Identification of the product of the major regulatory gene of the nitrogen control circuit of Neurospora crassa as a nuclear DNA-binding protein

Nitrogen Catabolite Repression in members of Paracoccidioides complex

Paracoccidioides complex is a genus that comprises pathogenic fungi which are responsible by systemic disease Paracoccidioidomycosis. In host tissues, pathogenic fungi need to acquire nutrients in order to survive, making the uptake of nitrogen essential for their establishment and dissemination. Nitrogen utilization is employed by the alleviation of Nitrogen Catabolite Repression (NCR) which ensures the use of non-preferential or alternative nitrogen sources when preferential sources are not available. NCR is controlled by GATA transcription factors which act through GATA binding sites on promoter regions in NCR-sensitive genes. This process is responsible for encoding proteins involved with the scavenge, uptake and catabolism of a wide variety of non-preferential nitrogen sources. In this work, we predict the existence of AreA GATA transcription factor and feature the zinc finger domain by three-dimensional structure in Paracoccidioides. Furthermore, we demonstrate the putative genes involved with NCR response by means of in silico analysis. The gene expression profile under NCR conditions was evaluated. Demonstrating that P. lutzii supported transcriptional regulation and alleviated NCR in non-preferential nitrogen-dependent medium. The elucidation of NCR in members of Paracoccidioides complex will provide new knowledge about survival, dissemination and virulence for these pathogens with regard to nitrogen-scavenging strategies in the interactions of host-pathogens.

Natural Variation of the Circadian Clock in Neurospora

Most living organisms on earth experience daily and expected changes from the rotation of the earth. For an organism, the ability to predict and prepare for incoming stresses or resources is a very important skill for survival. This cellular process of measuring daily time of the day is collectively called the circadian clock. Because of its fundamental role in survival in nature, there is a great interest in studying the natural variation of the circadian clock. However, characterizing the genetic and molecular mechanisms underlying natural variation of circadian clocks remains a challenging task. In this chapter, we will summarize the progress in studying natural variation of the circadian clock in the successful eukaryotic model Neurospora, which led to discovering many design principles of the molecular mechanisms of the eukaryotic circadian clock. Despite the success of the system in revealing the molecular mechanisms of the circadian clock, Neurospora has not been utilized to extensively study natural variation. We will review the challenges that hindered the natural variation studies in Neurospora, and how they were overcome. We will also review the advantages of Neurospora for natural variation studies. Since Neurospora is the model fungal species for circadian study, it represents over 5 million species of fungi on earth. These fungi play important roles in ecosystems on earth, and as such Neurospora could serve as an important model for understanding the ecological role of natural variation in fungal circadian clocks.

Trans-nuclear action of the nit-2 regulatory gene product and study of two additional nitrogen control genes in Neurospora crassa

The nit-2 gene of Neurospora crassa is a major regulatory gene for control of nitrogen metabolism. Synthesis of the enzyme L-amino acid oxidase requires a functional nit-2 gene product and is also controlled by amino acid induction and nitrogen catabolite repression. Electrophoretic variants of L-amino acid oxidase have been employed to demonstrate that in heterokaryons, a nit-2 (+) gene product can turn on the expression of this enzyme in its own nucleus and also in nuclei that possess a nit-2 mutant. This trans-nuclear effect is only partial since the variant coded for in the nucleus containing the nit-2 mutant allele is always present in lower amounts than the alternative form.Two additional putative nitrogen control genes, MS5 and en(am)1, have been found to have clear effects upon the expression of L-amino acid oxidase. The en(am)1 mutant appears to result in an unusual case of reversal of the control present in wild-type: the enzyme is expressed in a constitutive fashion and inducers, required for enzyme synthesis in wild-type, actually reduce the level of L-amino acid oxidase in en(am)1. The MS5 mutant shows a substantial release from the usual nitrogen catabolite repression exerted by glutamine in wild-type.

Modulation of the ligand binding properties of the transcription repressor NmrA by GATA-containing DNA and site-directed mutagenesis

NmrA is a negative transcription-regulating protein that binds to the C-terminal region of the GATA transcription-activating protein AreA. The proposed molecular mechanism of action for NmrA is to inhibit AreA binding to its target promoters. In contrast to this proposal, we report that a C-terminal fragment of AreA can bind individually to GATA-containing DNA and NmrA and that in the presence of a mixture of GATA-containing DNA and NmrA, the AreA fragment binds preferentially to the GATA-containing DNA in vitro. These observations are consistent with NmrA acting by an indirect route, such as by controlling entry into the nucleus. Deletion of the final nine amino acids of a C-terminal fragment of AreA does not affect NmrA binding. Wild-type NmrA binds NAD(+)(P+) with much greater affinity than NAD(P)H, despite the lack of the consensus GXXGXXG dinucleotide-binding motif. However, introducing the GXXGXXG sequence into the NmrA double mutant N12G/A18G causes an approximately 13-fold increase in the KD for NAD+ and a 2.3-fold increase for NADP+. An H37W mutant in NmrA designed to increase the interaction with the adenine ring of NAD+ has a decrease in KD of approximately 4.5-fold for NAD+ and a marginal 24% increase for NADP+. The crystal structure of the N12G/A18G mutant protein shows changes in main chain position as well as repositioning of H37, which disrupts contacts with the adenine ring of NAD+, changes which are predicted to reduce the binding affinity for this dinucleotide. The substitutions E193Q/D195N or Q202E/F204Y in the C-terminal domain of NmrA reduced the affinity for a C-terminal fragment of AreA, implying that this region of the protein interacts with AreA.

Biochemical pathways and mechanisms nitrogen, amino acid, and carbon metabolism

For both nitrogen and carbon metabolism there exist specific regulatory mechanisms to enable cells to assimilate a wide variety of nitrogen and carbon sources. Superimposed are regulatory circuits, the so called nitrogen and carbon catabolite regulation, to allow for selective use of "rich" sources first and "poor" sources later. Evidence points to the importance of specific regulatory mechanisms for short term adaptations, while generalized control circuits are used for long term modulation of nitrogen and carbon metabolism. Similarly a variety of regulatory mechanisms operate in amino acid metabolism. Modulation of enzyme activity and modulation of enzyme levels are the outstanding regulatory mechanisms. In prokaryotes, attenuation and repressor/operator control are predominant, besides a so called "metabolic control" which integrates amino acid metabolism into the overall nutritional status of the cells. In eukaryotic cells compartmentation of amino acid metabolites as well as of part of the pathways becomes an additional regulatory factor; pathway specific controls seem to be rare, but a complex regulatory network, the "general control of amino acid biosynthesis", coordinates the synthesis of enzymes of a number of amino acid biosynthetic pathways.

The negative transcriptional regulator NmrA discriminates between oxidized and reduced dinucleotides

NmrA, a transcription repressor involved in the regulation of nitrogen metabolism in Aspergillus nidulans,is a member of the short-chain dehydrogenase reductase superfamily. Isothermal titration calorimetry and differential scanning calorimetry have been used to show NmrA binds NAD+ and NADP+ with similar affinity (average KD 65 microM) but has a greatly reduced affinity for NADH and NADPH (average KD 6.0 mM). The structure of NmrA in a complex with NADP+ reveals how repositioning a His-37 side chain allows the different conformations of NAD+ and NADP+ to be accommodated. Modeling NAD(P)H into NmrA indicated that steric clashes, attenuation of electrostatic interactions, and loss of aromatic ring stacking can explain the differing affinities of NAD(P)+/NAD(P)H. The ability of NmrA to discriminate between the oxidized and reduced forms of the dinucleotides may be linked to a possible role in redox sensing. Isothermal titration calorimetry demonstrated that NmrA and a C-terminal fragment of the GATA transcription factor AreA interacted with a 1:1 stoichiometry and an apparent KD of 0.26 microM. NmrA was unable to bind the nitrogen metabolite repression signaling molecules ammonium or glutamine.

Multiple forms of arginase are differentially expressed from a single locus in Neurospora crassa

The Neurospora crassa catabolic enzyme, arginase (L-arginine amidinohydrolase, EC 3.5.3.1), exists in multiple forms. Multiple forms of arginase are found in many vertebrates, but this is the only reported example in a microbial organism. The two major forms are structurally similar with subunit sizes of 36 and 41 kDa, respectively. The larger form is produced by mycelia growing in arginine-supplemented medium. Both forms are localized in the cytosol. The structural gene for arginase, aga, has been cloned and sequenced; it contains a 358-codon open reading frame with three in-frame ATGs at the amino terminus. Mutagenesis of these ATGs revealed that the first ATG initiates the 41-kDa protein and the third ATG initiates the 36-kDa protein. Mutation of the second ATG has no effect on translation. Northern analysis demonstrated that a 1.4-kilobase (kb) transcript is synthesized in minimal medium and both a 1.4- and 1.7-kb transcript are produced in arginine-supplemented medium. Primer extension identified the 5' ends of each transcript and demonstrated that the first and third ATG of the open reading frame are the initial AUGs of the 1.7- and 1. 4-kb mRNA, respectively. The results suggest that a basal promoter produces the 1.4-kb transcript and an arginine "activated" promoter is responsible for the 1.7-kb transcript. Tandem promoters are rare in eukaryotic organisms, and they often regulate developmental or tissue-specific gene expression. The possibility that arginase has a role in differentiation in N. crassa is being investigated.

Deletion of the N-terminal region of the AREA protein is correlated with a derepressed phenotype with respect to nitrogen metabolite repression

The entire areA gene and a truncated version lacking the sequence encoding the N-terminal 389 amino acids were expressed from the qutE promoter and terminator in an Aspergillus nidulans strain with the endogenous areA gene deleted. This expression system was used to decouple the effects of transcription regulation and mRNA stability mediated by the native promoter and terminator from any posttranslational modulation of AREA activity. Both the full-length AREA protein and the truncated form were able to function in the deletion strain, conferring the ability to use alternate nitrogen sources. Transformants containing the entire areA gene had a repressible phenotype with respect to nitrogen metabolite repression, whereas those containing the truncated form of the areA gene had a derepressed phenotype. The truncated areA gene was expressed in an A. nidulans strain containing a normally regulated wild-type areA gene, and transformants displayed a quinate-inducible nitrogen metabolite derepressed phenotype. Northern blot analysis of transformed strains showed that areA-specific mRNAs of the expected sizes were being produced. The truncated AREA protein was overproduced in Escherichia coli as a fusion protein and purified to homogeneity by a single-step immobilized metal affinity chromatography, and the purified protein was shown to bind specifically to the niaD promoter. Revised sequences of the 5' region of the areA gene and the entire meaB gene are reported.

Evolution of transcription-regulating proteins by enzyme recruitment: molecular models for nitrogen metabolite repression and ethanol utilisation in eukaryotes

Studies on the quinic acid utilisation gene (qut) cluster in Aspergillus nidulans showed that the genes encoding transcriptional activator and repressor proteins evolved by co-opting duplicated copies of genes encoding metabolic enzymes. In order to test the hypothesis that this was a general route for the genesis of regulatory proteins, the origins of the major control protein mediating nitrogen metabolite repression (an example of inter-pathway regulation) and ethanol utilisation (an example of intra-pathway regulation) in filamentous fungi were sought. The regulatory proteins mediating nitrogen metabolite repression were deduced to have originated in a duplication of genes encoding the anthranilate synthase complex which is active in the shikimate pathway. The major protein regulating ethanol utilisation was deduced to have its origin in the fusion of duplicated genes encoding the aldehyde and alcohol dehydrogenases (ALDA and ALCA). These data strongly support the view that transcriptional regulatory proteins evolve by the recruitment of functional domains provided by metabolic enzymes.

Analysis of conventional and in vitro generated mutants of nmr, the negatively acting nitrogen regulatory gene of Neurospora crassa

The nmr gene is the major negative regulatory gene in the nitrogen control circuit of Neurospora crassa, which, together with positive regulatory genes, governs the expression of multiple unlinked structural genes of the circuit. Possible functional domains of the NMR protein were investigated by mutational analyses using three different approaches. First, the polymerase chain reaction was used to clone the nmr locus from two conventional mutants, V2M304 and MS5, and the mutant amino acid codons were identified. A single point mutation was shown to be responsible for the mutant phenotype in each of these strains. The V2M304 allele contains a nonsense codon, and in the MS5 allele an aspartate has been substituted for glycine at residue 386. Our second approach studied possible functionally important regions in the nmr gene by the use of site-directed mutagenesis. The region containing the naturally occurring substitution in MS5 appears to be essential for function whereas a region in the N-terminal part of the protein does not seem important for NMR function. Finally, over 50% of the protein coding region was randomly mutagenized and amino acid residues that are essential for function and others that are functionally unimportant were identified.

Induction and de novo synthesis of uricase, a nitrogen-regulated enzyme in Neurospora crassa

Two efficient procedures are presented for the purification of the purine catabolic enzyme uricase from Neurospora crassa. A specific antiserum for uricase was prepared and used to examine the regulation of uricase expression. Even when wild-type cells are growing under full nitrogen repression conditions, they possess a considerable basal level of uricase. Induction results in a severalfold increase in the level of this enzyme and reflects de novo enzyme synthesis. Identical forms of uricase were translated in vitro from RNA isolated from control and induced cells, but, unexpectedly, induced cells contained less translatable uricase mRNA than did control cells. Although uricase is localized in peroxisomes, the enzyme subunit appears to be synthesized in mature form without any requirement for processing.

Electron cytochemical demonstration of phosphatase activity with microbody membranes of Basidiobolus haptosporus

Growth of cells of the potentially zoopathogenic fungus Basidiobolus haptosporus on a nutritionally defined medium with xanthine or urate as the nitrogen source results in greatly increased populations of microbodies. Modified Gomori procedures at the electron microscopic level suggested the single limiting membrane (and in some cases the granular matrix) of immature microbodies to be the exclusive subcellular locale(s) of alkaline phosphatase, 5'-nucleotidase and nucleoside diphosphatase activities. When grown in the presence of low inorganic phosphate, additional alkaline phosphatase activity was further identified cytochemically at and along profiles of endoplasmic reticulum and on inclusions previously described as "double-membraned vesicles". Cytochemical localization of acid phosphatase at microbody membranes was minimal if not ambiguous; Mg++-dependent adenosine triphosphatase and glucose-6-phosphatase were not identified at these locales. Quantitative biochemical estimates of alkaline phosphatase activity levels in particulate fractions initially increased with age of cells, perhaps as a function of the cultural induction and marked increase in immature microbody populations. We suggest that this enzyme may participate in some manner with protein translocation mechanisms associated with microbody biogenesis, ontogeny, and/or physiological function.

Amber nonsense mutations in regulatory and structural genes of the nitrogen control circuit of Neurospora crassa

Neurospora crassa possesses a set of nitrogen-regulated enzymes whose expression requires a lifting of nitrogen catabolite repression and specific induction. The nit-2 gene is a major regulatory locus which appears to act in a positive way to turn on the expression of these nitrogen-related enzymes whereas the nit-4 gene appears to mediate nitrate induction of nitrate and nitrite reductase. The nit-3 gene specifies nitrate reductase and is subject to control by both nit-2 and nit-4. Many new nit-2, nit-3, and nit-4 mutants were isolated in order to obtain amber nonsense mutations in these loci which were suppressible by the suppressor gene, Ssu-1. A nit-2 nonsense mutant was isolated which has altered regulatory properties for control of nitrate reductase. L-amino acid oxidase, and uricase, and which may encode a truncated regulatory protein. Four nit-3 nonsense mutations were isolated, each of which completely lacks nitrate reductase activity, which is restored to markedly different levels by suppression with Ssu-1. Studies of heat activation and thermal lability of nitrate reductase suggest a qualitative alteration of the enzyme occurs in two of the Ssu-1 nit-3 strains.

Molecular cloning of nit-2, a regulatory gene required for nitrogen metabolite repression in Neurospora crassa

We used an efficient sib-selection procedure to isolate a cosmid clone that complemented a mutated nit-2 gene of Neurospora crassa. Restriction fragment length polymorphism mapping indicated that the cosmid DNA insert was derived from linkage group IL, between 5S rDNA locus 12 and mt, the region of the N. crassa genome that contains nit-2. We conclude that the cosmid carries the nit-2 gene.

Regulation of pyrimidine salvage in Aspergillus nidulans: a role for the major regulatory gene are A mediating nitrogen metabolite repression

The synthesis of thymine 7-hydroxylase, an alpha-ketoglutarate dependent dioxygenase, is subject both to nitrogen metabolite repression and to oxygen repression, while synthesis of the other pyrimidine salvage pathway dioxygenase, pyrimidine deoxyribonucleoside 2'-hydroxylase, is subject to neither. are A300, an allele of the positive acting regulatory gene are A mediating nitrogen metabolite repression in Aspergillus nidulans, considerably elevates levels of thymine 7-hydroxylase, probably alleviating at least partly both nitrogen metabolite repression and oxygen repression. are A300 has little or no effect on levels of pyrimidine deoxy-ribonucleoside 2'-hydroxylase but does elevate net uptake capacities for thymine, thymidine and deoxyuridine two-fold. are A300 was selected as allowing thymine to supplement a pyrimidine auxotrophy and was found to allow supplementation by thymidine, other pyrimidine nucleosides and pyrimidine salvage intermediates as well. This is the first reported evidence for are A control over an activity(-ies) not directly concerned with nitrogen source utilization.

Major extracellular protease of Neurospora crassa

The inducible extracellular alkaline protease of Neurospora crassa was demonstrated to be a glycoprotein containing D-galactose residues by use of the enzyme-lectin conjugate horseradish peroxidase-Ricinus communis-agglutinin-120. The carbohydrate moiety of the protease appears to be a poor antigen since an antiserum made to the native enzyme recognizes epitopes determined only by the polypeptide portion of the enzyme. Immunochemical techniques were used to quantitatively precipitate protease labeled in vivo for electrophoretic analysis. Protease synthesis could not be detected in control, uninduced cultures, whereas ca. 0.4% of total cellular protein synthesis is devoted to protease formation under inducing conditions.

Regulation of a Neurospora crassa extracellular RNase by phosphorus, nitrogen, and carbon derepressions

A new extracellular RNase, designated N4, was detected in culture filtrates from Neurospora crassa and its regulation was studied. Limitation of a nutrient obtainable from RNA alone was not sufficient to cause enzyme derepression. The addition of RNA to the medium had no inductive effect, but the addition of exogenous protein caused enzyme production. With protein in the medium, N4 was derepressible for all three elemental nutrients obtainable from RNA: carbon, nitrogen, and phosphorus. Successful carbon derepression required the addition of a small amount of proteolytic activity to the cultures, as has been reported for the carbon-derepressible proteases of N. crassa. Exogenous protein affected RNase production before translation. Effects of the exogenous protein appeared similar to those previously reported for N. crassa protease induction. N4 was under the control of the nit-2 and nuc-1 gene products. nit-2 and nuc-1 mutants were unable to derepress enzyme synthesis for nitrogen and phosphorus limitation, respectively; however, these mutants responded like wild types to the other two states of derepression. Enzyme synthesis was constitutive in the preg mutant. Results indicate that the transcription of the N4 structural gene responds to multiple regulatory gene products from different regulatory circuits and that external protein affects the synthesis of classes of hydrolases other than proteases.

Possible role of a regulatory gene product upon the myo-inositol-1-phosphate synthase production in Neurospora crassa

The regulatory effect of inositol on inositol-1-phosphate synthase in Neurospora crassa strains was studied. Inositol represses enzyme production in the cultures of the wild type and that of the thermosensitive inositol-requiring mutant grown at 22 degrees C. Enzyme activity as well as the quantity of enzyme protein decreased sharply in both strains by increasing concentrations of inositol in the medium. Inositol-requiring strains used in our experiments can be divided into two groups. The first group produces a protein related immunologically to inositol phosphate synthase, but which is enzymatically inactive. The synthesis of this defective enzyme was also repressed by inositol. In the second group, this protein was found to be completely lacking, in both the thermosensitive mutant grown at 37 degrees C, and in a strain requiring inositol due to a reciprocal translocation. The thermostability and the cross immunoelectrophoresis of the enzyme suggest that in the case of the thermosensitive inositol-requiring mutant, the mutation did not occur in the structural gene of the enzyme, but its regulation was probably affected.

Neurospora crassa mutant impaired in glutamine regulation

The final products of the catabolism of arginine that can be utilized as nitrogen sources by Neurospora crassa are ammonium, glutamic acid, and glutamine. Of these compounds, only glutamine represses arginase and glutamine synthetase. We report here the isolation and characterization of a mutant of N. crassa whose arginase, glutamine synthetase, and amino acid accumulations are resistant to glutamine repression (glnI). This mutant has a greater capacity than the wild type (glns) to accumulate most of the arginine and some of the glutamine in osmotically sensitive compartments while growing exponentially. Nonetheless, the major part of the glutamine remains soluble and metabolically available for repression. We propose that the lower repression of glutamine synthetase by glutamine in this mutant could be a necessary condition for sustaining the higher flow of nitrogen for the accumulation of amino acids observed in ammonium excess and that, if glutamine is the nitrogen signal that regulates the arginine accumulation of the vesicle, the glnr mutant has also escaped this control. Finally, in the glnr mutant, some glutamine resynthesis is necessary for arginine biosynthesis and accumulation.

A near terminal pericentric inversion leads to nitrogen metabolite derepression in Aspergillus nidulans

The mutation xprD-1, previously shown to be an allele of the areA gene and to lead to nitrogen metabolite derepression in Aspergillus nidulans, is shown to be associated with a near terminal pericentric inversion in linkage group III. The left arm break-point is between the adI and sC genes, and the right arm break-point is between the ornC and areA genes but just centromere proximal to areA. In crosses of xprD-1 strains to inversion-free strains one class of duplication-deficiency progeny is recovered. These progeny have two copies of the distal portion of the left arm beginning just before sC but lack a copy of areA and the region distal to it on the right arm. The viability of these duplication-deficiency progeny indicates that no indispensable gene can lie distal to areA, suggesting proximity of areA to the telomere. The inversion might increase expression of areA which, given the positive acting nature of this regulatory gene, would result in nitrogen metabolite derepression. If increased expression be the result of fusion to (or creation of) a more efficient promoter and/or ribosome binding sequence, areA must be transcribed towards the right arm telomere.

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.

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: 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.