Characterization of a glycerol kinase mutant of Aspergillus niger

A glycerol-kinase-deficient mutant of Aspergillus niger was isolated. Genetic analysis revealed that the mutation is located on linkage group VI. The phenotype of this mutant differed from that of a glycerol kinase mutant of Aspergillus nidulans in its ability to utilize dihydroxyacetone (DHA). The weak growth on glycerol of the A. niger glycerol kinase mutant showed that glycerol phosphorylation is an important step in glycerol catabolism. The mutant could still grow normally on DHA because of the presence of a DHA kinase. This enzyme, probably in combination with an NAD(+)-dependent glycerol dehydrogenase, present only in the mutant, is responsible for the weak growth of the mutant on glycerol. Enzymic analysis of both the mutant and the parental strain showed that at least three different glycerol dehydrogenases were formed under different physiological conditions: the NAD(+)-dependent enzyme described above, a constitutive NADP(+)-dependent enzyme and a D-glyceraldehyde-specific enzyme induced on D-galacturonate. The glycerol kinase mutant showed impaired growth on D-galacturonate.     

Glycerol catabolism in Aspergillus nidulans

Glycerol is catabolized in Aspergillus nidulans by glycerol kinase and a mitochondrial FAD-dependent sn-glycerol 3-phosphate dehydrogenase. The levels of both enzymes are controlled by carbon catabolite repression and by specific induction. Biochemical and genetical analyses show that dihydroxyacetone and D-glyceraldehyde are converted into glycerol and then catabolized by the same pathway. D-Glyceraldehyde can be reduced by NADP(+)-dependent glycerol dehydrogenase or by alcohol dehydrogenase I, while dihydroxyacetone is only reduced by the first enzyme. Three new glycerol non-utilizing mutants have been found. These three mutations define three hitherto unknown loci, glcE, glcF and glcG. The mutation in glcG leads to a greatly decreased sn-glycerol-3-phosphate dehydrogenase activity.     

Metabolism of 2-oxoaldehyde in mold. Purification and characterization of two methylglyoxal reductases from Aspergillus niger

Two kinds of methylglyoxal reductases were purified to apparent homogeneity from Aspergillus niger and designated MGR I and MGR II. Both enzymes consisted of a single polypeptide chain with a relative molecular mass of 36,000 (MGR I) and 38,000 (MGR II). NADPH was specifically required for the activities of both enzymes and Km values for NADPH were 54 microM (MGR I) and 6.8 microM (MGR II). MGR I was specific to 2-oxoaldehydes [glyoxal, methylglyoxal (Km = 15.4 mM) and phenylglyoxal], whereas MGR II was active on both 2-oxoaldehydes [glyoxal (Km = 10 mM), methylglyoxal (Km = 1.43 mM), phenylglyoxal (Km = 4.35 mM) and 4,5-dioxovalerate] and some aldehydes (propionaldehyde and acetaldehyde). Optimal pH values for MGR I and MGR II activities were 9.0 and 6.5 respectively. Both enzymes were inactivated by a brief incubation with 2-oxoaldehydes (glyoxal, methylglyoxal and phenylglyoxal) in the absence of NADPH. MGR I activity was competitively inhibited by NADP+ and the Ki value for NADP+ was calculated to be 0.49 mM. On the other hand, the inhibition of MGR II activity by NADP+ was of mixed type, the Ki value for NADP+ being 45 microM. MGR I was different from MGR II in amino acid composition.     

Mitochondrial location of pyruvate carboxylase in Aspergillus niger

Pyruvate carboxylase has been found in the mitochondrial fraction of two strains of Aspergillus niger along with the marker enzymes of citrate synthase and cytochrome c oxidase. The location of pyruvate carboxylase in A. nidulans was, however, confirmed to be in the cytosolic fraction. The enzyme from the former sources was dependent upon the presence of acetyl-CoA for full activity; the enzyme from A. nidulans was unaffected by the presence or absence of acetyl-CoA.     

The Aspergillus niger acuA and acuB genes correspond to the facA and facB genes in Aspergillus nidulans

Mutants in Aspergillus niger unable to grow on acetate as a sole carbon source were previously isolated by resistance to 1.2% propionate medium containing 0.1% glucose. AcuA mutants lacked acetyl-CoA synthetase (ACS) activity and acuB mutants lacked both ACS and isocitrate lyase activity. An acuA mutant was transformed to the acu+ phenotype with a clone of ACS (facA) from Aspergillus nidulans. The acuB mutant was transformed with the A. niger facB clone which has been identified by cross-hybridisation of an A. nidulans facB clone. These results confirm that acuA in A. niger is the gene for ACS and acuB is analogous to the A. nidulans facB regulatory gene.     

Metabolic control analysis of xylose catabolism in Aspergillus

A kinetic model for xylose catabolism in Aspergillus is proposed. From a thermodynamic analysis it was found that the intermediate xylitol will accumulate during xylose catabolism. Use of the kinetic model allowed metabolic control analysis (MCA) of the xylose catabolic pathway to be carried out, and flux control was shown to be dependent on the metabolite levels. Due to thermodynamic constraints, flux control may reside at the first step in the pathway, i.e., at the xylose reductase, even when the intracellular xylitol concentration is high. On the basis of the kinetic analysis, the general dogma specifying that flux control often resides at the step following an intermediate present at high concentrations was, therefore, shown not to hold. The intracellular xylitol concentration was measured in batch cultivations of two different strains of Aspergillus niger and two different strains of Aspergillus nidulans grown on media containing xylose, and a concentration up to 30 mM was found. Applying MCA showed that the first polyol dehydrogenase (XDH) in the catabolic pathway of xylose exerted the main flux control in the two strains of A. nidulans and A. niger NW324, but the flux control was exerted mainly at the first enzyme of the pathway (XR) of A. niger NW 296.     

Inhibition of fungal NADP+-isocitrate dehydrogenase by citric acid

The variations observed during earlier studies in the activity of NADP+-isocitrate dehydrogenase (EC. 1.1.1.42) in a strain of Aspergillus niger were found to be related to the extent of washing of mycelium. As a result the mycelium washed four times with phosphate buffer (0.05 M, pH 7.5), the enzyme activity present in 4 and 8 days old fungal mycelia increased five- and two-fold, respectively. In vivo studies showed a complete loss of enzyme activity in mycelia resuspended in HCl-KCl buffer (0.02 M, pH 2.2) containing citric acid (13 mM or more). The in vitro studies revealed 50% loss of enzyme activity in presence of 3.6 to 5.2 mM citric acid. However, in case of Aspergillus niger ATCC 1015, which produced less citric acid than the above strain, a much higher citric acid concentration (13 to 26 mM) was required to cause 50% loss of enzyme activity. These findings suggest a correlation between citric acid inhibition of NADP+-isocitrate dehydrogenase and the ability of A. niger to accumulate citric acid in the medium.     

pH dependence of kinetic parameters for oxalacetate decarboxylation and pyruvate reduction reactions catalyzed by malic enzyme

Both chicken liver NADP-malic enzyme and Ascaris suum NAD-malic enzyme catalyze the metal-dependent decarboxylation of oxalacetate. Both enzymes catalyze the reaction either in the presence or in the absence of dinucleotide. The presence of dinucleotide increases the affinity of oxalacetate for the chicken liver NADP-malic enzyme, but this information could not be obtained in the case of A. suum NAD-malic enzyme because of the low affinity of free enzyme for NAD. The kinetic mechanism for oxalacetate decarboxylation by the chicken liver NADP-malic enzyme is equilibrium ordered at pH values below 5.0 with NADP adding to enzyme first. The Ki for NADP increases by a factor of 10 per pH unit below pH 5.0. An enzyme residue is required protonated for oxalacetate decarboxylation (by both enzymes) and pyruvate reduction (by the NAD-malic enzyme), but the beta-carboxyl of oxalacetate must be unprotonated for reaction (by both enzymes). The pK of the enzyme residue of the chicken liver NADP-malic enzyme decreases from a value of 6.4 in the absence of NADP to about 5.5 with Mg2+ and 4.8 with Mn2+ in the presence of NADP. The pK value of the enzyme residue required protonated for either oxalacetate decarboxylation or pyruvate reduction for the A. suum NAD-malic enzyme is about 5.5-6.0. Although oxalacetate binds equally well to protonated and unprotonated forms of the NADP-enzyme, the NAD-enzyme requires that oxalacetate or pyruvate selectively bind to the protonated form of the enzyme. Both enzymes prefer Mn2+ over Mg2+ for oxalacetate decarboxylation.(ABSTRACT TRUNCATED AT 250 WORDS)     

Isolation and characterization of the Aspergillus niger trpC gene

The Aspergillus niger trpC gene was isolated by complementation experiments with an Escherichia coli trpC mutant. Plasmid DNA containing the A. niger trpC gene transforms an Aspergillus nidulans mutant strain, defective in all three enzymatic activities of the trpC gene, to Trp+, indicating the presence of a complete and functional trpC gene. Southern blot analysis of DNA from these Trp+ transformants showed that plasmid DNA was present but that this DNA was not integrated at the site of the chromosomal trpC locus. The A. niger trpC gene was localized on the cloned fragment by heterologous hybridization experiments and sequence analysis. These experiments suggest that the organization of the A. niger trpC gene is identical to that of the analogous A. nidulans trpC and the Neurospora crassa trp-1 genes.     

Transformation of Aspergillus niger by the amdS gene of Aspergillus nidulans.

Aspergillus niger grows poorly on acetamide as a nitrogen or carbon source and lacks sequences detectably homologous to the amdS gene encoding the acetamidase of Aspergillus nidulans. We have taken advantage of these observations to develop a transformation system for A. niger using the amdS gene as a dominant heterologous marker for selecting transformants on the basis of acetamide utilization. Transformants varied in their ability to grow on amide media and the number of integrated copies of the amdS plasmid ranged from 1 or 2 to greater than 100. Southern analysis of transformants revealed that the multiple copies were integrated into the chromosome in tandem arrays. This result indicates that transformation of A. niger is more similar to mammalian cells than to yeast. Analysis of enzyme activity levels and RNA levels showed that most of the copies of amdS were expressed. Mitotic stabilities of transformants were found to be high. A transformant containing greater than 100 copies of the amdS gene was impaired in omega-amino acid utilization, a result that has also been found in A. nidulans. Since, in A. nidulans, omega-amino acids induce acetamidase via a characterizied regulatory gene (amdR/intA) this observation implies that titration of an analogous A. niger regulatory gene product by multiple amdS copies has occurred. Additional evidence suggested that the amdS gene is regulated in A. niger. It has also been shown that an unselected plasmid can be co-transformed with the amdS plasmid into A. niger.     

The polygalacturonases of Aspergillus niger are encoded by a family of diverged genes.

Aspergillus niger produces several polygalacturonases that, with other enzymes, are involved in the degradation of pectin. One of the two previously characterized genes coding for the abundant polygalacturonases I and II (PGI and PGII) found in a commercial pectinase preparation was used as a probe to isolate five more genes by screening a genomic DNA library in phage lambda EMBL4 using conditions of moderate stringency. The products of these genes were detected in the culture medium of Aspergillus nidulans transformants on the basis of activity measurements and Western-blot analysis using a polyclonal antibody raised against PGI. These transformants were, with one exception, constructed using phage DNA. A. nidulans transformants secreted high amounts of PGI and PGII in comparison to the previously characterized A. niger transformants and a novel polygalacturonase (PGC) was produced at high levels by A. nidulans transformed with the subcloned pgaC gene. This gene was sequenced and the protein-coding region was found to be interrupted by three introns; the different intron/exon organization of the three sequenced A. niger polygalacturonase genes can be explained by the gain or loss of two single introns. The pgaC gene encodes a putative 383-amino-acid prepro-protein that is cleaved after a pair of basic amino acids and shows approximately 60% amino acid sequence similarity to the other polygalacturonases in the mature protein. The N-terminal amino acid sequences of the A. niger polygalacturonases display characteristic amino acid insertions or deletions that are also observed in polygalacturonases of phytopathogenic fungi. In the upstream regions of the A. niger polygalacturonase genes, a sequence of ten conserved nucleotides comprising a CCAAT sequence was found, which is likely to represent a binding site for a regulatory protein as it shows a high similarity to the yeast CYC1 upstream activation site recognized by the HAP2/3/4 activation complex.     

Production of cell wall-degrading enzymes by Aspergillus nidulans: a model system for fungal pathogenesis of plants.

The cell wall-degrading enzymes polygalacturonase and pectate lyase have been suggested to be crucial for penetration and colonization of plant tissues by some fungal pathogens. We have found that Aspergillus nidulans (= Emericella nidulans), a saprophytic Ascomycete, produces levels of these enzymes equal to those produced by soft-rotting Erwinia species. Induction of polygacturonase and pectate lyase in A. nidulans requires substrate and is completely repressed by glucose. Surprisingly, inoculation of excised plant tissues with A. nidulans conidia leads to formation of necrotic, water-soaked lesions within which the organism sporulates. Thus, A. nidulans has phytopathogenic potential. The release of glucose and other sugars from wounded tissues may repress pectolytic enzyme production and limit disease development. Therefore, we tested creA204, a mutation that relieves glucose repression of some A. nidulans carbon utilization enzymes, for its effect on production of pectolytic enzymes. creA204 failed to relieve catabolite repression of polygalacturonase or pectate lyase and had no effect on disease severity.     

Localization of pyruvate carboxylase in organic acid-producing Aspergillus strains.

The localization of pyruvate carboxylase (cytosolic or mitochondrial) was studied in nine different Aspergillus species (14 strains). In some species (A. aculeatus, A. flavus, A. foetidus, A. nidulans, A. ochraceus, and A. sojae), the pyruvate carboxylase activity could be detected only in the cytosolic fraction of the cells. Pyruvate carboxylase has been found only in the mitochondrial fraction of two strains of Aspergillus wentii. In Aspergillus oryzae and in five strains of Aspergillus niger, pyruvate carboxylase activity was detected both in the mitochondrial fraction and in the cytosol. There was no quantitative or qualitative correlation between the activities of pyruvate carboxylase in the mitochondrial and cytosolic fractions of the cells and the ability of the various Aspergillus strains to accumulate different organic acids.     

Esterification of ferulic acid with polyols using a ferulic acid esterase from Aspergillus niger

Commercially available enzyme preparations were screened for enzymes that have a high ability to catalyze direct ester-synthesis of ferulic acid with glycerol. Only a preparation, Pectinase PL "Amano" produced by Aspergillus niger, feruloylated glycerol under the experimental conditions. The enzyme responsible for the esterification was purified and characterized. This enzyme, called FAE-PL, was found to be quite similar to an A. niger ferulic acid esterase (FAE-III) in terms of molecular mass, pH and temperature optima, substrate specificity on synthetic substrates, and the N-terminal amino acid sequence. FAE-PL highly catalyzed direct esterification of ferulic acid and sinapinic acid with glycerol. FAE-PL could feruloylate monomeric sugars including arabinose, fructose, galactose, glucose, and xylose. We determined the suitable conditions for direct esterification of ferulic acid with glycerol to be as follows: 1% ferulic acid in the presence of 85% glycerol and 5% dimethyl sulfoxide at pH 4.0 and 50 degrees C. Under these conditions, 81% of ferulic acid could be converted to 1-glyceryl ferulate, which was identified by (1)H-NMR. The ability of 1-glyceryl ferulate to scavenge 1,1-diphenyl-2-picrylhydrazyl (DPPH) radicals was higher than that of the anti-oxidant butyl hydroxytoluene.     

An amdS-lacZ fusion for studying gene regulation in Aspergillus

A translational fusion has been constructed between the amdS gene of Aspergillus nidulans and the lacZ gene of Escherichia coli. Sequencing across the fusion junction confirmed the generation of an in-frame fusion at amino acid 34 of amdS and a novel protein has been detected in transformants carrying the fusion plasmid. Transformants of A. nidulans and Aspergillus niger carrying the fusion plasmid were obtained by co-transformation with a second selectable plasmid. These transformants were readily identified on media containing XGal. The intensity of the reaction on XGal media was indicative of the number of copies of the fusion plasmid carried by the transformants. The growth of highly expressing strains of A. nidulans was inhibited on XGal media. The fusion plasmid was used to develop a two-step gene replacement strategy in which the resident amdS gene was replaced with the fusion gene free of vector sequences. Plate tests and in vitro assays of the beta-galactosidase enzyme confirmed that expression of the fusion gene was regulated by amdS flanking sequences and trans-acting regulatory genes.     

Molecular and physiological characterization of the NAD-dependent glycerol 3-phosphate dehydrogenase in the filamentous fungus Aspergillus nidulans

In filamentous fungi, glycerol biosynthesis has been proposed to play an important role during conidiospore germination and in response to a hyperosmotic shock, but little is known about the genes involved. Here, we report on the characterization of the major Aspergillus nidulans glycerol 3-phosphate dehydrogenase (G3PDH)-encoding gene, gfdA. G3PDH is responsible for the conversion of dihydroxyacetone phosphate (DHAP) into glycerol 3-phosphate (G3P), which is subsequently converted into glycerol by an as yet uncharacterized phosphatase. Inactivation of gfdA does not abolish glycerol biosynthesis, showing that the other pathway from DHAP, via dihydroxyacetone (DHA), to glycerol is also functional in A. nidulans. The gfdA null mutant displays reduced G3P levels and an osmoremediable growth defect on various carbon sources except glycerol. This growth defect is associated with an abnormal hyphal morphology that is reminiscent of a cell wall defect. Furthermore, the growth defect at low osmolarity is enhanced in the presence of the chitin-interacting agent calcofluor and the membrane-destabilizing agent sodium dodecyl sulphate (SDS). As inactivation of gfdA has no impact on phospholipid biosynthesis or glycolytic intermediates levels, as might be expected from reduced G3P levels, a previously unsuspected link between G3P and cell wall integrity is proposed to occur in filamentous fungi.     

Localization of pyruvate carboxylase in organic acid-producing Aspergillus strains

The localization of pyruvate carboxylase (cytosolic or mitochondrial) was studied in nine different Aspergillus species (14 strains). In some species (A. aculeatus, A. flavus, A. foetidus, A. nidulans, A. ochraceus, and A. sojae), the pyruvate carboxylase activity could be detected only in the cytosolic fraction of the cells. Pyruvate carboxylase has been found only in the mitochondrial fraction of two strains of Aspergillus wentii. In Aspergillus oryzae and in five strains of Aspergillus niger, pyruvate carboxylase activity was detected both in the mitochondrial fraction and in the cytosol. There was no quantitative or qualitative correlation between the activities of pyruvate carboxylase in the mitochondrial and cytosolic fractions of the cells and the ability of the various Aspergillus strains to accumulate different organic acids.     

Purification and properties of glucose 6-phosphate dehydrogenase from Aspergillus aculeatus

Glucose 6-phosphate dehydrogenase (EC 1.1.1.49) was purified from Aspergillus aculeatus, a filamentous fungus previously isolated from infected tongue of a patient. The enzyme, apparently homogeneous, had a specific activity of 220 units mg(-1), a molecular weight of 105,000 +/- 5,000 Dal by gel filtration and subunit size of 52,000 +/- 1,100 Dal by sodium dodecyl sulphate-polyacrylamide gel electrophoresis. The substrate specificity was extremely strict, with glucose 6-phosphate (G6P) being oxidized by nicotinamide adenine dinucleotide phosphate (NADP) only. At assay pH of 7.5, the enzyme had K(m) values of 6 microM and 75 microM for NADP and G6P respectively. The k(cat) was 83 s(-1). Steady-state kinetics at pH 7.5 produced converging linear Lineweaver-Burk plots as expected for ternary-complex mechanism. The patterns of product and dead-end inhibition suggested that the enzyme can bind NADP and G6P separately to form a binary complex, indicating a random-order mechanism. The enzyme was irreversibly inactivated by heat in a linear fashion, with G6P providing a degree of protection. Phosphoenolpyruvate (PEP), adenosinetriphosphate (ATP), and fructose 6-phosphate (F6P), in decreasing order, are effective inhibitors. Zinc and Cobalt ions were effective inhibitors although cobalt ion was more potent; the two divalent metals were competitive inhibitors with respect to G6P, with Ki values of 6.6 microM and 4.7 microM respectively. It is proposed that inhibition by divalent metal ions, at low NADPH /NADP ratio, is another means of controlling pentosephosphate pathway.