NO3 −/NO2 − assimilation in halophilic archaea: physiological analysis, nasA and nasD expressions

The haloarchaeon Haloferax mediterranei is able to assimilate nitrate or nitrite using the assimilatory nitrate pathway. An assimilatory nitrate reductase (Nas) and an assimilatory nitrite reductase (NiR) catalyze the first and second reactions, respectively. The genes involved in this process are transcribed as two messengers, one polycistronic (nasABC; nasA encodes Nas) and one monocistronic (nasD; codes for NiR). Here we report the Hfx mediterranei growth as well as the Nas and NiR activities in presence of high nitrate, nitrite and salt concentrations, using different approaches such as physiological experiments and enzymatic activities assays. The nasA and nasD expression profiles are also analysed by real-time quantitative PCR. The results presented reveal that the assimilatory nitrate/nitrite pathway in Hfx mediterranei takes place even if the salt concentration is higher than those usually present in the environments where this microorganism inhabits. This haloarchaeon grows in presence of 2 M nitrate or 50 mM nitrite, which are the highest nitrate and nitrite concentrations described from a prokaryotic microorganism. Therefore, it could be attractive for bioremediation applications in sewage plants where high salt, nitrate and nitrite concentrations are detected in wastewaters and brines.     

Biogenesis of a respiratory complex is orchestrated by a single accessory protein

The biogenesis of respiratory complexes is a multistep process that requires finely tuned coordination of subunit assembly, metal cofactor insertion, and membrane-anchoring events. The dissimilatory nitrate reductase of the bacterial anaerobic respiratory chain is a membrane-bound heterotrimeric complex nitrate reductase A (NarGHI) carrying no less than eight redox centers. Here, we identified different stable folding assembly intermediates of the nitrate reductase complex and analyzed their redox cofactor contents using electron paramagnetic resonance spectroscopy. Upon the absence of the accessory protein NarJ, a global defect in metal incorporation was revealed. In addition to the molybdenum cofactor, we show that NarJ is required for specific insertion of the proximal iron-sulfur cluster (FS0) within the soluble nitrate reductase (NarGH) catalytic dimer. Further, we establish that NarJ ensures complete maturation of the b-type cytochrome subunit NarI by a proper timing for membrane anchoring of the NarGH complex. Our findings demonstrate that NarJ has a multifunctional role by orchestrating both the maturation and the assembly steps.     

Nitrate reductase-formate dehydrogenase couple involved in the fungal denitrification by Fusarium oxysporum

Dissimilatory nitrate reductase (Nar) was solubilized and partially purified from the large particle (mitochondrial) fraction of the denitrifying fungus Fusarium oxysporum and characterized. Many lines of evidence showed that the membrane-bound Nar is distinct from the soluble, assimilatory nitrate reductase. Further, the spectral and other properties of the fungal Nar were similar to those of dissimilatory Nars of Escherichia coli and denitrifying bacteria, which are comprised of a molybdoprotein, a cytochrome b, and an iron-sulfur protein. Formate-nitrate oxidoreductase activity was also detected in the mitochondrial fraction, which was shown to arise from the coupling of formate dehydrogenase (Fdh), Nar, and a ubiquinone/ubiquinol pool. This is the first report of the occurrence in a eukaryote of Fdh that is associated with the respiratory chain. The coupling with Fdh showed that the fungal Nar system is more similar to that involved in the nitrate respiration by Escherichia coli than that in the bacterial denitrifying system. Analyses of the mutant species of F. oxysporum that were defective in Nar and/or assimilatory nitrate reductase conclusively showed that Nar is essential for the fungal denitrification.     

Identification of an assimilatory nitrate reductase in mutants of Paracoccus denitrificans GB17 deficient in nitrate respiration

A Paracoccus denitrificans strain (M6Ω) unable to use nitrate as a terminal electron acceptor was constructed by insertional inactivation of the periplasmic and membrane-bound nitrate reductases. The mutant strain was able to grow aerobically with nitrate as the sole nitrogen source. It also grew anaerobically with nitrate as sole nitrogen source when nitrous oxide was provided as a respiratory electron acceptor. These growth characteristics are attributed to the presence of a third, assimilatory nitrate reductase. Nitrate reductase activity was detectable in intact cells and soluble fractions using nonphysiological electron donors. The enzyme activity was not detectable when ammonium was included in the growth medium. The results provide an unequivocal demonstration that P. denitrificans can express an assimilatory nitrate reductase in addition to the well-characterised periplasmic and membrane-bound nitrate reductases. Received: 12 August 1996 / Accepted: 29 October 1996     

A cytochrome c containing nitrate reductase plays a role in electron transport for denitrification in Thermus thermophilus without involvement of the bc respiratory complex

The bc(1) respiratory complex III constitutes a key energy-conserving respiratory electron transporter between complex I (type I NADH dehydrogenase) and II (succinate dehydrogenase) and the final nitrogen oxide reductases (Nir, Nor and Nos) in most denitrifying bacteria. However, we show that the expression of complex III from Thermus thermophilus is repressed under denitrification, and that its role as electron transporter is replaced by an unusual nitrate reductase (Nar) that contains a periplasmic cytochrome c (NarC). Several lines of evidence support this conclusion: (i) nitrite and NO are as effective signals as nitrate for the induction of Nar; (ii) narC mutants are defective in anaerobic growth with nitrite, NO and N2O; (iii) such mutants present decreased NADH oxidation coupled to these electron acceptors; and (iv) complementation assays of the mutants reveal that the membrane-distal heme c of NarC was necessary for anaerobic growth with nitrite, whereas the membrane-proximal heme c was not. Finally, we show evidence to support that Nrc, the main NADH oxidative activity in denitrification, interacts with Nar through their respective membrane subunits. Thus, we propose the existence of a Nrc-Nar respiratory super-complex that is required for the development of the whole denitrification pathway in T. thermophilus.     

Nitrate reductases: Structure,functions, and effect of stress factors

Structural and functional peculiarities of four types of nitrate reductases are considered: assimilatory nitrate reductase of eukaryotes, as well as cytoplasmic assimilatory, membrane-bound respiratory, and periplasmic dissimilatory bacterial nitrate reductases. Arguments are presented showing that eukaryotic organisms are capable of nitrate dissimilation. Data concerning new classes of extremophil nitrate reductases, whose active center does not contain molybdocofactor, are summarized.     

Regulation of Nitrate Assimilation and Nitrate Respiration in Aerobacter aerogenes

The influence of growth conditions on assimilatory and respiratory nitrate reduction in Aerobacter aerogenes was studied. The level of nitrate reductase activity in cells, growing in minimal medium with nitrate as the sole nitrogen source, was much lower under aerobic than anaerobic conditions. Further, the enzyme of the aerobic cultures was very sensitive to sonic disintegration, as distinct from the enzyme of anaerobic cultures. When a culture of A. aerogenes was shifted from anaerobic growth in minimal medium with nitrate and NH(4) (+) to aerobiosis in the same medium, but without NH(4) (+), the production of nitrite stopped instantaneously and the total activity of nitrate reductase decreased sharply. Moreover, there was a lag in growth of about 3 hr after such a shift. After resumption of growth, the total enzymatic activity increased again slowly and simultaneously became gradually sensitive to sonic disintegration. These findings show that oxygen inactivates the anaerobic nitrate reductase and represses its further formation; only after a de novo synthesis of nitrate reductase with an assimilatory function will growth be resumed. The enzyme in aerobic cultures was not significantly inactivated by air, only by pure oxygen. The formation of the assimilatory enzyme complex was repressed, however, by NH(4) (+), under both aerobic and anaerobic conditions. The results indicate that the formation of the assimilatory enzyme complex and that of the respiratory enzyme complex are regulated differently. We suggest that both complexes have a different composition, but that the nitrate reductase in both cases is the same protein.     

Look on the positive side! The orientation, identification and bioenergetics of 'Archaeal' membrane-bound nitrate reductases

Many species of Bacteria and Archaea respire nitrate using a molybdenum-dependent membrane-bound respiratory system called Nar. Classically, the 'Bacterial' Nar system is oriented such that nitrate reduction takes place on the inside of this membrane. However, the active site subunit of the 'Archaeal' Nar systems has a twin arginine ('RR') motif, which is a suggestion of translocation to the outside of the cytoplasmic membrane. These 'Archaeal' type of nitrate reductases are part of a group of molybdoenzymes with an 'RR' motif that are predicted to have an aspartate ligand to the molybdenum ion. This group includes selenate reductases and possible sequence signatures are described that serve to distinguish the Nar nitrate reductases from the selenate reductases. The 'RR' sequences of nitrate reductases of Archaea and some that have recently emerged in Bacteria are also considered and it is concluded that there is good evidence for there being both Archaeal and Bacterial examples of Nar-type nitrate reductases with an active site on the outside of the cytoplasmic membrane. Finally, the bioenergetic consequences of nitrate reduction on the outside of the cytoplasmic membrane have been explored.     

Effect of periplasmic nitrate reductase on diauxic lag of Paracoccus pantotrophus

Paracoccus pantotrophus expresses two nitrate reductases—membrane bound nitrate reductase (Nar) and periplasmic nitrate reductase (Nap). In growth experiments with two denitrifying species (Paracoccus pantotrophus and Alcaligenes eutrophus) that have both Nap and Nar and two species (Pseudomonas denitrificans and Pseudomonas fluorescens) with Nar only, it was found that diauxic lag is shorter for bacteria that express Nap. In P. pantotrophus, napEDABC encodes the periplasmic nitrate reductase. To analyze the effect of Nap on diauxic lag, the nap operon was deleted from P. pantotrophus. The growth experiments with nap^? mutant resulted in increased diauxic lag when switched from aerobic to anoxic respiration, suggesting Nap is responsible for shorter lags and helps in adaptation to anoxic metabolism after transition from aerobic conditions. © 2009 American Institute of Chemical Engineers Biotechnol. Prog., 2009     

Nitrate Reductase and Nitrous Oxide Production by Fusarium oxysporum 11dn1 Under Aerobic and Anaerobic Conditions

The fungus Fusarium oxysporum 11dn1 was found to be able to grow and produce nitrous oxide on nitrate-containing medium in anaerobic conditions. The rate of nitrous oxide formation was three to six orders of magnitude lower than the rates of molecular nitrogen production by common denitrifying bacteria. Acetylene and ammonia did not affect the release of nitrous oxide release. It was shown that under anaerobic conditions fast increase of nitrate reductase activity occurred, caused by the synthesis of enzyme de novo and protein dephosphorylation. Reverse transfer of the mycelium to aerobic conditions led to a decline in nitrate reductase activity and stopped nitrous oxide production. The presence of two nitrate reductases was shown, which differed in molecular mass, location, temperature optima, and activity in nitrate- and ammonium-containing media. Two enzymes represent assimilatory and dissimilatory nitrate reductases, which are active in aerobic and anaerobic conditions, respectively. Received: 2 February 2000 / Accepted: 28 February 2000     

Bacterial nitrate reductases: Molecular and biological aspects of nitrate reduction

Nitrogen is a vital component in living organisms as it participates in the making of essential biomolecules such as proteins, nucleic acids, etc. In the biosphere, nitrogen cycles between the oxidation states +V and -III producing many species that constitute the biogeochemical cycle of nitrogen. All reductive branches of this cycle involve the conversion of nitrate to nitrite, which is catalyzed by the enzyme nitrate reductase. The characterization of nitrate reductases from prokaryotic organisms has allowed us to gain considerable information on the molecular basis of nitrate reduction. Prokaryotic nitrate reductases are mononuclear Mo-containing enzymes sub-grouped as respiratory nitrate reductases, periplasmic nitrate reductases and assimilatory nitrate reductases. We review here the biological and molecular properties of these three enzymes along with their gene organization and expression, which are necessary to understand the biological processes involved in nitrate reduction.     

The mobA Gene Is Required for Assimilatory and Respiratory Nitrate Reduction but not Xanthine Dehydrogenase Activity in Pseudomonas aeruginosa

The requirement for the mobA gene in key assimilatory and respiratory nitrogen metabolism of Pseudomonas aeruginosa PAO1 was investigated by mutational analysis of PA3030 (mobA; MoCo guanylating enzyme), PA1779 (nasA; assimilatory nitrate reductase), and PA3875 (narG; respiratory nitrate reductase). The mobA mutant was deficient in both assimilatory and respiratory nitrate reductase activities, whereas xanthine dehydrogenase activity remained unaffected. Thus, P. aeruginosa requires both the molybdopterin (MPT) and molybdopterin guanine dinucleotide (MGD) forms of the molybdenum cofactor for a complete spectrum of nitrogen metabolism, and one form cannot substitute for the other. Regulation studies using a Φ(PA3030-lacZGm) reporter strain suggest that expression of mobA is not influenced by the type of nitrogen source or by anaerobiosis, whereas assimilatory nitrate reductase activity was detected only in the presence of nitrate.     

Structures of genes nasA and nasB, encoding assimilatory nitrate and nitrite reductases in Klebsiella pneumoniae M5al.

Klebsiella pneumoniae can use nitrate and nitrite as sole nitrogen sources during aerobic growth. Assimilatory nitrate and nitrite reductases convert nitrate through nitrite to ammonium. We report here the molecular cloning of the nasA and nasB genes, which encode assimilatory nitrate and nitrite reductase, respectively. These genes are tightly linked and probably form a nasBA operon. In vivo protein expression and DNA sequence analysis revealed that the nasA and nasB genes encode 92- and 104-kDa proteins, respectively. The NASA polypeptide is homologous to other prokaryotic molybdoenzymes, and the NASB polypeptide is homologous to eukaryotic and prokaryotic NADH-nitrite reductases. The narL gene product positively regulates expression of the structural genes for respiratory nitrate reductase, narGHJI. Surprisingly, we found that the nasBA operon is tightly linked to the narL-narGHJI region in K. pneumoniae, even though the nitrate assimilatory and respiratory enzymes serve different physiological functions.     

Disruption of narG, the Gene Encoding the Catalytic Subunit of Respiratory Nitrate Reductase, Also Affects Nitrite Respiration in Pseudomonas fluorescens YT101

The Pseudomonas fluorescens YT101 gene narG, which encodes the catalytic alpha subunit of the respiratory nitrate reductase, was disrupted by insertion of a gentamicin resistance cassette. In the Nar(-) mutants, nitrate reductase activity was not detectable under all the conditions tested, suggesting that P. fluorescens YT101 contains only one membrane-bound nitrate reductase and no periplasmic nitrate reductase. Whereas N(2)O respiration was not affected, anaerobic growth with NO(2) as the sole electron acceptor was delayed for all of the Nar(-) mutants following a transfer from oxic to anoxic conditions. These results provide the first demonstration of a regulatory link between nitrate and nitrite respiration in the denitrifying pathway.     

Iodate Reduction by Shewanella oneidensis Does Not Involve Nitrate Reductase

Microbial iodate (IO₃^?) reduction is a major component of iodine biogeochemical cycling and is the basis of alternative strategies for remediation of iodine-contaminated environments. The molecular mechanism of microbial IO₃^? reduction, however, is not well understood. In several microorganisms displaying IO₃^? and nitrate (NO₃^?) reduction activities, NO₃^? reductase is postulated to reduce IO₃^? as alternate electron acceptor. In the present study, whole genome analyses of 25 NO₃^?-reducing Shewanella strains identified various combinations of genes encoding one assimilatory (cytoplasmic Nas) and three dissimilatory (membrane-associated Nar and periplasmic Napα and Napβ) NO₃^? reductases. Shewanella oneidensis was the only Shewanella strain whose genome encoded a single NO₃^? reductase (Napβ). Terminal electron acceptor competition experiments in S. oneidensis batch cultures amended with both NO₃^? and IO₃^? demonstrated that neither NO₃^? nor IO₃^? reduction activities were competitively inhibited by the presence of the competing electron acceptor. The lack of involvement of S. oneidensis Napβ in IO₃^? reduction was confirmed via phenotypic analysis of an in-frame gene deletion mutant lacking napβA (encoding the NO₃^?-reducing NapβA catalytic subunit). S. oneidensis ΔnapβA was unable to reduce NO₃^?, yet reduced IO₃^? at rates higher than the wild-type strain. Thus, NapβA is required for dissimilatory NO₃^? reduction by S. oneidensis, while neither the assimilatory (Nas) nor dissimilatory (Napα, Napβ, and Nar) NO₃^? reductases are required for IO₃^? reduction. These findings provide the first genetic evidence that IO₃^? reduction by S. oneidensis does not involve nitrate reductase and indicate that S. oneidensis reduces IO₃^? via an as yet undiscovered enzymatic mechanism.     

The assimilatory and dissimilatory nitrate reductases of Pseudomonas aeruginosa are encoded by different genes

The phenotypes of certain mutant strains of Pseudomonas aeruginosa were reported to be pleiotropic for nitrate reduction; these strains were selected for their inability to dissimilate nitrate and were found also to have lost the ability to assimilate nitrate. We now report that the isolation procedure selected two mutations, one in genes encoding the synthesis of dissimilatory nitrate reductase (narA, narB or narE) and another in one of the genes (nas) encoding the synthesis of assimilatory nitrate reductase. Thus in P. aeruginosa dissimilatory and assimilatory nitrate reductases are genetically distinct. However, a loss of both enzymes is necessary to prevent slow dissimilatory growth on nitrate. Assimilatory nitrate reductase requires molybdenum to function, as does dissimilatory nitrate reductase. Lesions in narD affect incorporation of molybdenum into both enzymes, and hence exert a pleiotropic effect.     

Mutants of Azotobacter vinelandii altered in the regulation of nitrate assimilation

The two enzymes involved in the assimilatory pathway of nitrate in Azotobacter vinelandii are corregulated. Nitrate reductase and nitrite reductase are inducible by nitrate and nitrite. Ammonium represses induction by nitrate of both reductases. Repression by ammonium is higher in media containing 2-oxo-glutarate as carbon source than in media containing sucrose. Mutants in the gene ntrC lost nitrate and nitrite reductase simultaneously. Ten chlorate-resistant mutants with a new phenotype were isolated. In media without ammonium they had a normal phenotype, being sensitive to the toxic effect of chlorate. In media containing low ammonium concentrations they were resistant to chlorate. These mutants seem to be affected in the repression of nitrate and nitrite reductases by ammonium.     

Properties of nitrate reductase from Fusarium oxysporum 11dn1 fungi grown under aerobic and anaerobic conditions

Production of nitrate reductase was studied in 15 species of microscopic fungi grown on a nitrate-containing medium. Experiments were performed with Fusarium oxysporum 11dn1, a fungus capable of producing nitrous oxide as the end product of denitrification. Moreover, a shift from aerobic to anaerobic conditions of growth was accompanied by a sharp increase in the activity of nitrate reductase. Studies of nitrate reductase from the mycelium of Fusarium oxysporum 11dn1, grown under aerobic and anaerobic conditions, showed that this enzyme belongs to molybdenum-containing nitrate reductases. The enzymes under study differed in the molecular weight, temperature optimum, and other properties. Nitrate reductase from the mycelium grown under aerobic conditions was shown to belong to the class of assimilatory enzymes. However, nitrate reductase from the mycelium grown anaerobically had a dissimilatory function. An increase in the activity of dissimilatory nitrate reductase, observed under anaerobic conditions, was associated with de novo synthesis of the enzyme.     

Structural and mechanistic insights on nitrate reductases

Nitrate reductases (NR) belong to the DMSO reductase family of Mo‐containing enzymes and perform key roles in the metabolism of the nitrogen cycle, reducing nitrate to nitrite. Due to variable cell location, structure and function, they have been divided into periplasmic (Nap), cytoplasmic, and membrane‐bound (Nar) nitrate reductases. The first crystal structure obtained for a NR was that of the monomeric NapA from Desulfovibrio desulfuricans in 1999. Since then several new crystal structures were solved providing novel insights that led to the revision of the commonly accepted reaction mechanism for periplasmic nitrate reductases. The two crystal structures available for the NarGHI protein are from the same organism (Escherichia coli) and the combination with electrochemical and spectroscopic studies also lead to the proposal of a reaction mechanism for this group of enzymes. Here we present an overview on the current advances in structural and functional aspects of bacterial nitrate reductases, focusing on the mechanistic implications drawn from the crystallographic data.     

Three nitrate reductase activities in Alcaligenes eutrophus

Three nitrate reductase activities were detected in Alcaligenes eutrophus strain H16 by physiological and mutant analysis. The first (NAS) was subject to repression by ammonia and not affected by oxygen indicating a nitrate assimilatory function. The second (NAR) membrane-bound activity was only formed in the absence of oxygen and was insensitive to ammonia repression indicating a nitrate respiratory function. The third (NAP) activity of potential respiratory function occurred in the soluble fraction of cells grown to the stationary phase of growth. In contrast to NAR and NAS, expression of NAP did not require nitrate for induction and was independent of the rpoN gene product. Genes for the three reductases map at different loci. NAR and NAS are chromosomally encoded whereas NAP is a megaplasmid-borne activity in A. eutrophus.