Anaerobic energy metabolism of the sulfur-reducing bacterium “Spirillum” 5175 during dissimilatory nitrate reduction to ammonia

Sulfurospirillum diekertiae sp. nov., a tetrachloroethene-respiring bacterium isolated from contaminated soil

Two anaerobic, tetrachloroethene- (PCE-) respiring bacterial isolates, designated strain ACSDCE T and strain ACSTCE, were characterized using a polyphasic approach. Cells were Gram-stain-negative, motile, non-spore-forming and shared a vibrioid- to spirillum-shaped morphology. Optimum growth occurred at 30 °C and 0.1–0.4 % salinity. The pH range for growth was pH 5.5–7.5, with an optimum at pH 7.2. Hydrogen, formate, pyruvate and lactate as electron donors supported respiratory reductive dechlorination of PCE to cis-1,2-dichloroethene (cDCE) in strain ACSDCE T and of PCE to trichloroethene (TCE) in strain ACSTCE. Both strains were able to grow with pyruvate under microaerobic conditions. Nitrate, elemental sulphur, and thiosulphate were alternative electron acceptors. Autotrophic growth was not observed and acetate served as carbon source for both strains. The major cellular fatty acids were C16 : 1  ω7c, C16 : 0, C14 : 0 and C18 : 1  ω7c. Both genomes feature a circular plasmid. Strains ACSDCE T and ACSTCE were previously assigned to the candidate species 'Sulfurospirillum acididehalogenans'. Here, based on key genomic features and pairwise comparisons of whole-genome sequences, including average nucleotide identity, digital DNA–DNA hybridization and average amino acid identity, strains ACSDCE T and ACSTCE, 'Ca. Sulfurospirillum diekertiae' strains SL2-1 and SL2-2, and the unclassified Sulfurospirillum sp. strain SPD-1 are grouped into one distinct species separate from previously described Sulfurospirillum species. Compared to Sulfurospirillum multivorans and Sulfurospirillum halorespirans , which dechlorinate PCE to cDCE without substantial TCE accumulation, these five strains produce TCE or cDCE as the end product. In addition, some cellular fatty acids (e.g., C16 : 0 3OH, C17 : 0 iso 3OH, C17 : 0 2OH) were detected in strains ACSDCE T and ACSTCE but not in other Sulfurospirillum species. On the basis of phylogenetic, physiological and phenotypic characteristics, 'Ca. Sulfurospirillum acididehalogenans' and 'Ca. Sulfurospirillum diekertiae' are proposed to be merged into one novel species within the genus Sulfurospirillum , for which the name Sulfurospirillum diekertiae sp. nov. is proposed. The type strain is ACSDCE T (=JCM 33349T= KCTC 15819T=CGMCC 1.5292T).

Nature's nitrite-to-ammonia expressway, with no stop at dinitrogen

Since the characterization of cytochrome c₅₅₂ as a multiheme nitrite reductase, research on this enzyme has gained major interest. Today, it is known as pentaheme cytochrome c nitrite reductase (NrfA). Part of the NH₄⁺ produced from NO₂^- is released as NH₃ leading to nitrogen loss, similar to denitrification which generates NO, N₂O, and N₂. NH₄⁺ can also be used for assimilatory purposes, thus NrfA contributes to nitrogen retention. It catalyses the six-electron reduction of NO₂^- to NH₄⁺, hosting four His/His ligated c-type hemes for electron transfer and one structurally differentiated active site heme. Catalysis occurs at the distal side of a Fe(III) heme c proximally coordinated by lysine of a unique CXXCK motif (Sulfurospirillum deleyianum, Wolinella succinogenes) or, presumably, by the canonical histidine in Campylobacter jejeuni. Replacement of Lys by His in NrfA of W. succinogenes led to a significant loss of enzyme activity. NrfA forms homodimers as shown by high resolution X-ray crystallography, and there exist at least two distinct electron transfer systems to the enzyme. In γ-proteobacteria (Escherichia coli) NrfA is linked to the menaquinol pool in the cytoplasmic membrane through a pentaheme electron carrier (NrfB), in δ- and ε-proteobacteria (S. deleyianum, W. succinogenes), the NrfA dimer interacts with a tetraheme cytochrome c (NrfH). Both form a membrane-associated respiratory complex on the extracellular side of the cytoplasmic membrane to optimize electron transfer efficiency. This minireview traces important steps in understanding the nature of pentaheme cytochrome c nitrite reductases, and discusses their structural and functional features.

Nitrous Oxide Metabolism in Nitrate-Reducing Bacteria: Physiology and Regulatory Mechanisms

Nitrous oxide (N2O) is an important greenhouse gas (GHG) with substantial global warming potential and also contributes to ozone depletion through photochemical nitric oxide (NO) production in the stratosphere. The negative effects of N2O on climate and stratospheric ozone make N2O mitigation an international challenge. More than 60% of global N2O emissions are emitted from agricultural soils mainly due to the application of synthetic nitrogen-containing fertilizers. Thus, mitigation strategies must be developed which increase (or at least do not negatively impact) on agricultural efficiency whilst decrease the levels of N2O released. This aim is particularly important in the context of the ever expanding population and subsequent increased burden on the food chain. More than two-thirds of N2O emissions from soils can be attributed to bacterial and fungal denitrification and nitrification processes. In ammonia-oxidizing bacteria, N2O is formed through the oxidation of hydroxylamine to nitrite. In denitrifiers, nitrate is reduced to N2 via nitrite, NO and N2O production. In addition to denitrification, respiratory nitrate ammonification (also termed dissimilatory nitrate reduction to ammonium) is another important nitrate-reducing mechanism in soil, responsible for the loss of nitrate and production of N2O from reduction of NO that is formed as a by-product of the reduction process. This review will synthesize our current understanding of the environmental, regulatory and biochemical control of N2O emissions by nitrate-reducing bacteria and point to new solutions for agricultural GHG mitigation.

Production and consumption of nitrous oxide in nitrate-ammonifying Wolinella succinogenes cells

Global warming is moving more and more into the public consciousness. Besides the commonly mentioned carbon dioxide and methane, nitrous oxide (N2O) is a powerful greenhouse gas in addition to its contribution to depletion of stratospheric ozone. The increasing concern about N2O emission has focused interest on underlying microbial energy-converting processes and organisms harbouring N2O reductase (NosZ), such as denitrifiers and ammonifiers of nitrate and nitrite. Here, the epsilonproteobacterial model organism Wolinella succinogenes is investigated with regard to its capacity to produce and consume N2O during growth by anaerobic nitrate ammonification. This organism synthesizes an unconventional cytochrome c nitrous oxide reductase (cNosZ), which is encoded by the first gene of an atypical nos gene cluster. However, W. succinogenes lacks a nitric oxide (NO)-producing nitrite reductase of the NirS- or NirK-type as well as an NO reductase of the Nor-type. Using a robotized incubation system, the wild-type strain and suitable mutants of W. succinogenes that either produced or lacked cNosZ were analysed as to their production of NO, N2O and N2 in both nitrate-sufficient and nitrate-limited growth medium using formate as electron donor. It was found that cells growing in nitrate-sufficient medium produced small amounts of N2O, which derived from nitrite and, most likely, from the presence of NO. Furthermore, cells employing cNosZ were able to reduce N2O to N2. This reaction, which was fully inhibited by acetylene, was also observed after adding N2O to the culture headspace. The results indicate that W. succinogenes cells are competent in N2O and N2 production despite being correctly grouped as respiratory nitrate ammonifiers. N2O production is assumed to result from NO detoxification and nitrosative stress defence, while N2O serves as a terminal electron acceptor in anaerobic respiration. The ecological implications of these findings are discussed.

Refined NrfA phylogeny improves PCR-based nrfA gene detection

Dissimilatory nitrate reduction to ammonium (DNRA) and denitrification are contrasting microbial processes in the terrestrial nitrogen (N) cycle, in that the former promotes N retention and the latter leads to N loss (i.e., the formation of gaseous products). The nitrite reductase NrfA catalyzes nitrite reduction to ammonium, the enzyme associated with respiratory nitrite ammonification and the key step in DNRA. Although well studied biochemically, the diversity and phylogeny of this enzyme had not been rigorously analyzed. A phylogenetic analysis of 272 full-length NrfA protein sequences distinguished 18 NrfA clades with robust statistical support (>90% Bayesian posterior probabilities). Three clades possessed a CXXCH motif in the first heme-binding domain, whereas all other clades had a CXXCK motif in this location. The analysis further identified a KXRH or KXQH motif between the third and fourth heme-binding motifs as a conserved and diagnostic feature of all pentaheme NrfA proteins. PCR primers targeting a portion of the heme-binding motifs that flank this diagnostic region yielded the expected 250-bp-long amplicons with template DNA from eight pure cultures and 16 new nrfA-containing isolates. nrfA amplicons obtained with template DNA from two geomorphically distinct agricultural soils could be assigned to one of the 18 NrfA clades, providing support for this expanded classification. The extended NrfA phylogeny revealed novel diagnostic features of DNRA populations and will be useful to assess nitrate/nitrite fate in natural and engineered ecosystems.

Family 6 glycosyltransferases in vertebrates and bacteria: inactivation and horizontal gene transfer may enhance mutualism between vertebrates and bacteria

Glycosyltransferases (GTs) control the synthesis and structures of glycans. Inactivation and intense allelic variation in members of the GT6 family generate species-specific and individual variations in carbohydrate structures, including histo-blood group oligosaccharides, resulting in anti-glycan antibodies that target glycan-decorated pathogens. GT6 genes are ubiquitous in vertebrates but are otherwise rare, existing in a few bacteria, one protozoan, and cyanophages, suggesting lateral gene transfer. Prokaryotic GT6 genes correspond to one exon of vertebrate genes, yet their translated protein sequences are strikingly similar. Bacterial and phage GT6 genes influence the surface chemistry of bacteria, affecting their interactions, including those with vertebrate hosts.

Subsurface cycling of nitrogen and anaerobic aromatic hydrocarbon biodegradation revealed by nucleic Acid and metabolic biomarkers

Microbial processes are crucial for ecosystem maintenance, yet documentation of these processes in complex open field sites is challenging. Here we used a multidisciplinary strategy (site geochemistry, laboratory biodegradation assays, and field extraction of molecular biomarkers) to deduce an ongoing linkage between aromatic hydrocarbon biodegradation and nitrogen cycling in a contaminated subsurface site. Three site wells were monitored over a 10-month period, which revealed fluctuating concentrations of nitrate, ammonia, sulfate, sulfide, methane, and other constituents. Biodegradation assays performed under multiple redox conditions indicated that naphthalene metabolism was favored under aerobic conditions. To explore in situ field processes, we measured metabolites of anaerobic naphthalene metabolism and expressed mRNA transcripts selected to document aerobic and anaerobic microbial transformations of ammonia, nitrate, and methylated aromatic contaminants. Gas chromatography-mass spectrometry detection of two carboxylated naphthalene metabolites and transcribed benzylsuccinate synthase, cytochrome c nitrite reductase, and ammonia monooxygenase genes indicated that anaerobic metabolism of aromatic compounds and both dissimilatory nitrate reduction to ammonia (DNRA) and nitrification occurred in situ. These data link formation (via DNRA) and destruction (via nitrification) of ammonia to in situ cycling of nitrogen in this subsurface habitat, where metabolism of aromatic pollutants has led to accumulation of reduced metabolic end products (e.g., ammonia and methane).

Complete genome sequence of Sulfurospirillum deleyianum type strain (5175)

Sulfurospirillum deleyianum Schumacher et al. 1993 is the type species of the genus Sulfurospirillum. S. deleyianum is a model organism for studying sulfur reduction and dissimilatory nitrate reduction as an energy source for growth. Also, it is a prominent model organism for studying the structural and functional characteristics of cytochrome c nitrite reductase. Here, we describe the features of this organism, together with the complete genome sequence and annotation. This is the first completed genome sequence of the genus Sulfurospirillum. The 2,306,351 bp long genome with its 2,291 protein-coding and 52 RNA genes is part of the Genomic Encyclopedia of Bacteria and Archaea project.

Enzymology and bioenergetics of respiratory nitrite ammonification

Nitrite is widely used by bacteria as an electron acceptor under anaerobic conditions. In respiratory nitrite ammonification an electrochemical proton potential across the membrane is generated by electron transport from a non-fermentable substrate like formate or H(2) to nitrite. The corresponding electron transport chain minimally comprises formate dehydrogenase or hydrogenase, a respiratory quinone and cytochrome c nitrite reductase. The catalytic subunit of the latter enzyme (NrfA) catalyzes nitrite reduction to ammonia without liberating intermediate products. This review focuses on recent progress that has been made in understanding the enzymology and bioenergetics of respiratory nitrite ammonification. High-resolution structures of NrfA proteins from different bacteria have been determined, and many nrf operons sequenced, leading to the prediction of electron transfer pathways from the quinone pool to NrfA. Furthermore, the coupled electron transport chain from formate to nitrite of Wolinella succinogenes has been reconstituted by incorporating the purified enzymes into liposomes. The NrfH protein of W. succinogenes, a tetraheme c-type cytochrome of the NapC/NirT family, forms a stable complex with NrfA in the membrane and serves in passing electrons from menaquinol to NrfA. Proteins similar to NrfH are predicted by open reading frames of several bacterial nrf gene clusters. In gamma-proteobacteria, however, NrfH is thought to be replaced by the nrfBCD gene products. The active site heme c group of NrfA proteins from different bacteria is covalently bound via the cysteine residues of a unique CXXCK motif. The lysine residue of this motif serves as an axial ligand to the heme iron thus replacing the conventional histidine residue. The attachment of the lysine-ligated heme group requires specialized proteins in W. succinogenes and Escherichia coli that are encoded by accessory nrf genes. The proteins predicted by these genes are unrelated in the two bacteria but similar to proteins of the respective conventional cytochrome c biogenesis systems.

The nrfI gene is essential for the attachment of the active site haem group of Wolinella succinogenes cytochrome c nitrite reductase

The cytochrome c nitrite reductase complex (NrfHA) is the terminal enzyme in the electron transport chain catalysing nitrite respiration of Wolinella succinogenes. The catalytic subunit NrfA is a pentahaem cytochrome c containing an active site haem group which is covalently bound via the cysteine residues of a unique CWTCK motif. The lysine residue serves as the axial ligand of the haem iron. The other four haem groups of NrfA are bound by conventional haem-binding motifs (CXXCH). The nrfHAIJ locus was restored on the genome of the W. succinogenes DeltanrfAIJ deletion mutant by integration of a plasmid, thus enabling the expression of modified alleles of nrfA and nrfI. A mutant (K134H) was constructed which contained a nrfA gene encoding a CWTCH motif instead of CWTCK. NrfA of strain K134H was found to be synthesized with five bound haem groups, as judged by matrix-assisted laser-desorption/ionization (MALDI) mass spectrometry. Its nitrite reduction activity with reduced benzyl viologen was 40% of the wild-type activity. Ammonia was formed as the only product of nitrite reduction. The mutant did not grow by nitrite respiration and its electron transport activity from formate to nitrite was 5% of that of the wild-type strain. The predicted nrfI gene product is similar to proteins involved in system II cytochrome c biogenesis. A mutant of W. succinogenes (stopI) was constructed that contained a nrfHAIJ gene cluster with the nrfI codons 47 and 48 altered to stop codons. The NrfA protein of this mutant did not catalyse nitrite reduction and lacked the active site haem group, whereas the other four haem groups were present. Mutant (K134H/stopI) which contained the K134H modification in NrfA in addition to the inactivated nrfI gene had essentially the same properties as strain K134H. NrfA from strain K134H/stopI contained five haem groups. It is concluded that NrfI is involved in haem attachment to the CWTCK motif in NrfA but not to any of the CXXCH motifs. The nrfI gene is obviously dispensable for maturation of a modified NrfA protein containing a CWTCH motif instead of CWTCK. Therefore, NrfI might function as a specific haem lyase that recognizes the active site lysine residue of NrfA. A similar role has been proposed for NrfE, F and G of Escherichia coli, although these proteins share no overall sequence similarity to NrfI and belong to system I cytochrome c biogenesis, which differs fundamentally from system II.

Nitrate reduction in the periplasm of gram-negative bacteria

In contrast to the bacterial assimilatory and membrane-associated, respiratory nitrate reductases that have been studied for many years, it is only recently that periplasmic nitrate reductases have attracted growing interest. Recent research has shown that these soluble proteins are widely distributed, but vary greatly between species. All of those so far studied include four essential components: the periplasmic molybdoprotein, NapA, which is associated with a small, di-haem cytochrome, NapB; a putative quinol oxidase, NapC; and a possible pathway-specific chaperone, NapD. At least five other components have been found in different species. Other variations between species include the location of the nap genes on chromosomal or extrachromosomal DNA, and the environmental factors that regulate their expression. Despite the relatively small number of bacteria so far screened, striking correlations are beginning to emerge between the organization of the nap genes, the physiology of the host, the conditions under which the nap genes are expressed, and even the fate of nitrite, the product of Nap activity. Evidence is emerging that Nap fulfills a novel role in nitrate scavenging by some pathogenic bacteria.

Evidence for iron-dependent nitrate respiration in the dissimilatory iron-reducing bacterium Geobacter metallireducens

The dissimilatory iron-reducing bacterium Geobacter metallireducens was found to require iron at a concentration in excess of 50 microM for continuous cultivation on nitrate. Growth yield (approximately 3-fold), cytochrome c content (approximately 7-fold), and nitrate (approximately 4.5-fold) and nitrite (approximately 70-fold) reductase activities were all increased significantly when the growth medium was amended with 500 microM iron.

Bacterial cytochrome c nitrite reductase: new structural and functional aspects

Cytochrome c nitrite reductase catalyzes the six-electron reduction of nitrite to ammonia as a key step within the biological nitrogen cycle. Most recently, the crystal structure of the soluble enzyme from Sulfurospirillum deleyianum could be solved to 1.9 A resolution. This set the basis for new experiments on structural and functional aspects of the pentaheme protein which carries a Ca(2+) ion close to the active site heme. In the crystal, the protein was a homodimer with ten hemes in very close packing. The strong interaction between the nitrite reductase monomers also occurred in solution according to the dependence of the activity on the protein concentration. Addition of Ca(2+) to the enzyme as isolated had a stimulating effect on the activity. Ca(2+) could be removed from the enzyme by treatment with chelating agents such as EGTA or EDTA which led to a decrease in activity. In addition to nitrite, the enzyme converted NO, hydroxylamine and O-methyl hydroxylamine to ammonia at considerable rates. With N2O the activity was much lower; most likely dinitrogen was the product in this case. Cytochrome c nitrite reductase exhibited a remarkably high sulfite reductase activity, with hydrogen sulfide as the product. A paramagnetic Fe(II)-NO, S = 1/2 adduct was identified by rapid freeze EPR spectroscopy under turnover conditions with nitrite. This potential reaction intermediate of the reduction of nitrite to ammonia was also observed with PAPA NONOate and Spermine NONOate.

Isolation and characterisation of a strain of Pseudomonas putida that can express a periplasmic nitrate reductase

A strain of Pseudomonas putida that can express a nitrate reductase that is located in the periplasmic compartment was isolated from freshwater. The enzyme was active in vivo during arginine fermentation and at the onset of oxygen limitation in batch cultures. The activity of the enzyme increased the yield of bacteria following fermentative growth under anoxic conditions with arginine, but nitrate reduction did not support growth on non-fermentable carbon substrates under anoxic conditions. Cells expressing the periplasmic nitrate reductase were capable of reducing nitrate in the presence of oxygen. Nitrate reduction under oxic conditions was clearly coupled to a respiratory electron transport chain because: (1) the process was sensitive to the respiratory inhibitors rote-none and 2-n-heptyl-4-hydroxyquinoline N-oxide, and (2) membrane-bound and periplasmic cytochromes were involved. This is the first report of the presence of a periplasmic nitrate reductase in a member of the gamma proteobacteria.

Nitric oxide reductase from Pseudomonas stutzeri, a novel cytochrome bc complex. Phospholipid requirement, electron paramagnetic resonance and redox properties

The nitric oxide reductase (NOR) from Pseudomonas stutzeri is a cytochrome bc complex which shows on SDS/PAGE two subunits with apparent molecular masses of 17 kDa and 38 kDa. Two other species of approximately 45 kDa and 74-78 kDa represent the undissociated enzyme complex and an aggregate of the cytochrome b subunit, respectively. The cytochrome b subunit is highly hydrophobic and results in aberrant electrophoretic mobility. The stability of the enzyme in various detergents and at different pH was investigated. The highest specific activity of 60 mumol NO min-1 mg-1 protein was obtained after electrophoresis in the presence of laurylpropanediol-3-phosphorylcholine ether. Purified NOR contained cardiolipin, phosphatidylglycerol, and phosphatidylethanolamine, the latter as the major component. A phospholipid was required for high catalytic activity with either cardiolipin or phosphatidylglycerol increasing the activity of the enzyme as isolated by a factor of up to 5. Free fatty acids inhibited NOR, with cis-9-octadecenoic acid (oleic acid) showing the most pronounced effect. Certain detergents substituted for the phospholipid requirement of NOR. The enzyme, as isolated, in 0.1% Triton X-100, 20 mM Tris/HCl pH 8.5, exhibited a complex set of EPR resonances at low magnetic field, with a prominent peak at g 6.34 resulting from Fe(III) high-spin cytochrome b. The second prominent feature arose from a low-spin Fe(III) heme center with strong lines at apparent g values of 3.02 and 2.29, and a broad resonance at g approximately 1.5 which we assigned to the cytochrome c component of the enzyme. From spin quantitation and computer simulations of the various EPR signals a ratio close to 1:1 for the low-spin/high-spin heme centers in NOR was estimated. Shifting the pH from 8.5 to 5.0, replacing Triton X-100 by other detergents, or adding soybean phospholipids to the protein, led to pronounced changes of the EPR signals in the g = 6 region. In contrast, the strong inhibitor oleic acid did not cause significant spectral changes. NOR which had been reduced by L-ascorbate/phenazine methosulfate prior to incubation with its substrate NO gave the characteristic Fe(II) nitrosyl triplet centered at g approximately 2.01, with a hyperfine splitting of 1.70 mT. In the absence of dioxygen, NOR was quantitatively reduced by either sodium dithionite, or photochemically with deazaflavin and oxalate; the enzyme was reoxidizable by ferricyanide in a fully reversible reaction. Spectroelectrochemical oxidoreductive titrations gave E'o (versus standard hydrogen electrode) = +322 mV for the cytochrome b and +280 mV for the cytochrome c component.