The light-induced development of nitrate reductase in etiolated barley shoots: An inhibitory effect of laevulinic acid

Induction of nitrate reductase EC 1.6.6.1 in etiolated barley (Hordeum vulgare L., var. Proctor) required continuous illumination and showed a lag period of about three hours. During the first 16 h of illumination the ratio NADH/NAD and NADPH/NADP, taken as a measure of internal oxidation reduction potential, declined. The inhibitor DCMU applied to whole leaves at concentrations shown to inhibit the reduction of cytochrome f by Photosystem 2 light did not inhibit the induction of nitrate reductase nor did it diminish the ratio of reduced to oxidised puridine nucleotides in the early hours of greening. It was concluded that light driven electron flow was not necessary for nitrate reductase induction. Chloramphenicol gave a slight inhibition of nitrate reductase induction. Laevulinic acid was added to greening barley leaves to inhibit tetrapyrrole pigment biosynthesis and plastid development. It strongly inhibited chlorophyll synthesis and nitrate reductase induction, with relatively little effect upon Photosystem 1 and 2 activities in isolated plastids. The activities of other inducible enzymes and control enzymes were little affected by laevulinic acid. Laevulinic acid also inhibited nitrate reductase induction by added nitrate in fully-greened illuminated plants grown in nitrate-free medium and so is unlikely to be acting through inhibition of plastid development. This inhibitor lowered the level of protohaem in whole leaves and plastids of greening barley and it is postulated that it may diminish the protohaem available for the assembly of a cytochrome b component of nitrate reductase.Abbreviations DCMU 3-(3:4-Dichlorophenyl)-1:1-dimethylurea - LA laevulinic acid     

Cytochrome-c reductases from wild-type and mutant strains of Aspergillus nidulans

Summary The sedimentation coefficients of the NADPH: cytochrome-c oxidoreductase enzymes from wild-type and mutant strains of Aspergillus nidulans have been estimated by sucrose density gradient centrifugation. In the wild-type, two species of cytochrome-c reductase were found, with sedimentation coefficients of 13.7s and 7.6s respectively. The 13.7s species did not appear to be associated with the enzymes of nitrate reduction, whereas the 7.6s species was closely associated with NADPH: nitrate oxidoreductase. In mutant strains lacking nitrate reductase, a thir species of cytochrome-c reductase with a sedimentation coefficient of 4.5s was found. There is some evidence that this 4.5s cytochrome-c reductase is a subunit or breakdown product of nitrate reductase and a model is presented for the role of this 4.5s cytocnorome-c reductase in the assembly of the intact nitrate reductase molecule.     

A succinate-oxidising nitrate reductase is located at the plasma membrane of plant roots

Plasma-membrane-bound nitrate reductase (PM-NR) is located in roots and leaves of tobacco (Nicotiana tabacum L. cv. Samsun) and reduces nitrate with NADH as electron donor. When plasma membranes were prepared under specific protecting conditions, a PM-NR of roots was detected that accepts electrons from succinate to reduce nitrate. Comparison between the succinate dehydrogenase of mitochondria and the succinate-oxidising PM-NR of roots indicated that they are two different enzymes. Partial purification of the nitrate reductase forms by anion-exchange chromatography indicated that succinate and NADH supply electrons to the same plasma-membrane-bound protein. Received: 27 March 1997 / Accepted: 9 April 1997     

Isolation and characterization of two new negative regulatory mutants for nitrate assimilation inChlamydomonas reinhardtii obtained by insertional mutagenesis

Plasmid DNA carrying either the nitrate reductase (NR) gene or the argininosuccinate lyase gene as selectable markers and the correspondingChlamydomonas reinhardtii mutants as recipient strains have been used to isolate regulatory mutants for nitrate assimilation by insertional mutagenesis. Identification of putative regulatory mutants was based on their chlorate sensitivity in the presence of ammonium. Among 8975 transformants, two mutants, N1 and T1, were obtained. Genetic characterization of these mutants indicated that they carry recessive mutations at two different loci, namedNrg1 andNrg2. The mutation in N1 was shown to be linked to the plasmid insertion. Two copies of the nitrate reductase plasmid, one of them truncated, were inserted in the N1 genome in inverse orientation. In addition to the chlorate sensitivity phenotype in the presence of ammonium, these mutants expressed NR, nitrite reductase and nitrate transport activities in ammonium-nitrate media. Kinetic constants for ammonium (¹⁴C-methylammonium) transport, as well as enzymatic activities related to the ammonium-regulated metabolic pathway for xanthine utilization, were not affected in these strains. The data strongly suggest thatNrg1 andNrg2 are regulatory genes which specifically mediate the negative control exerted by ammonium on the nitrate assimilation pathway inC. reinhardtii.     

Genetic control of nitrate reduction in the cyanobacterium nostoc muscorum

Summary There were three kinds of chlorate resistant (Clr-R) mutants: Clr-R₁ type did not show nitrate reductase activity, Clr-R₂ type was deficient in nitrate uptake activity and Clr-R₃ type was defective in both. It is suggested that the genetic determinant of the uptake system is distinct from that of the reductase system.The NH ₄ ⁺ -repressible uptake system showed a requirement of nitrate as an activator of its activity. In comparison, NH ₄ ⁺ -repressible reductase enzyme showed requirement of nitrate neither as an inducer of its synthesis nor as an activator of its activity. Thus, the available mutational and physiological evidence suggest no involvement of the nitrate uptake system in control of nitrate reductase activity and vice versa.Abbreviations Tricine (N-tris[Hydroxymethyl] methyl glycine) - Tris Tris (Hydroxymethyl) amino methane - OD Optical Density - Chl. Chlorophyll     

Ammonium repression of the nitrite-nitrate ( nasAB ) assimilatory operon of Azotobacter vinelandii is enhanced in mutants expressing the nifO gene at high levels

A number of Tn5 mutants were isolated which were unable to fix nitrogen and showed enhanced ammonium repression of the nitrate/nitrite assimilation genes. They also had reduced nitrate reductase activity under fully inducing conditions. Insertions were localized within the nifB gene, and inability to fix nitrogen was shown to be due to disruption of the nifB gene. However, enhanced ammonium repression proved to be the result of constitutive expression of the downstream nifO gene from an `out' promoter present in Tn5. Our results suggest that molybdenum metabolism might function as a regulatory factor that acts through the nitrate reductase. Received: 4 December 1996 / Accepted: 27 March 1997     

Nitrate reductase and nitrite reductase activity in free-living cells and bacteroids of Rhizobium loti

Cells of Rhizobium loti strains T1 and U226 cultured in defined medium with glutamate as the only nitrogen source and bacteroids isolated from root nodules of Lotus corniculatus, L. pedunculatus and L. tenuis did not show constitutive (non-nitrate induced) nitrate reductase activity (NRA). In contrast, nitrite reductase activity (NiRA) was present in both free-living cells and bacteroids of either strain T1 or U226. Constitutive NRA and NiRA were detected in the cytosol fraction from nodules of all three symbioses examined. An induced NRA was expressed in bacteroids after a 10 h incubation in the presence of nitrate.     

Analysis of cytoplasmic membrane from wild type and mutant Paracoccus denitrificans containing excess nitrate reductase protein and cytochrome b

Cytoplasmic membranes were isolated from wild type and mutants strain M-1 of Paracoccus denitrificans grown with low aeration to promote synthesis of nitrate reductase protein and cytochrome b. The presence of 10-100-fold excess of nitrate reductase in the wild type or the corresponding enzymically inactive protein in the mutant did not significantly affect respiratory oxidase activities with NADH, succinate or TMPD-ascorbate as electron donor. A cytochrome b-nitrate reductase complex was resolved by isoelectric focussing of Triton X-100 solubilized membranes from the wild type grown with azide and from the mutant, whereas the enzyme complex from nitrate-grown wild type was not resolved from cytochrome c. Preparations from azideinduced wild type or from the mutant could be a suitable source of the cytochrome b associated with nitrate reductase for more detailed studies.Non standard abbreviations IEF isoelectric focussing - TMPD N, N, N, N-tetramethylphenylenediamine - SDS-PAGE Sodium dodecyl sulphate polyacrylamide gel electrophoresis     

Biochemical genetics of further chlorate resistant, molybdenum cofactor defective, conditional-lethal mutants of barley

Summary Three plants, R9201 and R11301 (from cv. Maris Mink) and R12202 (from cv. Golden Promise), were selected by screening M₂ populations of barley (Hordeum vulgare L.) seedlings (mutagenised with azide in the M₁) for resistance to 10 mM potassium chlorate. Selections R9201 and R11301 were crossed with the wild-type cv. Maris Mink and analysis of the F₂ progeny showed that one quarter lacked shoot nitrate reductase activity. These F₂ plants also withered and died in the continuous presence of nitrate as sole nitrogen source. Loss of nitrate reductase activity and withering and death were due in each case to a recessive mutation in a single nuclear gene. All F1 progeny derived from selfing selection R12202 lacked shoot nitrate reductase activity and also withered and subsequently died when maintained in the continuous presence of nitrate as sole nitrogen source. All homozygous mutant plants lacked not only shoot nitrate reductase activity but also shoot xanthine dehydrogenase activity. The plants took up nitrate, and possessed wild-type or higher levels of shoot nitrite reductase activity and NADH-cytochrome c reductase activity when treated with nitrate for 18 h. We conclude that loss of shoot nitrate reductase activity, xanthine dehydrogenase activity and withering and death, in the three mutants R9201, R11301 and R12202 is due to a mutation affecting the formation of a functional molybdenum cofactor. The mutants possessed wild-type levels of molybdenum and growth in the presence of unphysiologically high levels of molybdate did not restore shoot nitrate reductase or xanthine dehydrogenase activity. The shoot molybdenum cofactor of R9201 and of R12202 is unable to reconstitute NADPH nitrate reductase activity from extracts of the Neurospora crassa nit-1 mutant and dimerise the nitrate reductase subunits present in the respective barley mutant. The shoot molybdenum cofactor of R11301 is able to effect dimerisation of the R11301 nitrate reductase subunits and can reconstitute NADPH-nitrate reductase activity up to 40% of the wild-type molybdenum cofactor levels. The molybdenum cofactor of the roots of R9201 and R11301 is also defective. Genetic analysis demonstrated that R9201, but not R11301, is allelic to R9401 and Az34 (nar-2a), two mutants previously shown to be defective in synthesis of molybdenum cofactor. The mutations in R9401 and R9201 gave partial complementation of the nar-2a gene such that heterozygotes had higher levels of extractable nitrate reductase activity than the homozygous mutants.We conclude that: (a) the nar-2 gene locus encodes a step in molybdopterin biosynthesis; (b) the mutant R11301 represents a further locus involved in the synthesis of a functional molybdenum cofactor; (c) mutant Rl2202 is also defective in molybdopterin biosynthesis; and (d) the nar-2 gene locus and the gene locus defined by R11301 govern molybdenum cofactor biosynthesis in both shoot and root.     

Alleviation of nitrate inhibition of soybean nodulation by high inoculum does not involve bacterial nitrate metabolism

Inoculation of soybean (Glycine max. cv. Bragg) plants with high level inoculum partially alleviated the nitrate inhibition of nodule formation (3 to 4 fold), but not nodule growth. This alleviation did not require the bacterial nitrate reductase asBradyrhizobium japonicum mutant strains 110CR1 and 110CR2 (both lacking assimilatory nitrate reductase activity) gave the same results as the wild type parent 311b110. The study was carried out in the glasshouse, thereby confirming preliminary field data by Herridgeet al. (1984) using a wild type bacterial inoculant.     

Reduction of diverse electron acceptors by Aeromonas hydrophila

Aeromonas hydrophila ATCC 7966 grew anaerobically on glycerol with nitrate, fumarate, Fe(III), Co(III), or Se(VI) as the sole terminal electron acceptor, but did not ferment glycerol. Final cell yields were directly proportional to the amount of terminal electron acceptor provided. Twenty-four estuarine mesophilic aeromonads were isolated; all reduced nitrate, Fe(III), or Co(III), and five strains reduced Se(VI). Dissimilatory Fe(III) reduction by A. hydrophila may involve cytochromes. Difference spectra obtained with whole cells showed absorption maxima at wavelengths characteristic of c-type cytochromes (419, 522, and 553 nm). Hydrogen-reduced cytochromes within intact cells were oxidized by the addition of Fe(III) or nitrate. Studies with respiratory inhibitors yielded results consistent with a respiratory chain involving succinate (flavin-containing) dehydrogenase, quinones and cytochromes, and a single Fe(III) reductase. Neither anaerobic respiration nor dissimilatory metal reduction by members of the genus Aeromonas have been reported previously. Received: 24 June 1997 / Accepted: 20 October 1997     

Screening of monoclonal antibodies to scarce and labile enzymes: A functional immunoassay for isolating monoclonal antibodies to NADPH:nitrate reductase

A functional immunoassay, that has proved very useful, is described for screening and identifying monoclonal antibodies (McAbs) against scarce and labile enzymes. This method does not require purified enzyme or antigen and it has been successfully applied to isolate three hybridomas secreting McAbs to NADPH:nitrate reductase from the chloronema cells of the mossFunaria hygrometrica. Briefly, the protocol involves: adsorption of murine antibodies from hybridoma supernatants by rabbit antimouse IgG antibody pre-adsorbed toStaphylococcus aureus cells (SAC), reaction with crude extract for 15 min, sedimentation of the SAC complex by centrifugation and measurement of residual enzymatic activity in the supernatant. A depletion indicates the presence of antibodies that bind to the active enzyme. The method is rapid, sensitive and versatile enough to be used to isolate McAbs with exquisite specificities. The three isolated McAbs recognized nitrate reductase protein in a conformation-independent and/or a conformation-dependent manner.     

Molecular and Regulatory Properties of the Nitrate Reducing Systems of Rhodobacter

Phototrophic bacteria of the genus Rhodobacter possess several forms of nitrate reductase including assimilatory and dissimilatory enzymes. Assimilatory nitrate reductase from Rhodobacter capsulatus E1F1 is cytoplasmic, it uses NADH as the physiological electron donor and reduced viologens as artificial electron donors, and it is coupled to an ammonium-producing nitrite reductase. Nitrate reductase induction requires a high C/N balance and the presence of nitrate, nitrite, or nitroarenes. A periplasmic 47-kDa protein facilitates nitrate uptake, thus increasing nitrate reductase activity. Two types of dissimilatory nitrate reductases have been found in strains from Rhodobacter sphaeroides. One of them is coupled to a complete denitrifying pathway, and the other is a periplasmic protein whose physiological role seems to be the dissipation of excess reducing power, thus improving photoanaerobic growth. Periplasmic nitrate reductase does not use NADH as the physiological electron donor and is a 100-kDa heterodimeric hemoprotein that receives electrons through an electron transport chain spanning the plasma membrane. This nitrate reductase is regulated neither by the intracellular C/N balance nor by O₂ pressure. The enzyme also exhibits chlorate reductase activity, and both reaction products, nitrite and chlorite, are released almost stoichiometrically into the medium; this accounts for the high resistance to chlorate or nitrite exhibited by this bacterium. Nitrate reductases from both strains seem to be coded by genes located on megaplasmids. Received: 17 April 1996 / Accepted: 28 May 1996     

In vitro reconstitution of NADH: nitrate reductase in nitrate reductase-deficient mutants of barley

Summary In vitro complementation of the nitrate reductase-deficient barley mutant nar2a extracts with molybdenum cofactor from commercial xanthine oxidase resulted in reactivation of NADH: nitrate reductase activity. Maximum reactivation was achieved with 7.5 g/ml xanthine oxidase (final concentration), 10 mM glutathione (final concentration) and incubation for 30 min at room temperature (ca. 25°C). This in vitro complementation assay was used to determine the presence of functional apoprotein and molybdenum cofactor in 12 nitrate reductase-deficient barley mutants. Extracts of all nar1 alleles contained functional molybdenum cofactor (complemented with nar2a) but they lacked functional apoprotein (did not complement with molybdenum cofactor from xanthine oxidase). The nar2a, nar3a and nar3b extracts were able to donate functional apoprotein, but were poor sources of functional molybdenum cofactor. These data are in agreement with our previous assignment of nar1 to the barley NADH: nitrate reductase structural locus and nar2 and nar3 to molybdenum cofactor functions. Wild type cv. Steptoe barley seedlings grown in the absence of nitrate and lacking nitrate reductase activity contained low levels of molybdenum cofactor. Nitrate induction resulted in a several-fold increase in the measurable molybdenum cofactor levels that was correlated with the increase in nitrate reductase activity.Scientific Paper No. 6839. College of Agriculture Research Center, Washington State University, Pullman. Project Nos. 0430 and 0233. This work was supported in part by National Science Foundation Grant PCM 81-19096 and USDA Competitive Research Grant 82-CRCR-1-1112     

Effects of molybdenum and tungsten on induction of nitrate reductase and formate dehydrogenase in wild type and mutant Paracoccus denitrificans

Molybdenum is required for induction of nitrate reductase and of NAD-linked formate dehydrogenase activities in suspensions of wild type Paracoccus denitrificans; tungsten prevents the development of these enzyme activities. The wild type forms a membrane protein M _r150,000 when incubated with tungsten and inducers of nitrate reductase and this is presumed to represent an inactive form of the enzyme. Suspensions of mutant M-1 did not develop nitrate reductase or formate dehydrogenase activities but the membrane protein M _r150,000 was formed under all conditions tested, including without inducers and without molybdenum. Analysis of membranes, solubilized with deoxycholate, by polyacrylamide gel electrophoresis under nondenaturing conditions showed that the mutant protein had similar electrophoretic mobility to the active nitrate reductase formed by the wilde type. Autoradiography of preparations from cells incubated with ⁵⁵Fe showed that the mutant and wild type proteins contained iron. However, in similar experiments with ⁹⁹Mo, incorporation of molybdenum into the mutant protein was not detectable.We conclude that mutant M-1 is defective in one or more steps required to process molybdenum for incorporation into molybdoenzymes. This failure affects the normal regulation of nitrate reductase protein with respect to the role of inducers.Non-Standard Abbreviations DOC deoxycholate - PAGE polyacrylamide gel electrophoresis - SDS sodium dodecyl sulfate     

nir1, a conditional-lethal mutation in barley causing a defect in nitrite reduction

Summary Eleven green individuals were isolated when 95000 M₂ plants of barley (Hordeum vulgare L.), mutagenised with azide in the M₁, were screened for nitrite accumulation in their leaves after nitrate treatment in the light. The selected plants were maintained in aerated liquid culture solution containing glutamine as sole nitrogen source. Not all plants survived to flowering and some others that did were not fertile. One of the selected plants, STA3999, from the cultivar Tweed could be crossed to the wild-type cultivar and analysis of the F₂ progeny showed that leaf nitrite accumulation was due to a recessive mutation in a single nuclear gene, which has been designated Nir1. The homozygous nir1 mutant could be maintained to flowering in liquid culture with either glutamine or ammonium as sole nitrogen source, but died within 14 days after transfer to compost. The nitrite reductase cross-reacting material seen in nitrate-treated wild-type plants could not be detected in either the leaf or the root of the homozygous nir1 mutant. Nitrite reductase activity, measured with dithionite-reduced methyl viologen as electron donor, of the nitrate-treated homozygous nir1 mutant was much reduced but NADH-nitrate reductase activity was elevated compared to wild-type plants. We conclude that the Nir1 locus determines the formation of nitrite reductase apoprotein in both the leaf and root of barley and speculate that it represents either the nitrite reductase apoprotein gene locus or, less likely, a regulatory locus whose product is required for the synthesis of nitrite reductase, but not nitrate reductase. Elevation of NADH-nitrate reductase activity in the nir1 mutant suggests a regulatory perturbation in the expression of the Narl gene.     

Inheritance and expression of NAD(P)H nitrate reductase in barley

Summary NADH-specific and NAD(P)H bispecific nitrate reductases are present in barley (Hordeum vulgare L.). Wild-type leaves have only the NADH-specific enzyme while mutants with defects in the NADH nitrate reductase structural gene (nar1) have the NAD(P)H bispecific enzyme. A mutant deficient in the NAD(P)H nitrate reductase was isolated in a line (nar1a) deficient in the NADH nitrate reductase structural gene. The double mutant (nar1a;nar7w) lacks NAD(P)H nitrate reductase activity and has xanthine dehydrogenase and nitrite reductase activities similar to nar1a. NAD(P)H nitrate reductase activity in this mutant is controlled by a single codominant gene designated nar7. The nar7 locus appears to be the NAD(P)H nitrate reductase structural gene and is not closely linked to nar1. From segregating progeny of a cross between the wild type and nar1a;nar7w, a line was obtained which has the same NADH nitrate reductase activity as the wild type in both the roots and leaves but lacks NADPH nitrate reductase activity in the roots. This line is assumed to have the genotype Nar1Nar1nar7nar7. Roots of wild type seedlings have both nitrate reductases as shown by differential inactivation of the NADH and NAD(P)H nitrate reductases by a monospecific NADH-nitrate reductase antiserum. Thus, nar7 controls the NAD(P)H nitrate reductase in roots and in leaves of barley.Scientific Paper No. 7617, College of Agriculture Research Center and Home Economics, Washington State University, Pullman, WA, USA. Project Nos. 0233 and 0745     

Effect of DCMU, Simazine and Atrazine on Nitrate Reductase Activity in Hordeum vulgare in vitro

The effect of three herbicides—DCMU (1,1-dimethyl-3- (3,4-dichlorophenyl) -urea), Simazine (2,4-bis(ethylamino)- 6-chloro-s-triazine), and Atrazine (2-chloro-4-ethylamino-6-iso-propylamino-5-triazine)—on the induction of nitrate reduc–tase and its in vivo activity was studied in detached leaves of Hordeum vulgare L. All increased both extractable nitrate reductase activity and nitrate content. The increases occurred at optimum temperatures for growth and at several concentrations of nitrate. It was also determined that the herbicides did not protect the enzyme against inactivation in vivo. Although the extractable nitrate reductase was greater, the in vivo activity of nitrate reductase was decreased in the presence of the herbicides resulting in a higher internal concentration of nitrate. Since in viva nitrate reduction is dependent upon photosynthesis it is reasonable that reduction is decreased by these known inhibitors of photosynthesis. Hence, the effect of the inhibitors on induction of nitrate reductase activity may be secondary. The higher concentration of nitrate resulting from a decreased rate of in vivo reduction in the presence of the inhibitors could conceivably be responsible for the greater corutent of nitrate reductase.     

Isolation of the transposable element hupfer from the entomopathogenic fungus Beauveria bassiana by insertion mutagenesis of the nitrate reductase structural gene

A transposable element has been isolated from the entomopathogenic fungus Beauveria bassiana by trapping it in the nitrate reductase structural gene, which has been cloned from this species. The element had inserted in the first exon of the nia gene and appeared to have duplicated the sequence TA at the site of insertion. It was 3336 bp long with 30-bp imperfect, inverted, terminal repeats. The element, called hupfer, contained an open reading frame encoding a 321-amino acid protein similar to the IS630- or mariner-Tc1-like transposases, and a residual sequence of about 2 kb which was not significantly similar to any published sequence. There are fewer than five copies of this transposable element present per genome in the fungus. Received: 12 February 1997 / Accepted: 2 May 1997     

Reassessment of the in vivo Assay for Nitrate Reductase in Leaves

The in vivo assay procedure for nitrate reductase and its dependence on the concentration of nitrate and other ions were examined. It was found that high ion concentrations led to an increased release of nitrite to the reaction media which could be interpreted as a stimulated nitrate reductase activity. This phenomenon is not an osmotic effect, since equivalent concentrations of mannitol did not lead to identical results. The effect of ions on the enhanced nitrite production was attributed to changes in cell membrane permeability rather than to a supply of substrate. This conclusion is based on several findings: (a) in in vitro assays, the rate of nitrite production was not affected by ion concentrations: (b) the stimulation of nitrite production was obtained by various ions and not only by nitrate; (c) pretreatment of alfalfa leaves with nitrate did not increase the NO₂^? release rate to the external solution; and (d) nitrate and nitrite export from leaf discs to the solution was stimulated even in discs which were enzymatically inactive. Calcium ions in the presence of KNO₃ inhibited the enhanced nitrite production, probably due to alteration of membrane stability. The effect of ions on the rate of nitrite production was reversible and the high rate of nitrite production was reduced to the control rate when discs were transferred to a solution of low concentration.