P.J.I. van de Vondervoort

Oxalic acid production by Aspergillus niger : an oxalate-non-producing mutant produces citric acid at pH 5 and in the presence of manganese

The external pH appeared to be the main factor governing oxalic acid production by Aspergillus niger. A glucose-oxidase-negative mutant produced substantial amounts of oxalic acid as long as the pH of the culture was 3 or higher. When pH was decreased below 2, no oxalic acid was formed. The activity of oxaloacetate acetylhydrolase (OAH), the enzyme believed to be responsible for oxalate formation in A. niger, correlated with oxalate production. OAH was purified from A. niger and characterized. OAH cleaves oxaloacetate to oxalate and acetate, but A. niger never accumulated any acetate in the culture broth. Since an A. niger acuA mutant, which lacks acetyl-CoA synthase, did produce some acetate, wild-type A. niger is apparently able to catabolize acetate sufficiently fast to prevent its production. An A. niger mutant, prtF28, previously isolated in a screen for strains deficient in extracellular protease expression, was shown here to be oxalate non-producing. The prtF28 mutant lacked OAH, implying that OAH is the only enzyme involved in oxalate production in A. niger. In a traditional citric acid fermentation low pH and absence of Mn2+ are prerequisites. Remarkably, a strain lacking both glucose oxidase (goxC) and OAH (prtF) produced citric acid from sugar substrates in a regular synthetic medium at pH 5 and under these conditions production was completely insensitive to Mn2+.

L- Arabinose and D-Xylose Catabolism in Aspergillus niger

SUMMARY: A mutant of Aspergillus niger unable to grow on d-xylose and l-arabinose has been isolated. Genetic analysis revealed that the mutation is located on linkage group IV. Enzymic analysis revealed a deficiency in d-xylulose kinase activity. After transfer of growing mycelium to a medium containing either d-xylose or l-arabinose, the mutant accumulates large amounts of arabitol and xylitol, as shown by 13C NMR spectroscopy. These data and an analysis of enzyme activities induced by d-xylose and l-arabinose in the wild-type strain led to the following catabolic pathway for d-xylose: d-xylose - xylitol - d-xylulose - d-xylulose 5-phosphate; and for l-arabinose: l-arabinose - l-arabitol - l-xylulose - xylitol - d-xylulose - d-xylulose 5-phosphate. The reduction steps of the sugars to the corresponding polyols are all NADPH dependent. The oxidation steps of the polyols to the sugars are all NAD+ dependent. Fractionation of cell-free extracts gave information about the specificity of the enzymes and showed that all the reactions are catalysed by different enzymes.

Isolation of Aspergillus niger creA mutants and effects of the mutations on expression of arabinases and L-arabinose catabolic enzymes.

Aspergillus niger mutants relieved of carbon repression were isolated from an areA parental strain by selection of colonies that exhibited improved growth on a combination of 4-aminobutanoic acid (GABA) and D-glucose. In addition to derepression of the utilization of GABA as a nitrogen source in the presence of D-glucose, three of the four mutants also showed derepression of L-alanine and L-proline utilization. Transformation of the mutants with the A. niger creA gene, encoding the repressor protein CREA, re-established the areA phenotype on GABA/D-glucose, identifying the mutations as creAd. The creA gene mapped on chromosome IV by linkage analysis and contour-clamped homogeneous electric field hybridization. The creA mutants obtained were used to study the involvement of CREA in repression by D-glucose of arabinases and L-arabinose catabolism in A. niger. In wild-type A. niger, alpha-L-arabinofuranosidase A, alpha-L-arabinofuranosidase B, endo-arabinase, L-arabinose reductase and L-arabitol dehydrogenase were induced on L-arabinose, but addition of D-glucose prevented this induction. Repression was relieved to varying degrees in the creA mutants, showing that biosynthesis of arabinases and L-arabinose catabolic enzymes is under control of CREA.

Regulation of the alpha-glucuronidase-encoding gene ( aguA) from Aspergillus niger.

Abstract. The α-glucuronidase gene aguA from Aspergillus niger was cloned and characterised. Analysis of the promoter region of aguA revealed the presence of four putative binding sites for the major carbon catabolite repressor protein CREA and one putative binding site for the transcriptional activator XLNR. In addition, a sequence motif was detected which differed only in the last nucleotide from the XLNR consensus site. A construct in which part of the aguA coding region was deleted still resulted in production of a stable mRNA upon transformation of A. niger. The putative XLNR binding sites and two of the putative CREA binding sites were mutated individually in this construct and the effects on expression were examined in A. niger transformants. Northern analysis of the transformants revealed that the consensus XLNR site is not actually functional in the aguA promoter, whereas the sequence that diverges from the consensus at a single position is functional. This indicates that XLNR is also able to bind to the sequence GGCTAG, and the XLNR binding site consensus should therefore be changed to GGCTAR. Both CREA sites are functional, indicating that CREA has a strong influence on aguA expression. A detailed expression analysis of aguA in four genetic backgrounds revealed a second regulatory system involved in activation of aguA gene expression. This system responds to the presence of glucuronic and galacturonic acids, and is not dependent on XLNR.

The abfB gene encoding the major alpha-L-arabinofuranosidase of Aspergillus nidulans: nucleotide sequence, regulation and construction of a disrupted strain.

Using a DNA fragment containing the Aspergillus niger abfB gene as a probe, the homologous Aspergillus nidulans gene, designated abfB, has been cloned from a genomic library containing size-selected HindIII fragments. The nucleotide sequence of the A. nidulans abfB gene shows strong homology with the A. niger abfB, Trichoderma reesei abf-1 and Trichoderma koningii alpha-L-arabinofuranosidase/beta-xylosidase genes. Regulation of abfB expression has been investigated in cultures induced with L-arabitol. The accumulation of abfB mRNA, total alpha-L-arabinofuranosidase activity and AbfB protein levels have been determined in a wild-type A. nidulans strain as well as in different mutant strains. These strains are affected either in their response to ambient pH (paIA1 and pacC(c)14 mutants), carbon catabolite repression (creA(d)4 mutant), the ability to utilize L-arabitol as a carbon source (araA1 mutant) or a combination of both latter genotypes (araA1 creA(d)4). The results obtained indicate that the expression of the A. nidulans abfB gene was higher at acidic pHs and was superinduced in this double mutant. Furthermore, disruption of the abfB gene demonstrated that in A. nidulans AbfB is the major p-nitrophenyl alpha-L-arabinofuranoside-hydrolysing activity but at least one minor activity is expressed, which is involved in the release of L-arabinose from polysaccharides.

Aspergillus vadensis, a new species of the group of black Aspergilli

A strain from the group of black Aspergilli was analysed in detail to determine the species to which it belongs. A detailed analysis of morphology, RFLP patterns and metabolite profiles was carried out. In addition, a phylogenetic tree was constructed for the black Aspergilli using the ITS and the β-tubulin sequences of the individual strains. The new species differs by its poor growth on glycerol and galacturonate and its unique extrolite profile consisting of aurasperone B, nigragillin, asperazine and kotanins. RFLP analysis using three genes as probes also resulted in a unique pattern. These data indicate that the strain was closely related but not identical to Aspergillus foetidus, Aspergillus niger and Aspergillus tubingensis. It was therefore designated as a novel species and named Aspergillus vadensis.

Cyclic AMP-dependent protein kinase is involved in morphogenesis of Aspergillus niger

The cAMP signal transduction pathway controls many processes in fungi. The pkaR gene, encoding the regulatory subunit (PKA-R) of cAMP-dependent protein kinase (PKA), was cloned from the industrially important filamentous fungus Aspergillus niger. To investigate the involvement of PKA in morphology of A. niger, a set of transformants which overexpressed pkaR or pkaC (encoding the catalytic subunit of PKA) either individually or simultaneously was prepared as well as mutants in which pkaR and/or pkaC were disrupted. Strains overexpressing pkaR or both pkaC and pkaR could not be distinguished from the wild-type, suggesting that regulation of PKA activity is normal in these strains. Absence of PKA activity resulted in a two- to threefold reduction in colony diameter on plates. The most severe phenotype was observed in the absence of PKA-R, i.e., very small colonies on plates, absence of sporulation and complete loss of growth polarity during submerged growth. Suppressor mutations easily developed in the DeltapkaR mutant and one of these mutants appeared to lack PKA-C activity. These data suggest that cAMP-dependent protein phosphorylation in A. niger regulates growth polarity and formation of conidiospores.

Regulation of acid phosphatases in an Aspergillus niger pacC disruption strain.

AnAspergillus niger strain has been constructed in which the pH-dependent regulatory gene,pacC, was disrupted. ThepacC gene ofA. niger, like that ofA. nidulans, is involved in the regulation of acid phosphatase expression. Disruptants were identified by a reduction in acid phosphatase staining of colonies. Southern analysis demonstrated integration of the disruption plasmid at thepacC locus and Northern analysis showed that the disruption strain produced a truncatedpacC mRNA of 2.2 kb (as compared to 2.8 kb in the wild type). The strain carrying thepacC disruption was used to assign thepacC gene to linkage group IV; this was confirmed by CHEF electrophoresis and Southern analysis. This strain further allowed us to determine which extracellular enzyme and transport systems are under the control ofpacC inA. niger. Expression of theA. niger pacC wild-type gene and the truncatedpacC gene showed that, in contrast to the auto-regulated wild-type expression, which was elevated only at alkaline pH, the truncatedpacC gene was deregulated, as high-level expression occurred regardless of the pH of the culture medium. Analysis of the phosphatase spectrum by isoelectric focussing and enzyme activity staining both in the wild-type and thepacC disruptant showed that at least three acid phosphatases are regulated by thepacC. For the single alkaline phosphatase no pH regulation was observed.

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.

Heterologous expression of the Aspergillus nidulans alcR-alcA system in Aspergillus niger.

The inducible and strongly expressed alcA gene encoding alcohol dehydrogenase I from Aspergillus nidulans was transferred together with the activator gene alcR, in the industrial fungus Aspergillus niger. This latter organism does not possess an inducible alc system but has an endogenously constitutive lowly expressed alcohol dehydrogenase activity. The overall induced expression of the alcA gene was of the same order in both fungi, as monitored by alcA transcription, alcohol dehydrogenase activity and heterologous expression of the reporter enzyme, beta-glucuronidase. However, important differences in the pattern of alcA regulation were observed between the two fungi. A high basal level of alcA transcription was observed in A. niger resulting in a lower ratio of alcA inducibility. This may be due to higher levels of the physiological inducer of the alc regulon, acetaldehyde, from general metabolism in A. niger which differs from that of A. nidulans.