Dissimilatory nitrate uptake in Paracoccus denitrificans via a Δ\̃gmH+-dependent system and a nitrate-nitrite antiport system

The Paracoccus denitrificans NarK-like nitrate and nitrite transporters-probing nitrate uptake and nitrate/nitrite exchange mechanisms

Nitrate and nitrite transport across biological membranes is often facilitated by protein transporters that are members of the major facilitator superfamily. Paracoccus denitrificans contains an unusual arrangement whereby two of these transporters, NarK1 and NarK2, are fused into a single protein, NarK, which delivers nitrate to the respiratory nitrate reductase and transfers the product, nitrite, to the periplasm. Our complementation studies, using a mutant lacking the nitrate/proton symporter NasA from the assimilatory nitrate reductase pathway, support that NarK1 functions as a nitrate/proton symporter while NarK2 is a nitrate/nitrite antiporter. Through the same experimental system, we find that Escherichia coli NarK and NarU can complement deletions in both narK and nasA in P. denitrificans, suggesting that, while these proteins are most likely nitrate/nitrite antiporters, they can also act in the net uptake of nitrate. Finally, we argue that primary sequence analysis and structural modelling do not readily explain why NasA, NarK1 and NarK2, as well as other transporters from this protein family, have such different functions, ranging from net nitrate uptake to nitrate/nitrite exchange.

Structure and mechanism of a nitrate transporter

The nitrate/nitrite transporters NarK and NarU play an important role in nitrogen homeostasis in bacteria and belong to the nitrate/nitrite porter family (NNP) of the major facilitator superfamily (MFS) fold. The structure and functional mechanism of NarK and NarU remain unknown. Here, we report the crystal structure of NarU at a resolution of 3.1 Å and systematic biochemical characterization. The two molecules of NarU in an asymmetric unit exhibit two distinct conformational states: occluded and partially inward-open. The substrate molecule nitrate appears to be coordinated by four highly conserved, charged, or polar amino acids. Structural and biochemical analyses allowed the identification of key amino acids that are involved in substrate gating and transport. The observed conformational differences of NarU, together with unique sequence features of the NNP family transporters, suggest a transport mechanism that might deviate from the canonical rocker-switch model.

Energy coupling to nitrate uptake into the denitrifying cells of Paracoccus denitrificans

This study deals with the effects of the agents that dissipate the individual components of the proton motive force (short-chain fatty acids, nigericin, and valinomycin) upon the methyl viologen-coupled nitrate reductase activity in intact cells. Substitution of butyrate or acetate for chloride in Tris-buffered assay media resulted in a marked inhibition at pH 7. In a Tris--chloride buffer of neutral pH, the reaction was almost fully inhibitable by nigericin. Alkalinisation increased the IC(50) value for nigericin and decreased the maximal inhibition attained. Both types of inhibitions could be reversed by the permeabilisation of cells or by the addition of nitrite, and that caused by nigericin disappeared at high extracellular concentrations of potassium. These data indicate that nitrate transport step relies heavily on the pH gradient at neutral pH. Since the affinity of cells for nitrate was strongly diminished by imposing an inside-positive potassium (or lithium) diffusion potential at alkaline external pH, a potential dependent step may be of significance in the transporter cycle under these conditions. Experiments with sodium-depleted media provided no hints for Na(+) as a possible H(+) substitute.

Characterization of the chlorate reductase from Pseudomonas chloritidismutans

A chlorate reductase has been purified from the chlorate-reducing strain Pseudomonas chloritidismutans. Comparison with the periplasmic (per)chlorate reductase of strain GR-1 showed that the cytoplasmic chlorate reductase of P. chloritidismutans reduced only chlorate and bromate. Differences were also found in N-terminal sequences, molecular weight, and subunit composition. Metal analysis and electron paramagnetic resonance measurements showed the presence of iron and molybdenum, which are also found in other dissimilatory oxyanion reductases.

Inhibition by phenylglyoxal of nitrate transport in Paracoccus denitrificans: a comparison with the effect of a protonophorous uncoupler

The amino acid modifier phenylglyoxal (PG) gradually inactivated the methyl viologen-coupled nitrate reductase activity of the anoxically grown whole cells of Paracoccus denitrificans. A double log plot of the pseudo-first-order inactivation rate constant versus PG concentration was linear with a mean slope of 1.4 (0.1M sodium phosphate) or 0.87 (0.1M sodium borate). Phenylglyoxalation of cells lowered the limiting velocity (V), while hardly affecting the apparent half-saturation concentration (K(m)) of nitrate. Nitrate afforded no protection against inactivation. The inhibition by PG could be removed by the detergent Triton X-100 or by the lipid-soluble tetraphenylphosphonium countercation, suggesting that PG exerts its effect at the level of nitrate transport. Based on studies with membrane potential- and pH-sensitive fluorescent probes, the inhibition was shown not to be due to changes in the electrochemical gradient of hydrogen ions. Both K(m) and V values for nitrate uptake increased in a hyperbolic fashion in response to exogenously added nitrite. Nitrite promoted a bypass of the inhibition caused by low concentrations of the proton-conducting agent carbonyl cyanide m-chlorophenylhydrazone (CCCP), but was almost ineffective in the case of the PG block. These results are rationalized in terms of two nitrate import pathways that are comparably inhibited by PG and differ in their sensitivities to CCCP. A simplified kinetic model for phenylglyoxalation is proposed to account for the observed nonintegral reaction orders.

Two domains of a dual-function NarK protein are required for nitrate uptake, the first step of denitrification in Paracoccus pantotrophus

Uptake of nitrate into the cytoplasm is the first but least well understood step of denitrification; no gene has previously been identified to be necessary for this process. Upstream from the structural genes of the membrane-bound nitrate reductase (narGHJI) in Paracoccus pantotrophus there is a fusion of two genes, each homologous to members of the narK family. The single open reading frame is predicted to encode 24 transmembrane helices, comprising two domains, NarK1 and NarK2. Analysis of both the accumulation of intracellular nitrite and electron transport through the nitrate reductase enzyme in narK mutants reveals that NarK1 and NarK2 are both involved in nitrate uptake. Maximal rate of nitrate transport via NarK2 was dependent upon nitrite, indicating that NarK2 encodes a nitrate/nitrite antiporter. The uncouplers S13 and dinitrophenol showed that NarK2 was not dependent on the proton motive force for maximal nitrate transport activity. Nitrate transport via NarK1 was dependent on proton motive force, indicating that it is likely to be a nitrate/proton symporter. Low expression of membrane-bound nitrate reductase in narK mutants was counteracted by azide, which induced nitrate reductase expression only if the transcriptional activator NarR was present.

Cell biology and molecular basis of denitrification

Denitrification is a distinct means of energy conservation, making use of N oxides as terminal electron acceptors for cellular bioenergetics under anaerobic, microaerophilic, and occasionally aerobic conditions. The process is an essential branch of the global N cycle, reversing dinitrogen fixation, and is associated with chemolithotrophic, phototrophic, diazotrophic, or organotrophic metabolism but generally not with obligately anaerobic life. Discovered more than a century ago and believed to be exclusively a bacterial trait, denitrification has now been found in halophilic and hyperthermophilic archaea and in the mitochondria of fungi, raising evolutionarily intriguing vistas. Important advances in the biochemical characterization of denitrification and the underlying genetics have been achieved with Pseudomonas stutzeri, Pseudomonas aeruginosa, Paracoccus denitrificans, Ralstonia eutropha, and Rhodobacter sphaeroides. Pseudomonads represent one of the largest assemblies of the denitrifying bacteria within a single genus, favoring their use as model organisms. Around 50 genes are required within a single bacterium to encode the core structures of the denitrification apparatus. Much of the denitrification process of gram-negative bacteria has been found confined to the periplasm, whereas the topology and enzymology of the gram-positive bacteria are less well established. The activation and enzymatic transformation of N oxides is based on the redox chemistry of Fe, Cu, and Mo. Biochemical breakthroughs have included the X-ray structures of the two types of respiratory nitrite reductases and the isolation of the novel enzymes nitric oxide reductase and nitrous oxide reductase, as well as their structural characterization by indirect spectroscopic means. This revealed unexpected relationships among denitrification enzymes and respiratory oxygen reductases. Denitrification is intimately related to fundamental cellular processes that include primary and secondary transport, protein translocation, cytochrome c biogenesis, anaerobic gene regulation, metalloprotein assembly, and the biosynthesis of the cofactors molybdopterin and heme D1. An important class of regulators for the anaerobic expression of the denitrification apparatus are transcription factors of the greater FNR family. Nitrate and nitric oxide, in addition to being respiratory substrates, have been identified as signaling molecules for the induction of distinct N oxide-metabolizing enzymes.

Regulation of oxidative phosphorylation: the flexible respiratory network of Paracoccus denitrificans

Paracoccus denitrificans is a facultative anaerobic bacterium that has the capacity to adjust its metabolic infrastructure, quantitatively and/or qualitatively, to the prevailing growth condition. In this bacterium the relative activity of distinct catabolic pathways is subject to a hierarchical control. In the presence of oxygen the aerobic respiration, the most efficient way of electron transfer-linked phosphorylation, has priority. At high oxygen tensions P. denitrificans synthesizes an oxidase with a relatively low affinity for oxygen, whereas under oxygen limitation a high-affinity oxidase appears specifically induced. During anaerobiosis, the pathways with lower free energy-transducing efficiency are induced. In the presence of nitrate, the expression of a number of dehydrogenases ensures the continuation of oxidative phosphorylation via denitrification. After identification of the structural components that are involved in both the aerobic and the anaerobic respiratory networks of P. denitrificans, the intriguing next challenge is to get insight in its regulation. Two transcription regulators have recently been demonstrated to be involved in the expression of a number of aerobic and/or anaerobic respiratory complexes in P. denitrificans. Understanding of the regulation machinery is beginning to emerge and promises much excitement in discovery.

Effect of ionophores on denitrification inFlexibacter canadensis

Denitrification by Flexibacter canadensis was investigated by measuring the production and (or) consumption of nitrite, nitric oxide (NO), and nitrous oxide (N2O) under anaerobic conditions. Carbonyl cyanide m-chlorophenylhydrazone (CCCP), carbonyl cyanide p-trifluoromethoxyphenylhydrazone (FCCP), 2,4-dinitrophenol, and nigericin, but not valinomycin-K+inhibited the production of nitrite and N2O from nitrate by intact cells. However, CCCP, FCCP, 2,4-dinitrophenol, nigericin, and valinomycin-K+did not affect nitrite production from nitrate by cell-free extracts. These results suggest that nitrate transport was dependent on the transmembrane pH gradient but not on the membrane potential. CCCP, FCCP, and nigericin but not 2,4-dinitrophenol and valinomycin-K+caused NO accumulation during the reduction of nitrite, and also inhibited NO consumption and N2O production from nitrite by intact cells. These results preclude an explanation for NO accumulation based on the collapse of the proton motive force by ionophores, and imply that CCCP, FCCP, and nigericin perhaps dissociated a nitrite reductase–nitric oxide reductase complex, and (or) inhibited nitric oxide reductase specifically. 2,4-Dinitrophenol and CCCP did not inhibit the reduction of N2O to dinitrogen. Addition of ≤ 1.16 μM dissolved NO did not affect the production of nitrite from nitrate, or the disappearance of nitrite or N2O. The rate of NO consumption was linear with concentrations of dissolved NO up to 67 nM. Above 67 nM NO, NO consumption was inhibited, suggesting that NO is toxic to nitric oxide reductase.Key words: ionophores, denitrification, nitric oxide, Flexibacter canadensis.

Denitrification and its control

Denitrification in bacteria comprises a series of four reduction reactions; for nitrate, nitrite, nitric oxide and nitrous oxide. Nitrogen gas is the final product. The nature of the enzymes catalysing these reactions is described along with the the properties of the underlying electron transport systems. The factors influencing the expression of the reductases for the four reactions are reviewed along with the effect of oxygen on the activities of the enzymes of denitrification. The main emphasis is on observations made with Paracoccus denitrificans and Pseudomonas stutzeri.

Metabolic pathways in Paracoccus denitrificans and closely related bacteria in relation to the phylogeny of prokaryotes

Denitrification and methylotrophy in Paracoccus denitrificans are discussed. The properties of the enzymes of denitrification: the nitrate-nitrite antiporter, nitrate reductase, nitrite reductase, nitric oxide reductase and nitrous oxide reductase are described. The genes for none of these proteins have yet been cloned and sequenced from P. denitrificans. A number of sequences are available for enzymes from Escherichia coli, Pseudomonas stutzeri and Pseudomonas aeruginosa. It is concluded that pathway specific c-type cytochromes are involved in denitrification. At least 40 genes are involved in denitrification. In methanol oxidation at least 20 genes are involved. In this case too pathway specific c-type cytochromes are involved. The sequence homology between the quinoproteins methanol dehydrogenase, alcoholde-hydrogenase and glucose dehydrogenase is discussed. This superfamily of proteins is believed to be derived from a common ancestor. The moxFJGI operon determines the structural components of methanol dehydrogenase and the associated c-type cytochrome. Upstream of this operon 3 regulatory proteins were found. The moxY protein shows the general features of a sensor protein and the moxX protein those of a regulatory protein. Thus a two component regulatory system is involved in both denitrification and methylotrophy. The phylogeny of prokaryotes based on 16S rRNA sequence is discussed. It is remarkable that the 16S rRNA of Thiosphaera pantotropha is identical to that of P. denitrificans. Still these bacteria show a number of differences. T. pantotropha is able to denitrify under aerobic circumstances and it shows heterotrophic nitrification. Nitrification and heterotrophic nitrification are found in species belonging to the beta-and gamma-subdivisions of purple non-sulfur bacteria. Thus the occurrence of heterotrophic nitrification in T. pantotropha, which belongs to the alpha-subdivision of purple non-sulfur bacteria is a remarkable property. Furthermore T. pantotropha contains two nitrate reductases of which the periplasmic one is supposed to be involved in aerobic denitrification. The nitrite reductase is of the Cu-type and not of the cytochrome cd1 type as in P. denitrificans. Also the cytochrome b of the Qbc complex of T. pantotropha is highly similar to its counterpart in P. denitrificans. It is hypothesized that the differences between these two organisms which both contain large megaplasmids is due to a combination of loss of genetic information and plasmid-coded properties. The distribution of a number of complex metabolic systems in eubacteria and in a number of species belonging to the alpha-group of purple non sulphur bacteria is reviewed.(ABSTRACT TRUNCATED AT 400 WORDS)

NarK enhances nitrate uptake and nitrite excretion in Escherichia coli

narK mutants of Escherichia coli produce wild-type levels of nitrate reductase but, unlike the wild-type strain, do not accumulate nitrite when grown anaerobically on a glucose-nitrate medium. Comparison of the rates of nitrate and nitrite metabolism in cultures growing anaerobically on glucose-nitrate medium revealed that a narK mutant reduced nitrate at a rate only slightly slower than that in the NarK+ parental strain. Although the specific activities of nitrate reductase and nitrite reductase were similar in the two strains, the parental strain accumulated nitrite in the medium in almost stoichiometric amounts before it was further reduced, while the narK mutant did not accumulate nitrite in the medium but apparently reduced it as rapidly as it was formed. Under conditions in which nitrite reductase was not produced, the narK mutant excreted the nitrite formed from nitrate into the medium; however, the rate of reduction of nitrate to nitrite was significantly slower than that of the parental strain or that which occurred when nitrite reductase was present. These results demonstrate that E. coli is capable of taking up nitrate and excreting nitrite in the absence of a functional NarK protein; however, in growing cells, a functional NarK promotes a more rapid rate of anaerobic nitrate reduction and the continuous excretion of the nitrite formed. Based on the kinetics of nitrate reduction and of nitrite reduction and excretion in growing cultures and in washed cell suspensions, it is proposed that the narK gene encodes a nitrate/nitrite antiporter which facilitates anaerobic nitrate respiration by coupling the excretion of nitrite to nitrate uptake. The failure of nitrate to suppress the reduction of trimethylamine N-oxide in narK mutants was not due to a change in the level of trimethylamine N-oxide reductase but apparently resulted from a relative decrease in the rate of anaerobic nitrate reduction caused by the loss of the antiporter system.

Metabolic regulation including anaerobic metabolism in Paracoccus denitrificans

Under anaerobic circumstances in the presence of nitrate Paracoccus denitrificans is able to denitrify. The properties of the reductases involved in nitrate reductase, nitrite reductase, nitric oxide reductase, and nitrous oxide reductase are described. For that purpose not only the properties of the enzymes of P. denitrificans are considered but also those from Escherichia coli, Pseudomonas aeruginosa, and Pseudomonas stutzeri. Nitrate reductase consists of three subunits: the alpha subunit contains the molybdenum cofactor, the beta subunit contains the iron sulfur clusters, and the gamma subunit is a special cytochrome b. Nitrate is reduced at the cytoplasmic side of the membrane and evidence for the presence of a nitrate-nitrite antiporter is presented. Electron flow is from ubiquinol via the specific cytochrome b to the nitrate reductase. Nitrite reductase (which is identical to cytochrome cd1) and nitrous oxide reductase are periplasmic proteins. Nitric oxide reductase is a membrane-bound enzyme. The bc1 complex is involved in electron flow to these reductases and the whole reaction takes place at the periplasmic side of the membrane. It is now firmly established that NO is an obligatory intermediate between nitrite and nitrous oxide. Nitrous oxide reductase is a multi-copper protein. A large number of genes is involved in the acquisition of molybdenum and copper, the formation of the molybdenum cofactor, and the insertion of the metals. It is estimated that at least 40 genes are involved in the process of denitrification. The control of the expression of these genes in P. denitrificans is totally unknown. As an example of such complex regulatory systems the function of the fnr, narX, and narL gene products in the expression of nitrate reductase in E. coli is described. The control of the effects of oxygen on the reduction of nitrate, nitrite, and nitrous oxide are discussed. Oxygen inhibits reduction of nitrate by prevention of nitrate uptake in the cell. In the case of nitrite and nitrous oxide a competition between reductases and oxidases for a limited supply of electrons from primary dehydrogenases seems to play an important role. Under some circumstances NO formed from nitrite may inhibit oxidases, resulting in a redistribution of electron flow from oxygen to nitrite. P. denitrificans contains three main oxidases: cytochrome aa3, cytochrome o, and cytochrome co. Cytochrome o is proton translocating and receives its electrons from ubiquinol. Some properties of cytochrome co, which receives its electrons from cytochrome c, are reported.(ABSTRACT TRUNCATED AT 400 WORDS)

Localization of the cytochrome cd1 and copper nitrite reductases in denitrifying bacteria

The locations of cytochrome cd1 nitrite reductases in Pseudomonas aeruginosa and Pseudomonas fluorescens and copper nitrite reductases in Achromobacter cycloclastes and Achromobacter xylosoxidans were identified. Immunogold labeling with colloidal-gold probes showed that the nitrite reductases were synthesized exclusively in anaerobically grown (denitrifying) cells. Little immunogold label occurred in the cytoplasm of these four strains; most was found in the periplasmic space or was associated with cell membranes. Immunogold labeling of thin sections was superior to fractionation by osmotic shock for locating nitrite reductases. The results support models of dentrification energetics that require a periplasmic, not a cytoplasmic, location for nitrite reductases.

The azurin gene from Pseudomonas aeruginosa. Cloning and characterization

We have cloned and sequenced the Pseudomonas aeruginosa azurin structural gene and its flanking regions. The DNA sequence predicts a pre-protein with a signal peptide of 19 amino acids followed by the 128-amino-acid mature azurin protein. Nuclease-S1 mapping and primer elongation experiments indicated two 5' termini of the azurin transcript. The major transcript of the azurin gene is initiated around 35 base pairs upstream from the translational start. The minor transcript, with a promoter region sharing homology with a consensus nif promoter of Klebsiella pneumoniae and also with other Pseudomonas genes, is initiated 145 base pairs upstreams of the azurin initiation codon. Downstream from the azurin structural gene a sequence similar to a transcriptional terminator is found. Northern blot analysis indicated two sizes of the azurin mRNA (0.54 kb and 0.65 kb) confirming the S1 mapping and the predictions from the nucleotide sequence.

The respiratory nitrate reductase from Paracoccus denitrificans. Molecular characterisation and kinetic properties

The respiratory nitrate reductase from Paracoccus denitrificans has been purified in the non-ionic detergent Nonidet P-40. The enzyme comprises three polypeptides, alpha, beta and gamma with estimated relative molecular masses of 127 000, 61 000 and 21 000. Duroquinol or reduced-viologen compounds acted as the reducing substrates. The nitrate reductase contained a b-type cytochrome that was reduced by duroquinol and oxidised by nitrate. A preparation of the enzyme that lacked both detectable b-type cytochrome and the gamma subunit was obtained from a trailing peak of nitrate reductase activity collected from a gel filtration column. Absence of the gamma subunit correlated with failure to use duroquinol as reductant; activity with reduced viologens was retained. It is concluded that in the plasma membrane of P. denitrificans the gamma subunit catalyses electron transfer to the alpha and beta subunits of nitrate reductase from ubiquinol which acts as a branch point in the respiratory chain. A new assay was introduced for both nitrate and quinol-nitrate oxidoreductase activity. Diaphorase was used to couple the oxidation of NADH to the production of duroquinol which acted as electron donor to nitrate reductase. Under anaerobic conditions absorbance changes at 340 nm were sensitive to nitrate concentrations in the low micromolar range. This coupled assay was used to determine that the purified enzyme had Km(NO-3) of 13 microM and a Km of 470 microM for ClO-3, an alternative substrate. With viologen substrates Km(NO-3) of 283 microM and Km(ClO-3) of 470 microM were determined; the enzymes possessed a considerably higher Vmax with either NO-3 or ClO-3 than was found when duroquinol was substrate. Azide was a competitive inhibitor of nitrate reduction in either assay system (Ki = 0.55 microM) but 2-n-heptyl-4-hydroxyquinoline N-oxide was effective only with the complete three-subunit enzyme and duroquinol as substrate, consistent with a site of action for this inhibitor on the b-type cytochrome. The low Km for nitrate observed in the duriquinol assay is comparable with the apparent Km(NO-3) recently reported for intact cells of P. denitrificans [Parsonage, D., Greenfield, A. J. & Ferguson, S. J. (1985) Biochim. Biophys. Acta 807, 81-95]. This similarity is discussed in terms of a possible requirement for a nitrate transport system. The nitrate reductase system from P. denitrificans is compared with that from Escherichia coli.

Hydrogen oxidation and nitrogen fixation in rhizobia, with special attention focused on strain ORS 571

In this survey we describe the influence of hydrogen oxidation on the physiology of Rhizobium ORS 571. The presence of hydrogen is required for the synthesis of hydrogenase. Carbon substrates do not repress the synthesis of hydrogenase. The respiratory system contains cytrochromes of the b- and c-type. Cytochrome alpha 600 is present after growth at high oxygen tensions. The nature of the terminal oxidases functioning at low oxygen tensions has not been established yet----H+/O values with endogenous substrates are between 6 and 7. The results show the presence of two phosphorylation sites: site 1 (ATP/2e = 1.0) and site 2(ATP/2e = 1.33). By measuring molar growth yields it has been demonstrated that carbon-limited, nitrogen-fixing cultures obtain additional ATP from hydrogen oxidation, and that site 2 of oxidative phosphorylation is passed during hydrogen oxidation. A method is described to calculate ATP/N2 values (the total amount of ATP used by nitrogenase during the fixation of 1 mol N2) and H2/N2 ratios (mol hydrogen formed per mol N2 fixed) in aerobic organisms. For Rhizobium ORS 571 the ATP/N2 value is about 40 and the H2/N2 ratio is between 5 and 7.5. Cells obtained from oxygen-limited nitrogen-fixing cultures contain 30-40% poly-beta-hydroxybutyrate, which explains the high molar growth yields found. Hydrogen has not been detected in the effluent gas of these cultures, which may point to reoxidation of the hydrogen formed at nitrogen fixation. Calculations show that the effect of hydrogen reoxidation on the efficiency of nitrogen fixation (g N fixed X mol-1 substrate converted) is not very large and that the actual H2/N2 ratio is of much more importance. After addition of hydrogen to succinate-limited, ammonia-assimilating cultures, an initial increase of the Ysuccinate value (g dry wt X mol-1 succinate) is followed by a gradual decrease. This is accompanied by a large decrease of the YO2 value, and an increased permeability of the cytoplasmic membrane to protons. The results may be explained by a transition of the culture from an energy-limited state to a carbon-limited state.

Denitrification and nitrite reduction: Pseudomonas aeruginosa nitrite-reductase

Present knowledge of the different enzymatic steps of the denitrification chains in various bacteria, particularly Paracoccus denitrificans and Pseudomonas aeruginosa has been briefly reviewed. The question whether nitric oxide (NO), nitrous oxide (N2O) and other nitrogen derivatives are obligatory intermediates has been discussed. The second part is an extensive review of the structure and the function of a key enzyme in denitrification, cytochrome c551-nitrite-oxidoreductase from P. aeruginosa. Recent results on the stoichiometry of nitrite reduction have been discussed.