphogluconate aldolase) were detected, but those of the Embden-Meyerhof-Parnas pathway were not. While several tricarboxylic acid cycle enzymes were present under both aerobic and anaerobic conditions, two key enzymes (2-oxoglutarate dehydrogenase and pyruvate dehydrogenase) were greatly diminished under anaerobic conditions. Extracts of cell grown anaerobically on formate as the sole source of carbon and energy were positive for hydroxypyruvate reductase, the key enzyme of the serine pathway in other methylotrophs, while no hexulose synthase activity was seen.

Siderophore-Mediated Iron Sequestering by Shewanella putrefaciens

The iron-sequestering abilities of 51 strains of Shewanella putrefaciens isolated from different sources (fish, water, and warm-blooded animals) were assessed. Thirty strains (60%) produced siderophores in heat-sterilized fish juice as determined by the chrome-azurol-S assay. All cultures were negative for the catechol-type siderophore, whereas 24 of the 30 siderophore-producing strains tested positive in the Csáky test, indicating the production of siderophores of the hydroxamate type. Siderophore-producing S. putrefaciens could to some degree cross-feed on the siderophores of other S. putrefaciens strains and on compounds produced by an Aeromonas salmonicida strain under iron-limited conditions. The siderophores of S. putrefaciens were not sufficiently strong to inhibit growth of other bacteria under iron-restricted conditions. However, siderophore-producing Pseudomonas bacteria were always inhibitory to S. putrefaciens under iron-limited conditions. Growth of siderophore-producing strains under iron-limited conditions induced the formation of one major new outer membrane protein of approximately 72 kDa. Two outer membrane proteins of approximately 53 and 23 kDa were not seen when iron was restricted.

Ferric iron reduction-linked growth yields of Shewanella putrefaciens MR-1

The anaerobic reduction of ferric citrate by Shewanella putrefaciens MR-1 cells was inhibited markedly by p-chloromercuriphenylsulphonate, moderately by potassium cyanide, and to a small extent by 2-heptyl-4-hydroxyquinolone-N-oxide. Iron reduction was accompanied by increases in total cellular protein, with values of 0.33-7.54 g cell protein produced per mol Fe(III) reduced. The growth yields were dependent upon the growth conditions of the inoculum and the initial concentration of Fe(III) citrate in the medium. Specifically, maximum growth yields were obtained when the inoculum was pregrown anaerobically and when the initial Fe(III) citrate concentrations were 5-10 mmol l-1. Lower growth yields were obtained with initial Fe(III) citrate concentrations of 20-30 mmol l-1, suggesting that cell growth was partially inhibited by higher concentrations of Fe(III) or Fe(II). Maximal growth yields were also observed early (6-24 h), after which continued increases in cell protein were minimal.

Isolation of anaerobic respiratory mutants of Shewannella putrefaciens and genetic analysis of mutants deficient in anaerobic growth on Fe3+

A genetic approach was used to study (dissimilatory) ferric iron (Fe3+) reduction in Shewanella putrefaciens 200. Chemical mutagenesis procedures and two rapid plate assays were developed to facilitate the screening of Fe3+ reduction-deficient mutants. Sixty-two putative Fe3+ reduction-deficient mutants were identified, and each was subsequently tested for its ability to grow anaerobically on various compounds as sole terminal electron acceptors, including Fe3+, nitrate (NO3-), nitrite (NO2-), manganese oxide (Mn4+), sulfite (SO3(2-)), thiosulfate (S2O3(2-)), trimethylamine N-oxide, and fumarate. A broad spectrum of mutants deficient in anaerobic growth on one or more electron acceptors was identified. Nine of the 62 mutants (designated Fer mutants) were deficient only in anaerobic growth on Fe3+ and retained the ability to grow on all other electron acceptors. These results suggest that S. putrefaciens expresses at least one terminal Fe3+ reductase that is distinct from other terminal reductases coupled to anaerobic growth. The nine Fer mutants were conjugally mated with an S. putrefaciens genomic library harbored in Escherichia coli S17-1. Complemented S. putrefaciens transconjugants were identified by the acquired ability to grow anaerobically on Fe3+ as the sole terminal electron acceptor. All recombinant cosmids that conferred the Fer+ phenotype appeared to carry a common internal region.