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

Enhancement of a novel extracellular uricase production by media optimization and partial purification by aqueous three-phase system

Uricase (urate: oxygen oxidoreductase, EC 1.7.3.3), an enzyme belonging to the class of oxidoreductases, catalyzes the enzymatic oxidation of uric acid to allantoin and finds a wide variety of application as therapeutic and clinical reagent. In this study, uricase production ability of the bacterial strains isolated from deep litter poultry soil is investigated. The strain with maximum extracellular uricase production capability was identified as Xanthomonas fuscans subsp. aurantifolii based on 16S rRNA sequencing. Effect of various carbon and nitrogen sources on uricase productivity was investigated. The uricase production for this strain was optimized using statistically based experimental designs and resulted in uricase activity of 306 U/L, which is 2 times higher than initial uricase activity. Two-step purification, such as ammonium sulfate precipitation and aqueous two-phase system, was carried out and a twofold increase in yield and specific activity was observed.

A eukaryote without catalase-containing microbodies: Neurospora crassa exhibits a unique cellular distribution of its four catalases

Microbodies usually house catalase to decompose hydrogen peroxide generated within the organelle by the action of various oxidases. Here we have analyzed whether peroxisomes (i.e., catalase-containing microbodies) exist in Neurospora crassa. Three distinct catalase isoforms were identified by native catalase activity gels under various peroxisome-inducing conditions. Subcellular fractionation by density gradient centrifugation revealed that most of the spectrophotometrically measured activity was present in the light upper fractions, with an additional small peak coinciding with the peak fractions of HEX-1, the marker protein for Woronin bodies, a compartment related to the microbody family. However, neither in-gel assays nor monospecific antibodies generated against the three purified catalases detected the enzymes in any dense organellar fraction. Furthermore, staining of an N. crassa wild-type strain with 3,3'-diaminobenzidine and H(2)O(2) did not lead to catalase-dependent reaction products within microbodies. Nonetheless, N. crassa does possess a gene (cat-4) whose product is most similar to the peroxisomal type of monofunctional catalases. This novel protein indeed exhibited catalase activity, but was not localized to microbodies either. We conclude that N. crassa lacks catalase-containing peroxisomes, a characteristic that is probably restricted to a few filamentous fungi that produce little hydrogen peroxide within microbodies.

The global nitrogen regulator, FNR1, regulates fungal nutrition-genes and fitness during Fusarium oxysporum pathogenesis

SUMMARY Fusarium oxysporum is a soil-borne pathogen that infects plants through the roots and uses the vascular system for host ingress. Specialized for this route of infection, F. oxysporum is able to adapt to the scarce nutrient environment in the xylem vessels. Here we report the cloning of the F. oxysporum global nitrogen regulator, Fnr1, and show that it is one of the determinants for fungal fitness during in planta growth. The Fnr1 gene has a single conserved GATA-type zinc finger domain and is 96% and 48% identical to AREA-GF from Gibberella fujikuroi, and NIT2 from Neurospora crassa, respectively. Fnr1 cDNA, expressed under a constitutive promoter, was able to complement functionally an N. crassa nit-2(RIP) mutant, restoring the ability of the mutant to utilize nitrate. Fnr1 disruption mutants showed high tolerance to chlorate and reduced ability to utilize several secondary nitrogen sources such as amino acids, hypoxanthine and uric acid, whereas growth on favourable nitrogen sources was not affected. Fnr1 disruption also abolished in vitro expression of nutrition genes, normally induced during the early phase of infection. In an infection assay on tomato seedlings, infection rate of disruption mutants was significantly delayed in comparison with the parental strain. Our results indicate that FNR1 mediates adaptation to nitrogen-poor conditions in planta through the regulation of secondary nitrogen acquisition, and as such acts as a determinant for fungal fitness during infection.

Genetic regulation of nitrogen metabolism in the fungi

In the fungi, nitrogen metabolism is controlled by a complex genetic regulatory circuit which ensures the preferential use of primary nitrogen sources and also confers the ability to use many different secondary nitrogen sources when appropriate. Most structural genes encoding nitrogen catabolic enzymes are subject to nitrogen catabolite repression, mediated by positive-acting transcription factors of the GATA family of proteins. However, certain GATA family members, such as the yeast DAL80 factor, act negatively to repress gene expression. Selective expression of the genes which encode enzymes for the metabolism of secondary nitrogen sources is often achieved by induction, mediated by pathway-specific factors, many of which have a GAL4-like C6/Zn2 DNA binding domain. Regulation within the nitrogen circuit also involves specific protein-protein interactions, as exemplified by the specific binding of the negative-acting NMR protein with the positive-acting NIT2 protein of Neurospora crassa. Nitrogen metabolic regulation appears to play a significant role in the pathogenicity of certain animal and plant fungal pathogens.

nit-2, the major positive-acting nitrogen regulatory gene of Neurospora crassa, encodes a sequence-specific DNA-binding protein

The nit-2 major nitrogen regulatory gene of Neurospora crassa turns on the expression of various unlinked structural genes that specify nitrogen-catabolic enzymes under nitrogen-limitation conditions. The nit-2 gene encodes a protein of 1036 amino acid residues with a single zinc finger and a downstream basic region that may make up a DNA-binding domain. The zinc-finger domain of the NIT2 protein was synthesized in two ways to examine its DNA-binding activity with gel-band-mobility shift and DNA-footprint experiments. The NIT2 protein binds to specific DNA recognition elements that are located upstream of nitrogen-regulated structural genes. Each recognition element contains at least two copies of a core sequence whose consensus is TATCTA.

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.

Nucleotide sequence and analysis of NMR, a negative-acting regulatory gene in the nitrogen circuit of Neurospora crassa

In Neurospora the expression of a set of unlinked structural genes, which allows utilization of various nitrogen-containing compounds, is controlled by the positive-acting nit-2 gene and the negative-acting nmr gene. The nucleotide sequence of the nmr gene has been determined and a long open reading frame which encodes a putative protein of 54854 daltons has been identified. A full-length cDNA clone was obtained and its the sequence revealed that the nmr gene contains no introns. The transcriptional start and stop sites have been mapped by S1 nuclease and primer extension. Site-directed mutagenesis was used to introduce stop codons at various locations in the nmr coding region. Transformation assays showed that the proteins lacking up to 16% of the carboxyl-terminus were still functional. Homology searches showed that the nmr protein is homologous to the yeast arginine regulatory gene AR-GRII.

nit-2, the major nitrogen regulatory gene of Neurospora crassa, encodes a protein with a putative zinc finger DNA-binding domain

The nitrogen regulatory circuit of Neurospora crassa consists of a set of unlinked structural genes which specify various nitrogen catabolic enzymes plus control genes and metabolic effectors which regulate their expression. The positive-acting nit-2 regulatory gene is required to turn on the expression of the nitrogen catabolic enzymes during conditions of nitrogen limitation. The complete nucleotide sequence of the nit-2 gene was determined. The nit-2 mRNA is 4.3 kilobases long and has a long nontranslated sequence at both its 5' and 3' ends. The nit-2 gene nucleotide sequence can be translated to yield a protein containing 1,036 amino acid residues with a molecular weight of approximately 110,000. Deletion analyses demonstrated that approximately 21% of the NIT2 protein at its carboxy terminus can be removed without loss of function. The nit-2 protein contains a single putative Cys2/Cys2 zinc finger domain which appears to function in DNA binding and which has striking homology to a mammalian trans-acting factor, GF-1.

nit-2, the major nitrogen regulatory gene of Neurospora crassa, encodes a protein with a putative zinc finger DNA-binding domain

The nitrogen regulatory circuit of Neurospora crassa consists of a set of unlinked structural genes which specify various nitrogen catabolic enzymes plus control genes and metabolic effectors which regulate their expression. The positive-acting nit-2 regulatory gene is required to turn on the expression of the nitrogen catabolic enzymes during conditions of nitrogen limitation. The complete nucleotide sequence of the nit-2 gene was determined. The nit-2 mRNA is 4.3 kilobases long and has a long nontranslated sequence at both its 5' and 3' ends. The nit-2 gene nucleotide sequence can be translated to yield a protein containing 1,036 amino acid residues with a molecular weight of approximately 110,000. Deletion analyses demonstrated that approximately 21% of the NIT2 protein at its carboxy terminus can be removed without loss of function. The nit-2 protein contains a single putative Cys2/Cys2 zinc finger domain which appears to function in DNA binding and which has striking homology to a mammalian trans-acting factor, GF-1.

Liver uricase in Camelus dromedarius: purification and properties

1. Uricase (urate: oxygen oxidoreductase, EC 1.7.3.3) was purified 750-fold from the liver of Camelus dromedarius. 2. The enzyme is a tetramer with a Mr of 100,000, displays high specificity for uric acid with a Km of 12 microM and is inhibited by a selected number of purine derivatives carrying oxygen at the C2 position. 3. The effect of pH and the inhibition by thiol compounds and chelating agents on the enzyme activity is reported. 4. Some lines of evidence suggesting the possibility of interaction of camel liver uricase with oligonucleotides are presented.

Molecular cloning and characterization of a negative-acting nitrogen regulatory gene of Neurospora crassa

Expression of the structural genes of the nitrogen control circuit of Neurospora crassa is regulated by the positive-acting nit-2 control gene and by the negative-acting nmr control gene. Nitrate reductase is expressed in a constitutive fashion in nmr mutant strains, which appear to be largely insensitive to nitrogen catabolite repression. Thus, nmr mutants are sensitive to chlorate in the presence of ammonia or glutamine, whereas the wild type is chlorate resistant under these conditions. A cosmid library was screened for the presence of the nmr+ gene by the sib selection procedure, and a single cosmid was isolated which transforms the nmr mutant to chlorate resistance at a high frequency. A restriction fragment length polymorphism analysis revealed that the cloned DNA segment maps to the precise genomic location of nmr. Northern blot analyses revealed that the nmr gene is itself not regulated but is expressed constitutively to give a single transcript of approximately 1.8 kb.

Xanthine dehydrogenase expression in Neurospora crassa does not require a functional nit-2 regulatory gene

Xanthine dehydrogenase (XDH) is the initial enzyme in the purine catabolic pathway of N. crassa. Secondary nitrogen sources such as purines are metabolized when preferred sources of reduced nitrogen (ammonium or glutamine) are unavailable. XDH synthesis is regulated by glutamine repression and uric acid induction. The nit-2 locus is believed to encode a trans-acting positive regulator essential for the expression of genes encoding enzymes involved in secondary pathways of nitrogen acquisition, such as XDH and nitrate reductase. However, immunoblot analyses and enzyme assays reveal that XDH protein is synthesized and XDH activity is expressed in nit-2 mutants. Nevertheless, XDH responds to nitrogen metabolite repression. The generality that nit-2 is an obligate control element in nitrogen metabolite repression is questioned. Additionally, mutants defective in XDH activity, namely, xdh-1 and the molybdenum cofactor mutants nit-1, -7, -8 and -9, are observed to grow on xanthine but not hypoxanthine.