Biochemical studies on the nit mutants of Neurospora crassa

Xanthine Oxidase-A Personal History

A personal perspective is provided regarding the work in several laboratories, including the author's, that has established the reaction mechanism of xanthine oxidase and related enzymes.

Isolation and characterization of a methylammonium resistant mutant of Neurospora crassa

A mutant of Neurospora crassa has been isolated which is resistant to methylammonium, a structural analog of ammonium. In contrast to wild type, this mutant, mea-1, has derepressed nitrate reductase and nitrite reductase activities in the presence of ammonium. However, glutamine still represses these nitrate assimilation enzymes in mea-1. The nit-2 mutant was epistatic to mea-1 since the mea-1; nit-2 double mutant has the nit-2 mutant phenotype. In addition, mea-1; nit-2 double mutants cannot utilize ammonium as a nitrogen source. We suggest therefore that nit-2 and mea-1 loci play a role in ammonia/methylamine uptake.

A Neurospora crassa mutant altered in the regulation of L-amino acid oxidase

The isolation and characterization of a Neurospora crassa mutant altered in L-amino oxidase regulation is reported. The previously isolated gln-1bR8 strain, which only synthesizes the glutamine synthetase alpha monomer and lacks the beta monomer, was used as parental strain. A mutant derivative of strain was selected for its ability to grow on minimal medium in the presence of DL-methionine-SR-sulfoximine (MSO), an inhibitor of glutamine synthetase activity. This gln-1bR8;MSOR mutant overcame the inhibitory effect of MSO by increasing the activity of L-amino acid oxidase, an enzyme capable of degrading this compound. In contrast with the wild-type strain, the L-amino acid oxidase of the MSOR mutant was resistant to glutamine repression; in fact, it was induced by this amino acid but repressed by ammonium. This mutant is different from other nitrogen regulatory N. crassa mutants reported and is only altered in the regulation of L-amino acid oxidase. The MSOR mutation is epistatic to nit-2 since the nit2;MSOR double mutant regulated the L-amino acid oxidase in the same way as the MSOR single mutant.

Regulation of ammonium ion assimilation enzymes in Neurospora crassa nit-2 and ms-5 mutant strains

In Neurospora crassa the nit-2 and nmr-1 (ms-5) loci represent the major control genes encoding regulatory proteins that allow the coordinated expression of various systems involved with the utilization of a secondary nitrogen source. In this paper we examined the effect of the nit-2 and ms-5 (nmr-1 locus) mutations on the regulation of the ammonium assimilation enzymes, glutamine synthetase and glutamate dehydrogenase, which are regulated by the products of these genes; however, glutamate synthase is not so regulated. Glutamine synthetase and glutamate dehydrogenase levels are also regulated by the amino nitrogen content. We present evidence that the ms-5 and glnr strains, which behave very similarly in their resistance to glutamine repression, are different and map in different loci.

Molecular cloning and analysis of the regulation of nit-3, the structural gene for nitrate reductase in Neurospora crassa

The nit-3 gene of Neurospora crassa encodes the enzyme nitrate reductase and is regulated by nitrogen catabolite repression and by specific induction with nitrate. The nit-3 gene was isolated from a cosmid-based genomic library by dual selection for benomyl resistance and for the ability to complement a nit-3 mutant strain using the sibling-selection procedure. The nit-3 gene was subcloned as a 3.8-kilobase DNA fragment from a cosmid that carried an approximately 40-kilobase N. crassa DNA insert. A restriction fragment length polymorphism analysis revealed that the cloned segment displayed tight linkage to two linkage-group-4 markers that flank the genomic location of nit-3. RNA gel blot analyses of RNA from wild-type and various mutant strains were carried out to examine the molecular mechanism for regulation of nit-3 gene expression. The nit-3 gene was transcribed to give a large mRNA of approximately 3.4 kilobases, the expected size to encode nitrate reductase. The nit-3 gene was only expressed in wild-type cells subject to simultaneous nitrogen derepression and nitrate induction. A mutant of nit-2, the major nitrogen regulatory gene of Neurospora, did not have detectable levels of nit-3 gene transcripts under the exact conditions in which these transcripts were highly expressed in wild type. Similarly, a mutant of nit-4, which defines a minor positive-acting nitrogen control gene, failed to express detectable levels of the nit-3 transcript. Nitrate reductase gene expression was partially resistant to nitrogen repression in a mutant of the nmr gene, which appears to be a regulatory gene with a direct role in nitrogen catabolite repression. Results are presented that suggest that the enzyme glutamine synthetase does not serve any regulatory role in controlling nitrate reductase gene expression.

Nitrogen regulation of amino acid utilization by Neurospora crassa

The production of an extracellular deaminase activity involved with the utilization of amino acids as sole sources of nitrogen is under the control of the nit-2 locus of Neurospora crassa. This locus is the sole major nitrogen regulatory locus described for N. crassa and is believed to encode a positive effector required for induction of activities involved with the utilization of alternate nitrogen sources. Production of deaminase activity requires the lifting of nitrogen metabolite repression, the presence of a functional nit-2 gene product, and specific induction by amino acids. Additional parameters of enzyme production are described.

Regulation of amino acid utilization in Neurospora crassa: effect of nmr-1 and ms-5 mutations

The effect of the nmr-1 and ms-5 mutations, which lead to insensitivity to glutamine-mediated nitrogen metabolite repression, was examined with respect to extracellular deaminase production by Neurospora crassa. Deaminase production normally requires nitrogen limitation, but these mutations eliminated this requirement and allowed production of deaminase activity under nitrogen metabolite repressing conditions. Demonstration of normal glutamine transport by both strains eliminated the possibility that these mutations exerted their effects through repressor exclusion. We have proposed a new working model for nitrogen regulation in Neurospora based on the findings that these mutations affected a nitrogen-regulated activity in addition to those activities originally reported and that the mutations are genetically very closely linked and likely allelic.

Quantitative transfer of the molybdenum cofactor from xanthine oxidase and from sulphite oxidase to the deficient enzyme of the nit-1 mutant of Neurospora crassa to yield active nitrate reductase

An assay method is described for measurement of absolute concentrations of the molybdenum cofactor, based on complementation of the defective nitrate reductase ('apo nitrate reductase') in extracts of the nit-1 mutant of Neurospora crassa. A number of alternative methods are described for preparing, anaerobically, molybdenum-cofactor-containing solutions from sulphite oxidase, xanthine oxidase and desulpho xanthine oxidase. For assay, these were mixed with an excess of extract of the nit-1 mutant, incubated for 24 h at 3.5 degrees C then assayed for NADPH:nitrate reductase activity. In all cases, the specific activity of the molybdenum cofactor, expressed as mumol of NO2-formed/min per ng-atom of Mo added from the denatured molybdoenzyme , was 25 +/- 4, a value that agrees with the known catalytic activity of the nitrate reductase of wild-type Neurospora crassa. This indicates that, under our conditions, there was quantitative transfer of the molybdenum cofactor from denatured molybdoenzyme to yield fully active nitrate reductase. Comparable cofactor assay methods of previous workers, apparently indicating transfer efficiencies of at best a few per cent, have never excluded satisfactorily the possibility that cofactor activity arose, not from stoichiometric constituents of the molybdoenzymes , but from contaminants. The following factors were investigated separately in developing the assay:the efficiency of extraction of the cofactor from the original enzyme, the efficiency of the complementation reaction between cofactor and apo nitrate reductase, and the assay of the resultant nitrate reductase, which must be carried out under non-inhibitory conditions. Though the cofactor is unstable in air (t1/2 about 15 min at 3.5 degrees C), it is stable when kept anaerobic in the presence of sodium dithionite, in aqueous solution or in dimethyl sulphoxide (activity lost at the rate of about 3%/24 h at 20-25 degrees C). Studies of stabilities, and investigations of the effect of added molybdate on the assay, permit conclusions to be drawn about the ligation of molybdenum to the cofactor and about steps in incorporation of the cofactor into the apoenzyme. Though the development of nitrate reductase activity is slow at 3.5 degrees C (t1/2 1.5-3 h) the complementation reaction may be carried out in high yield, aerobically. This is ascribed to rapid formation of an air-stable but catalytically inactive complex of the cofactor, as a precursor of the active nitrate reductase.(ABSTRACT TRUNCATED AT 400 WORDS)

A regulatory phenotype associated with the en-am 1 mutant of Neurospora crassa

We have investigated the nature of the en-am1 mutant of Neurospora crassa and have found that it affects the regulation of proline oxidase and utilisation of other nitrogen sources. This mutant is closely linked to the gln gene but not allelic with it. Data from crosses suggest that the two genes he on opposite sides of the in1 gene on linkage group VR.

Reverse mutants of the nitrate reductase-deficient mutant B 25 of Arabidopsis thaliana

Revertants of B25, a nitrate reductase-deficient mutant of Arabidopsis thaliana (L.) Heinh, were isolated with a high frequency. All 7 independently arisen revertants were mutations in the same suppressor gene su, which is unlinked to the originally mutated gene rgn. The mutant character shows up both in growth on nitrate as the sole nitrogen source and in susceptibility to chlorate. When judged for these properties the mutant alleles are either dominant for both, recessive for both or dominant for growth on nitrate and recessive for the effect of chlorate, when compared to the wildtype allele. Whereas the original mutant B25 exhibits no or very little nitrate reductase activity, the activities of the revenants were in the range of 0.4 to 1.5 of the wildtype activity. Physiological characteristics of nitrate reductase from the revertants are the same as those from the wildtype. Probably rgn is not the structural gene for nitrate reductase. The ability to assemble the nitrate reductase complex from its subunits, which was absent in mutant B25, appears to have been restored in the revertants.

Regulation of the nitrate-reducing system enzymes in wild-type and mutant strains of Chlamydomonas reinhardii

Six mutant strains (301, 102, 203, 104, 305, and 307) affected in their nitrate assimilation capability and their corresponding parental wild-type strains (6145c and 21gr) from Chlamydomonas reinhardii have been studied on different nitrogen sources with respect to NAD(P)H-nitrate reductase and its associated activities (NAD(P)H-cytochrome c reductase and reduced benzyl viologen-nitrate reductase) and to nitrite reductase activity. The mutant strains lack NAD(P)H-nitrate reductase activity in all the nitrogen sources. Mutants 301, 102, 104, and 307 have only NAD(P)H-cytochrome c reductase activity whereas mutant 305 solely has reduced benzyl viologen-nitrate reductase activity. Both activities are repressible by ammonia but, in contrast to the nitrate reductase complex of wild-type strains, require neither nitrate nor nitrite for their induction. Moreover, the enzyme from mutant 305 is always obtained in active form whereas nitrate reductase from wild-types needs to be reactivated previously with ferricyanide to be fully detected. Wild-type strains and mutants 301, 102, 104, and 307, when properly induced, exhibit an NAD(P)H-cytochrome c reductase distinguishable electrophoretically from constitutive diaphorases as a rapidly migrating band. Nitrite reductase from wild-type and mutant strains is also repressible by ammonia and does not require nitrate or nitrite for its synthesis. These facts are explained in terms of a regulation of nitrate reductase synthesis by the enzyme itself.

Regulation of L-amino acid oxidase and of D-amino acid oxidase in Neurospora crassa

Neurospora crassa possesses an inducible L-amino acid oxidase that is expressed only when cells are derepressed for nitrogen in the presence of an amino acid. Enzyme synthesis requires both induction by an amino acid and simultaneous nitrogen catabolite derepression. Carbon limition in the presence of an amino acid does not permit induction of L-amino acid oxidase. The nit-2 gene is a major regulatory locus which is believed to mediate nitrogen catabolite repression in Neurospora. Mutants of nit-2 are repressed for L-amino acid oxidase activity under conditions which lead to good enzyme induction in wild type and nit-2 revertants. The loss of the enzyme in nit-2 mutants does not result from inducer exclusion, which suggests that the nit-2 gene product has a direct role in controlling the expression of this enzyme. Substantial amounts of L-amino acid oxidase were detected in the growth medium as well as in cell extracts of the wild type strain. Biochemical data indicates that the intracellular and the extracellular L-amino acid oxidases are identical. Inhibitors of protein and of RNA synthesis block accumulation of L-amino acid oxidase, suggesting that enzyme expression is controlled at the level of transcription. D-amino acid oxidase can be detected in cell extracts of Neurospora grown in the presence of a D-amino acid. The enzyme is present in cys-3 mutants and is not repressed by high concentrations of sulfate or nitrogen indicating that D-amino acid oxidase is not a member of the sulfur or nitrogen regulatory circuits of this organism.

Regulation of L-phenylalanine ammonia-lyase by L-phenylalanine and nitrogen in Neurospora crassa

Neurospora crassa possesses an inducible L-phenylalanine ammonia-lyase that is expressed only when cells are derepressed for nitrogen in the presence of L-phenylalanine. Enzyme synthesis requires both induction by L-phenylalanine and simultaneous nitrogen catabolite derepression. Carbon limitation in the presence of phenylalanine does not elicit induction of L-phenylalanine ammonia-lyase. Specific induction by L-phenylalanine is required, and other amino acids completely failed to induce any lyase activity. The nit-2 gene is a major regulatory locus which is believed to mediate nitrogen catabolite repression in Neurospora. Mutants of nit-2 fail to express any phenylalanine ammonia-lyase activity under conditions of derepression and induction which lead to good enzyme induction in the wild type and in nit-2 revertants. The loss of lyase activity in nit-2 mutants does not result from inducer exclusion, which suggests that the nit-2 gene product has a direct role in controlling the expression of this enzyme. Substantial amounts of the enzyme were detected in the growth medium as well as in cell extracts. Inhibitors of protein synthesis or RNA synthesis block the induction of L-phenylalanine ammonia-lyase, suggesting that expression of this enzyme is controlled at the level of transcription.

Isolation and characterization of nitrate reductase-deficient mutants of Arabidopsis thaliana

Chlorate resistant mutants of Arabidopsis thaliana were isolated, of which 10 exhibited a lowered nitrate reductase activity and 51 were chlorate-resistant because of an impaired uptake of chlorate. The 51 mutants of this type are all affected in the same gene. The mutants with a lowered nitrate reductase activity fall into 7 different complementation groups. Three of these mutants grow poorly on media with nitrate as the sole nitrogen source, while the others apparently can reduce sufficient nitrate to bring about growth. In all cases a low nitrate reductase activity coincides with an enhanced nitrite reductase activity. After sucrose gradient centrifugation of wildtype extracts nitrate reductase is found at the 8S position, whereas cytochrome-c reductase is found both at 4 and 8S positions. It is suggested that the functional nitrate reductase is a complex consisting of 4S subunits showing cytochrome-c reductase activity and a Mo-bearing cofactor. All mutants except B25 are capable of assembling the 4S subunits into complexes which for most mutants have a lower S value and exhibit a lower nitrate reductase activity than the wildtype complexes. Since the mutants B25 and B73 exhibit a low xanthine dehydrogenase activity, the Mo-bearing cofactor is probably less available in these mutants than in the wildtype. B73 appears to be the only mutant which is partly repaired by excessive Mo. The possible role of several genes is discussed.

Biochemical analysis of mutants defective in nitrate assimilation in Neurospora crassa: evidence for autogenous control by nitrate reductase

A biochemical analysis of mutants altered for nitrate assimilation in Neurospora crassa is described. Mutant alleles at each of the nine nit (nitrate-nonutilizing) loci were assayed for nitrite reductase activity, for three partial activities of nitrate reductase, and for nitrite reductase activity. In each case, the enzyme deficiency was consistent with data obtained from growth tests and complementation tests in previous studies. The mutant strains at these nit loci were also examined for altered regulation of enzyme synthesis. Such experiments revealed that mutations which affect the structural integrity of the native nitrate reductase molecule can result in constitutive synthesis of this enzyme protein and of nitrite reductase. These results provide very strong evidence that, as in Aspergillus nidulans, nitrate reductase autogenously regulates the pathway of nitrate assimilation. However, only mutants at the nit-2 locus affect the regulation of this pathway by nitrogen metabolite repression.

The regulation of nitrate assimilation in Neurospora crassa: biochemical analysis of the nmr-1 mutants

Neurospora crassa nmr-1 mutants, selected on the basis of their sensitivity to chlorate in the presence of glutamine, have elevated levels of the nitrate assimilation enzymes, NADPH-nitrate reductase and NAD(P)H-nitrite reductase. Immunoelectrophoretic determinations show that the higher nitrate reductase activities in nmr-1 mutants are due to greater enzyme concentrations. The half-life of nitrate reductase in these mutants is unaltered. As in wild-type, expression of nitrate assimilation in nmr-1 mutants is dependent on induction by nitrate. Reduced nitrogen metabolites like ammonium and glutamine still repress this expression in nmr-1 mutants, but not as effectively as in wild-type. Enzymatic activity measurements in double mutant strains confirm that the nit regulatory loci, nit-2 and nit-4/5, are epistatic to nmr-1, but nmr-1 is epistatic to nit-3, the nitrate reductase structural gene. The results imply that nmr-1 is involved in post-transcriptional control of nitrate assimilation.

The regulation of nitrate assimilation in Neurospora crassa: the isolation and genetic analysis of nmr-1 mutants

Four mutants of Neurospora crassa have been isolated which have altered regulation of nitrate reductase. They each carry a mutation which results in derepressed synthesis of nitrate reductase even in the presence of glutamine. They map to a single locus which has been designated nmr-1 and which is located between am and gln on linkage group VR. The mutations appear to affect only nitrate assimilation. The nit-2, nit-3 and nit-4/5 mutations are epistatic to nmr-1 since the double mutants have the single nit mutant phenotype. For nitrate reductase synthesis, the nmr-1 mutation is epistatic to am such that the double mutant is derepressed even in the presence of glutamate or glutamine. In all other respects however, the double mutant exhibits the am phenotype. We suggest therefore that the nmr-1 mutations do not directly affect the regulation of nitrate reductase at the level of transcription but instead act post-transcriptionally.

In vitro complementation of assimilatory NAD(P)H-nitrate reductase from mutants of Chlamydomonas reinhardii

In vitro complementation of the soluble assimilatory NAD(P)H-nitrate reductase (NAD(P)H:nitrate oxidoreductase, EC 1.6.6.2) was attained by mixing cell-free preparations of Chlamydomonas reinhardii mutant 104, uniquely possessing nitrate-inducible NAD(P)H-cytochrome c reductase, and mutant 305 which possesses solely the nitrate-inducible FMNH2- and reduced benzyl viologen-nitrate reductase activities. Full activity and integrity of NAD(P)H-cytochrome c reductase from mutant 104 and reduced benzyl viologen-nitrate reductase from mutant 305 are needed for the complementation to take place. A constitutive and heat-labile molybdenum-containing cofactor, that reconstitutes the NAD(P)H-nitrate reductase activity of nit-1 Neurospora crassa but is incapable of complementing with 104 from C. reinhardii, is present in the wild type and 305 algal strains. The complemented NAD(P)H-nitrate reductase has been purified 100-fold and was found to be similar to the wild enzyme in sucrose density sedimentation, molecular size, pH optimum, kinetic parameters, substrate affinity and sensitivity to inhibitors and temperature. From previous data and data presented in this article on 104 and 305 mutant activities, it is concluded that C. reinhardii NAD(P)H-nitrate reductase is a heteromultimeric complex consisting of, at least, two types of subunits separately responsible for the NAD(P)H-cytochrome c reductase and the reduced benzyl viologen-nitrate reductase activities.

Physiological characterization of a Neurospora crassa mutant with impaired regulation of nitrate reductase

This report describes the isolation and characterization of a Neurospora crassa mutant with an impaired regulation of nitrate reductase. Glutamine, which prevents the induction of nitrate reductase in N. crassa, did so relatively ineffectively in this mutant. The mutation did not affect the regulation of all enzymes regulated by "nitrogen metabolite regulation"; it did affect the regulation of nitrate reductase, nitrite reductase, histidase, and acetamidase, as well as that of thiourea sensitivity. The mutation was not allelic with nit-2, the gene controlling a general positive effector of nitrogen metabolite-regulated enzyme formation.

The roles of nitrate and ammonium in the regulation of the development of nitrate reductase in Chlamydomonas reinhardii

The regulation of the development of nitrate reductase (NR) activity in Chlamydomonas reinhardii has been compared in a wild-type strain and in a mutant (nit-A) which possesses a modified nitrate reductase enzyme that is non-functional in vivo. The modified enzyme cannot use NAD(P)H as an electron donor for nitrate reduction and it differs from wild-type enzyme in that NR activity is not inactivated in vitro by incubation with NAD(P)H and small quantities of cyanide; it is inactivated when reduced benzyl viologen or flavin mononucleotide is present. After short periods of nitrogen starvation mutant organisms contain much higher levels of terminal-NR activity than do similarly treated wild-type ones. Despite the inability of the mutant to utilize nitrate, no nitrate or nitrite was found in nitrogen-starved cultures; it is therefore concluded that the appearance of NR activity is not a consequence of nitrification. After prolonged nitrogen starvation (22 h) the NR level in the mutant is low. It increases rapidly if nitrate is then added and this increase in activity does not occur in the presence of ammonium, tungstate or cycloheximide. Disappearance of preformed NR activity is stimulated by addition of tungstate and even more by addition of ammonium. The results are interpreted as evidence for a continuous turnover of NR in cells of the mutant with ammonium both stimulating NR breakdown and stopping NR synthesis. Nitrate protects the enzyme from breakdown. Reversible inactivation of NR activity is thought to play an insignificant rôle in the mutant.

Nitrogen metabolite repression of nitrate reductase in Neurospora crassa: effect of the gln-1a locus

Nicotinamide adenine dinucleotide phosphate (reduced form)-nitrate reductase was freed from ammonium repression in a Neurospora crassa mutant having drastically lowered glutamine synthetase activity, gln-1a. The general phenomenon of nitrogen metabolite repression required glutamine or some aspect of glutamine metabolism.

Further characterization of the reduced nicotinamide adenine dinucleotide phosphate: nitrate oxidoreductase in Aspergillus nidulans

The reduced nicotinamide adenine dinucleotide phosphate (NADPH):nitrate oxidoreductase (EC 1.6.6.2) from Aspergillus nidulans wild-type bi-1 was purified by means of salt fractionation, gel filtration, affinity chromatography, and polyacrylamide gel electrophoresis. Enzyme which was adsorbed on Cibacron blue agarose could be eluted with 2 mM NADPH or 2 mM oxidized NADP (NADP(+)), the former being about three times more effective than the latter. About half the total NADPH:nitrate reductase activity adsorbed on agarose required elution with 1 M NaCl. This salt-elutable form remained active with NADPH and was not converted to the NADPH-elutable form after readsorption on Cibacron blue agarose. The NADPH-eluted enzyme exhibited a markedly different electrophoretic mobility than the enzyme eluted with NADP(+) or NaCl. After electrophoresis on polyacrylamide gels, the NADPH-eluted NADPH:nitrate reductase was separated into four proteins, two of which contained nonheme iron and exhibited reduced methyl viologen-nitrate reductase activity. None of these proteins, singly or in combination, reduced nitrate with NADPH as substrate. Difference spectra analyses and specific heme iron stains revealed the presence of cytochrome b(557) in the largest of the proteins. The molecular weights of the four proteins, which were determined from the relationship of their mobilities on varied concentrations of acrylamide gel, were 360,000, 300,000, 240,000, and 118,000. The subunit molecular weights of these, which are determined via sodium dodecyl sulfate slab gel electrophoresis, were 49,000, 50,000, and 75,000. The key role of NADPH in maintenance of the active form of the heteromultimer is further substantiated.

Demonstration in vitro of two intracellular inactivators of nitrate reductase from Neurospora

Two different inactivators of nitrate reductase have been found in cell free preparations of Neurospora. The first (Inactivator I) is very active at pH 9, is inhibited by disodium ethylene diamine tetraacetate (EDTA) and is present in all mycelia incubated under all conditions tested; the second (Inactivator II) is very active at pH 5, is repressed by ammonia or by a metabolic product of ammonia and derepressed by nitrogen starvation, cannot be derepressed by nitrogen starvation in strain nit-2, in which a number of "ammonia-represible" enzymes are permanently repressed, and is sensitive to phenyl methyl sulfonyl fluoride. Crude extracts of mycelia contain inhibitor(s) of both inactivators.

Nitrogen regulation of amino acid catabolism in Neurospora crassa

Neurospora crassa can utilize numerous compounds including certain amino acids as a sole nitrogen source. Mutants of the nit-2 locus, a regulatory gene which is postulated to mediate nitrogen catabolite repression, are deficient in the ability to utilize several amino acids as well as other nitrogen sources used by wild type. Various enzymes involved in amino acid catabolism were found to be regulated in distinct ways. Arginase, ornithine transaminase, and pyrroline-5-carboxylate dehydrogenase are all inducible enzymes but are not subject to nitrogen catabolite repression. By contrast, proline oxidase and the amino acid transport system(s) are controlled by nitrogen repression and their synthesis is increased markedly when nitrogen source is limiting. Unlike wild type, the nit-2 mutant cannot derepress amino acid transport, although proline oxidase is regulated in a normal fashion.

Formation of NADPH-nitrate reductase activity in vitro from Aspergillus nidulans niaD and cnx mutants

Mutants of A. nidulans at several loci lack detectable NADPH-nitrate reductase activity. These loci include niaD, the structural gene for the nitrate reductase polypeptide, and five other loci termed cnxABC, E, F, G and H which are presumed to be involved in the formation of a molybdenum-containing component (MCC) necessary for nitrate reductase activity. When forzen mycelia from A. nidulans deletion mutant niaD26 were homogenized in a Ten Broeck homogenizer together with frozen mycelia from either cnxA6, cnxE29, cnsF12, cnxG4 or cnxH3 strains grown on urea + nitrate as the nitrogen source, nitrate reductase activity was detectable in the extract. Similar results were obtained by co-homogenizind niaD mycelia with Neurospora crassa nit-1 mycelia induced on nitrate. Thus, all A. nidulans cnx mutants are similar to the N. crassa nit-1 strain in their capacity to yield NADPH-nitrate reductase in the presence of the presumed MCC. As judged by the amounts of nitrate reductase formed, niaD26 mycelia grown on urea +/- nitrate contained much more available MCC than ammonium-grown mycelia. No NADPH-nitrate reductase activity was found in extracts prepared by co-homogenizing mycelia from all five A. nidulans cnx strains. Wild-type A. nidulans NADPH-nitrate reductase acid dissociated by adjustment to pH 2.0-2.5 AND RE-ADJUSTED TO PH 7 could itself re-assemble to form active nitrate reductase and thus was not a useful source of MCC for these experiments. These results are consistent with the conclusion that the active nitrate reductase complex is composed of polypeptide components which are the niaD gene product, plus the MCC which is formed through the combined action of the cnx gene products. Further, the production of MCC may be regulated in response to the nitrogen nutrition available to the organism.