Induction of glucose oxidase, catalase, and lactonase in Aspergillus niger

Identification of Aspergillus niger Aquaporins Involved in Hydrogen Peroxide Signaling

Aspergillus niger is a robust microbial cell factory for organic acid production. However, the regulation of many industrially important pathways is still poorly understood. The regulation of the glucose oxidase (Gox) expression system, involved in the biosynthesis of gluconic acid, has recently been uncovered. The results of that study show hydrogen peroxide, a by-product of the extracellular conversion of glucose to gluconate, has a pivotal role as a signaling molecule in the induction of this system. In this study, the facilitated diffusion of hydrogen peroxide via aquaporin water channels (AQPs) was studied. AQPs are transmembrane proteins of the major intrinsic proteins (MIPs) superfamily. In addition to water and glycerol, they may also transport small solutes such as hydrogen peroxide. The genome sequence of A. niger N402 was screened for putative AQPs. Seven AQPs were found and could be classified into three main groups. One protein (AQPA) belonged to orthodox AQP, three (AQPB, AQPD, and AQPE) were grouped in aquaglyceroporins (AQGP), two (AQPC and AQPF) were in X-intrinsic proteins (XIPs), and the other (AQPG) could not be classified. Their ability to facilitate diffusion of hydrogen peroxide was identified using yeast phenotypic growth assays and by studying AQP gene knock-outs in A. niger. The X-intrinsic protein AQPF appears to play roles in facilitating hydrogen peroxide transport across the cellular membrane in both Saccharomyces cerevisiae and A. niger experiments.

Proteomic enzyme analysis of the marine fungus Paradendryphiella salina reveals alginate lyase as a minimal adaptation strategy for brown algae degradation

We set out to investigate the genetic adaptations of the marine fungus Paradendryphiella salina CBS112865 for degradation of brown macroalgae. We performed whole genome and transcriptome sequencing and shotgun proteomic analysis of the secretome of P. salina grown on three species of brown algae and under carbon limitation. Genome comparison with closely related terrestrial fungi revealed that P. salina had a similar but reduced CAZyme profile relative to the terrestrial fungi except for the presence of three putative alginate lyases from Polysaccharide Lyase (PL) family 7 and a putative PL8 with similarity to ascomycete chondroitin AC lyases. Phylogenetic and homology analyses place the PL7 sequences amongst mannuronic acid specific PL7 proteins from marine bacteria. Recombinant expression, purification and characterization of one of the PL7 genes confirmed the specificity. Proteomic analysis of the P. salina secretome when growing on brown algae, revealed the PL7 and PL8 enzymes abundantly secreted together with enzymes necessary for degradation of laminarin, cellulose, lipids and peptides. Our findings indicate that the basic CAZyme repertoire of saprobic and plant pathogenic ascomycetes, with the addition of PL7 alginate lyases, provide P. salina with sufficient enzymatic capabilities to degrade several types of brown algae polysaccharides.

Forward Genetics by Genome Sequencing Uncovers the Central Role of the Aspergillus niger goxB Locus in Hydrogen Peroxide Induced Glucose Oxidase Expression

Aspergillus niger is an industrially important source for gluconic acid and glucose oxidase (GOx), a secreted commercially important flavoprotein which catalyses the oxidation of β-D-glucose by molecular oxygen to D-glucolactone and hydrogen peroxide. Expression of goxC, the GOx encoding gene and the concomitant two step conversion of glucose to gluconic acid requires oxygen and the presence of significant amounts of glucose in the medium and is optimally induced at pH 5.5. The molecular mechanisms underlying regulation of goxC expression are, however, still enigmatic. Genetic studies aimed at understanding GOx induction have indicated the involvement of at least seven complementation groups, for none of which the molecular basis has been resolved. In this study, a mapping-by-sequencing forward genetics approach was used to uncover the molecular role of the goxB locus in goxC expression. Using the Illumina and PacBio sequencing platforms a hybrid high quality draft genome assembly of laboratory strain N402 was obtained and used as a reference for mapping of genomic reads obtained from the derivative NW103:goxB mutant strain. The goxB locus encodes a thioredoxin reductase. A deletion of the encoding gene in the N402 parent strain led to a high constitutive expression level of the GOx and the lactonase encoding genes required for the two-step conversion of glucose in gluconic acid and of the catR gene encoding catalase R. This high constitutive level of expression was observed to be irrespective of the carbon source and oxidative stress applied. A model clarifying the role of GoxB in the regulation of the expression of goxC involving hydrogen peroxide as second messenger is presented.

Diversity, Application, and Synthetic Biology of Industrially Important Aspergillus Fungi

The filamentous fungal genus Aspergillus consists of over 340 officially recognized species. A handful of these Aspergillus fungi are predominantly used for food fermentation and large-scale production of enzymes, organic acids, and bioactive compounds. These industrially important Aspergilli primarily belong to the two major Aspergillus sections, Nigri and Flavi. Aspergillus oryzae (section Flavi) is the most commonly used mold for the fermentation of soybeans, rice, grains, and potatoes. Aspergillus niger (section Nigri) is used in the industrial production of various enzymes and organic acids, including 99% (1.4 million tons per year) of citric acid produced worldwide. Better understanding of the genomes and the signaling mechanisms of key Aspergillus species can help identify novel approaches to enhance these commercially significant strains. This review summarizes the diversity, current applications, key products, and synthetic biology of Aspergillus fungi commonly used in industry.

Identification of a transcription factor controlling pH-dependent organic acid response in Aspergillus niger

Acid formation in Aspergillus niger is known to be subjected to tight regulation, and the acid production profiles are fine-tuned to respond to the ambient pH. Based on transcriptome data, putative trans-acting pH responding transcription factors were listed and through knock out studies, mutants exhibiting an oxalate overproducing phenotype were identified. The yield of oxalate was increased up to 158% compared to the wild type and the corresponding transcription factor was therefore entitled Oxalic Acid repression Factor, OafA. Detailed physiological characterization of one of the ΔoafA mutants, compared to the wild type, showed that both strains produced substantial amounts of gluconic acid, but the mutant strain was more efficient in re-uptake of gluconic acid and converting it to oxalic acid, particularly at high pH (pH 5.0). Transcriptional profiles showed that 241 genes were differentially expressed due to the deletion of oafA and this supported the argument of OafA being a trans-acting transcription factor. Furthermore, expression of two phosphoketolases was down-regulated in the ΔoafA mutant, one of which has not previously been described in fungi. It was argued that the observed oxalate overproducing phenotype was a consequence of the efficient re-uptake of gluconic acid and thereby a higher flux through glycolysis. This results in a lower flux through the pentose phosphate pathway, demonstrated by the down-regulation of the phosphoketolases. Finally, the physiological data, in terms of the specific oxygen consumption, indicated a connection between the oxidative phosphorylation and oxalate production and this was further substantiated through transcription analysis.

The solubilization of potassium-bearing rock powder by Aspergillus niger in small-scale batch fermentations

The fungus Aspergillus niger was cultivated in culture medium with an alkaline ultramafic rock powder to evaluate the solubilization of potassium for biofertilizer production. The assays were carried out with 2 strains (CCT4355 and CCT911) in small-scale batch fermentations using 125, 500, 1000, and 2000 mL Erlenmeyer flasks, with a nominal volume of 40%, and rock powder at 0.4%, shaken at 160 r/min, incubated at 30 degrees C, and sampled every 7 days for 35 days. The amount of soluble K(+), the pH of the culture medium, and the acidity were determined. Both strains solubilized K(+) from the rock powder to the same extent (approximately 62%-70% after 35 days) in the 125 mL flasks; however, the percent solubilization decreased at higher volumetric scales. The results also indicated a difference in strain sensitivity to the increase in volumetric scales in batch fermentation. When filter-sterilized air was injected into the medium, the K(+) percent solubilization obtained after 4 days of cultivation was similar to that obtained after a 28 day period. The acid production by the fungus may be a mechanism of rock solubilization, in spite of the elevation in pH values probably caused by the increasing hydrolysis of the silicates. Both strains of A. niger are recommended for solubilizing potassium from ultramafic rocks, but it is necessary to optimize the oxygen transfer, which seemed to affect the rock solubilization at higher volumetric scales.

Systemic analysis of the response of Aspergillus niger to ambient pH

BACKGROUND

The filamentous fungus Aspergillus niger is an exceptionally efficient producer of organic acids, which is one of the reasons for its relevance to industrial processes and commercial importance. While it is known that the mechanisms regulating this production are tied to the levels of ambient pH, the reasons and mechanisms for this are poorly understood.

METHODS

To cast light on the connection between extracellular pH and acid production, we integrate results from two genome-based strategies: A novel method of genome-scale modeling of the response, and transcriptome analysis across three levels of pH.

RESULTS

With genome scale modeling with an optimization for extracellular proton-production, it was possible to reproduce the preferred pH levels for citrate and oxalate. Transcriptome analysis and clustering expanded upon these results and allowed the identification of 162 clusters with distinct transcription patterns across the different pH-levels examined. New and previously described pH-dependent cis-acting promoter elements were identified. Combining transcriptome data with genomic coordinates identified four pH-regulated secondary metabolite gene clusters. Integration of regulatory profiles with functional genomics led to the identification of candidate genes for all steps of the pal/pacC pH signalling pathway.

CONCLUSIONS

The combination of genome-scale modeling with comparative genomics and transcriptome analysis has provided systems-wide insights into the evolution of highly efficient acidification as well as production process applicable knowledge on the transcriptional regulation of pH response in the industrially important A. niger. It has also made clear that filamentous fungi have evolved to employ several offensive strategies for out-competing rival organisms.

Filamentous fungi for production of food additives and processing aids

Filamentous fungi are metabolically versatile organisms with a very wide distribution in nature. They exist in association with other species, e.g. as lichens or mycorrhiza, as pathogens of animals and plants or as free-living species. Many are regarded as nature's primary degraders because they secrete a wide variety of hydrolytic enzymes that degrade waste organic materials. Many species produce secondary metabolites such as polyketides or peptides and an increasing range of fungal species is exploited commercially as sources of enzymes and metabolites for food or pharmaceutical applications. The recent availability of fungal genome sequences has provided a major opportunity to explore and further exploit fungi as sources of enzymes and metabolites. In this review chapter we focus on the use of fungi in the production of food additives but take a largely pre-genomic, albeit a mainly molecular, view of the topic.

Secretory and continuous expression of Aspergillus niger glucose oxidase gene in Pichia pastoris

We proposed a yeast transformant cell incorporating the Aspergillus niger glucose oxidase gene (GOX gene), which is capable of constitutively as well as secretory expression. The GOX gene has been cloned in this study. This conclusion is based on the following: first, the ligated DNA determined by electrophoresis, was a 1489-1882bp fragment, close to the size of glucose oxidase (GOD), which is 1818bp. Secondly, the single open reading frame encoded a protein of 605 amino acids. Thirdly, secreted GOD recombinant proteins in the culture supernatants of the GOX gene transformant migrated as a single band in SDS-PAGE with an apparent molecular mass of between 75,000 and 100,000 Da, which is glycosylated GOD by the Pichia pastoris X-33 host machinery during the secretion process. Finally, the clones were cultured and secreted a protein, which possessed the GOD activity of catalyzing beta-d-glucose oxidation. With regard to the pH characteristics, the activity was more than 80% of the maximum activity in the range between pH 5 and pH 7. As for the temperature characteristics, the activity was not less than 92% of the maximum in the temperature range between 10 and 45 degrees C. The GOX gene transformant was able to maintain the GOD enzyme activity and produce recombinant GOD continuously for at least 2 weeks.

Involvement of Gluconic Acid and Glucose Oxidase in the Pathogenicity of Penicillium expansum in Apples

ABSTRACT The contribution of gluconic acid secretion to the colonization of apple tissue by Penicillium expansum was analyzed by modulation (increase or decrease) of gluconic acid accumulation at the infection court. P. expansum isolates that express the most gox2 transcripts and concomitant glucose oxidase (GOX) activity and that secrete the most gluconic acid cause disease of apple at the fastest rate. Cultures grown under reduced oxygen concentration generated fewer gox2 transcripts, produced less gluconic acid, and led to a 15% reduction in disease. Furthermore, the detection of significantly high levels of transcripts of gox2 and GOX activity at the edge of the decaying tissue emphasize the involvement of GOX in tissue acidification of the decaying tissue. Taken together, these results emphasize the importance of GOX in the production of the gluconic acid that leads, in turn, to host tissue acidification. This acidification enhanced the expression of pectolytic enzymes and the establishment of conditions for necrotrophic development of P. expansum.

Biotechnological production of gluconic acid: future implications

Gluconic acid (GA) is a multifunctional carbonic acid regarded as a bulk chemical in the food, feed, beverage, textile, pharmaceutical, and construction industries. The favored production process is submerged fermentation by Aspergillus niger utilizing glucose as a major carbohydrate source, which accompanied product yield of 98%. However, use of GA and its derivatives is currently restricted because of high prices: about US$ 1.20-8.50/kg. Advancements in biotechnology such as screening of microorganisms, immobilization techniques, and modifications in fermentation process for continuous fermentation, including genetic engineering programmes, could lead to cost-effective production of GA. Among alternative carbohydrate sources, sugarcane molasses, grape must show highest GA yield of 95.8%, and banana must may assist reducing the overall cost of GA production. These methodologies would open new markets and increase applications of GA.

Bioconversion of grape must into modulated gluconic acid production by Aspergillus niger ORS-4.410

AIMS

Analysis of regulators for modulated gluconic acid production under surface fermentation (SF) condition using grape must as the cheap carbohydrate source, by mutant Aspergillus niger ORS-4.410. Replacement of conventional fermentation condition by solid-state surface fermentation (SSF) for semi-continuous production of gluconic acid by pseudo-immobilization of A. niger ORS-4.410.

METHODS AND RESULTS

Grape must after rectification was utilized for gluconic acid production in batch fermentation in SF and SSF processes using mutant strain of A. niger ORS-4.410. Use of rectified grape must led to the improved levels of gluconic acid production (80-85 g l(-1)) in the fermentation medium containing 0.075% (NH4)2HPO4; 0.1% KH2PO4 and 0.015% MgSO4.7H2O at an initial pH 6.6 (+/-0.1) under surface fermentation. Gluconic acid production was modulated by incorporating the 2% soybean oil, 2% starch and 1% H2O2 in fermentation medium at continuously high aeration rate (2.0 l min(-1)). Interestingly, 95.8% yield of gluconic acid was obtained when A. niger ORS-4.410 was pseudo-immobilized on cellulose fibres (bagasse) under SSF. Four consecutive fermentation cycles were achieved with a conversion rate of 0.752-0.804 g g(-1) of substrate into gluconic acid under SSF.

CONCLUSIONS

Use of additives modulated the gluconic acid production under SF condition. Semi-continuous production of gluconic acid was achieved with pseudo-immobilized mycelia of A. niger ORS-4.410 having a promising yield (95.8%) under SSF condition.

SIGNIFICANCE AND IMPACT OF THE STUDY

The bioconversion of grape must into modulated gluconic acid production under SSF conditions can further be employed in fermentation industries by replacing the conventional carbohydrate sources and expensive, energy consuming fermentation processes.

Screening, mutagenesis and protoplast fusion of Aspergillus niger for the enhancement of extracellular glucose oxidase production

Various strains of Aspergillus niger were screened for extracellular glucose oxidase (GOD) activity. The most effective producer, strain FS-3 (15.9 U mL(-1)), was mutagenized using UV-irradiation or ethyl methane sulfonate. Of the 400 mutants obtained, 32 were found to be resistant to 2-deoxy D: -glucose, and 17 of these exhibited higher GOD activities (from 114.5 to 332.1%) than the original FS-3 strain. Following determination of antifungal resistance of the highest producing mutants, four mutants were selected and used in protoplast fusions in three different intraspecific crosses. All fusants showed higher activities (from 285.5 to 394.2%) than the original strain. Moreover, of the 30 fusants isolated, 19 showed higher GOD activity than their corresponding higher-producing parent strain.

Cloning and characterization of gluconolactone oxidase of Penicillium cyaneo-fulvum ATCC 10431 and evaluation of its use for production of D-erythorbic acid in recombinant Pichia pastoris

A D-erythorbic acid-forming soluble flavoprotein, gluconolactone oxidase (GLO), was purified from Penicillium cyaneo-fulvum strain ATCC 10431 and partially sequenced. Peptide sequences were used to isolate a cDNA clone encoding the enzyme. The cloned gene exhibits high levels of similarity with the genes encoding other known eukaryotic lactone oxidases and also with the genes encoding some putative prokaryotic lactone oxidases. Analysis of the coding sequence of the GLO gene indicated the presence of a typical secretion signal sequence at the N terminus of GLO. No other targeting or anchoring signals were found, suggesting that GLO is the first known lactone oxidase that is secreted rather than targeted to the membranes of the endoplasmic reticulum or mitochondria. Experimental evidence, including the N-terminal sequence of mature GLO and data on glycosylation and localization of the enzyme in native and recombinant hosts, supports this analysis. The GLO gene was expressed in Pichia pastoris, and recombinant GLO was produced by using the strong methanol-induced AOX1 promoter. In order to evaluate the suitability of purified GLO for production of D-erythorbic acid, we immobilized it on N-hydroxysuccinimide-activated Sepharose and found that the immobilized GLO retained full activity during immobilization but was rather unstable under reaction conditions. Our results show that both soluble and immobilized forms of GLO can, in principle, be used for production of D-erythorbic acid from D-glucono-delta-lactone or (in combination with glucose oxidase and catalase) from glucose. We also demonstrated the feasibility of glucose-D-erythorbic acid fermentation with recombinant strains coexpressing GLO and glucose oxidase genes, and we analyzed problems associated with construction of efficient D-erythorbic acid-producing hosts.

Aspergillus niger Protein EstA Defines a New Class of Fungal Esterases within the α/β Hydrolase Fold Superfamily of Proteins

From the fungus Aspergillus niger, we identified a new gene encoding protein EstA, a member of the alpha/beta-hydrolase fold superfamily but of unknown substrate specificity. EstA was overexpressed and its crystal structure was solved by molecular replacement using a lipase-acetylcholinesterase chimera template. The 2.1 A resolution structure of EstA reveals a canonical Ser/Glu/His catalytic triad located in a small pocket at the bottom of a large solvent-accessible, bowl-shaped cavity. Potential substrates selected by manual docking procedures were assayed for EstA activity. Consistent with the pocket geometry, preference for hydrolysis of short acyl/propyl chain substrates was found. Identification of close homologs from the genome of other fungi, of which some are broad host-range pathogens, defines EstA as the first member of a novel class of fungal esterases within the superfamily. Hence the structure of EstA constitutes a lead template in the design of new antifungal agents directed toward its pathogenic homologs.

Autolysis and ageing of Penicillium chrysogenum under carbon starvation: respiration and glucose oxidase production

During the exponential growth phase of Penicillium chrysogenum NCAIM 00237 the effective conversion of glucose and O2 to gluconate and H2O2 by glucose oxidase (GOX) was the most likely source of intracellular ROS measured. In glucose-supplemented autolysing cultures, the increased of intracellular ROS concentration was attributed to respiration in the absence of any significant GOX activity. The induction of GOX and catalase by glucose and H2O2 was clearly age-dependent in P. chrysogenum. In ageing cryptic growth phase cultures, superoxide dismutase and cyanide-resistant respiration were the major elements of antioxidative defence but these activities were insufficient to prevent the progressive accumulation of ROS and the concomitant decrease in cell vitality.

The effect of pH on glucoamylase production, glycosylation and chemostat evolution of Aspergillus niger

The effect of ambient pH on production and glycosylation of glucoamylase (GAM) and on the generation of a morphological mutant produced by Aspergillus niger strain B1 (a transformant containing an additional 20 copies of the homologous GAM glaA gene) was studied. We have shown that a change in the pH from 4 to 5.4 during continuous cultivation of the A. niger B1 strain instigates or accelerates the spontaneous generation of a morphological mutant (LB). This mutant strain produced approx. 50% less extracellular protein and GAM during both chemostat and batch cultivation compared to another strain with parental-type morphology (PS). The intracellular levels of GAM were also lower in the LB strain. In addition, cultivation of the original parent B1 strain in a batch-pulse bioreactor at pH 5.5 resulted in a 9-fold drop in GAM production and a 5-fold drop in extracellular protein compared to that obtained at pH 4. Glycosylation analysis of the glucoamylases purified from shake-flask cultivation showed that both principal forms of GAM secreted by the LB strain possessed enhanced galactosylation (2-fold), compared to those of the PS. Four diagnostic methods (immunostaining, mild methanolysis, mild acid hydrolysis and beta-galactofuranosidase digestion) provided evidence that the majority of this galactose was of the furanoic conformation. The GAMs produced during batch-pulse cultivation at pH 5.5 similarly showed an approx. 2-fold increase in galactofuranosylation compared to pH 4. Interestingly, in both cases the increased galactofuranosylation appears primarily restricted to the O-linked glycan component. Ambient pH therefore regulates both GAM production and influences its glycosylation.

"Oxidative stress" response in submerged cultures of a recombinant Aspergillus niger (B1-D)

A recombinant strain of Aspergillus niger (B1-D), engineered to produce the marker protein hen egg white lysozyme, was investigated with regard to its susceptibility to "oxidative stress" in submerged culture in bioreactor systems. The culture response to oxidative stress, produced either by addition of exogenous hydrogen peroxide or by high-dissolved oxygen tensions, was examined in terms of the activities of two key defensive enzymes: catalase (CAT) and superoxide dismutase (SOD). Batch cultures in the bioreactor were generally found to have maximum specific activities of CAT and SOD (Umg x protein(-1)) in the stationary/early-decline phase. Continuous addition of H2O2 (16 mmole L(-1) h(-1)), starting in the early exponential phase, induced CAT but did not increase SOD significantly. Gassing an early exponential-phase culture with O2 enriched (25 vol%) air resulted in increased activities of both SOD and CAT relative to control processes gassed continuously with air, while gassing the culture with 25 vol% O2 enriched air throughout the experiment, although inducing a higher base level of enzyme activities, did not increase the maximum SOD activity obtained relative to control processes gassed continuously with air. The profile of the specific activity of SOD (U mg CDW(-1)) appeared to correlate with dissolved oxygen levels in processes where no H2O2 addition occurred. These findings indicate that it is unsound to use the term "oxidative stress" to encompass a stress response produced by addition of a chemical (H2O2) or by elevated dissolved oxygen levels because the response to each might be quite different.

Production of catalase and glucose oxidase by Aspergillus niger using unconventional oxygenation of culture

A novel method for increasing dissolved oxygen concentration in culture media has been developed. It involves adding hydrogen peroxide (H2O2) to the medium which is then decomposed to oxygen and water by catalase. Some factors affecting oxygenation of culture were investigated. Maximal oxygen concentration occurred in 50 ml of the medium containing 0-2 g wet mycelium and 0.2% glucose at pH 5.0. A new apparatus for automated addition of H2O2 to the bioreactor to keep the dissolved oxygen concentration constant over the range 1-100% +/- 2% was tested. A significant increase (over sixfold) of intracellular catalase activity was obtained while the dissolved oxygen concentration remained stable (30% +/- 2%).

Properties of Aspergillus niger citrate synthase and effects of citA overexpression on citric acid production

Using a combination of dye adsorption and affinity elution we purified Aspergillus niger citrate synthase to homogeneity using a single column and characterised the enzyme. An A. niger citrate synthase cDNA was isolated by immunological screening and used to clone the corresponding citA gene. The deduced amino acid sequence showed high similarity to other fungal citrate synthases. After processing upon mitochondrial import, the calculated M(r) of A. niger citrate synthase is 48501, which agrees well with the estimated molecular mass of the purified protein (48 kDa). In addition to an N-terminal mitochondrial import signal, a peroxisomal target sequence (AKL) was found at the C-terminus of the protein. Whether both signals are functional in vivo is not clear. Strains overexpressing citA were made by transformation and cultured under citric acid-producing conditions. Up to 11-fold overproduction of citrate synthase did not increase the rate of citric acid production by the fungus, suggesting that citrate synthase contributes little to flux control in the pathway involved in citric acid biosynthesis by a non-commercial strain.

Cloning and characterization of two rhamnogalacturonan hydrolase genes from Aspergillus niger

A rhamnogalacturonan hydrolase gene of Aspergillus aculeatus was used as a probe for the cloning of two rhamnogalacturonan hydrolase genes of Aspergillus niger. The corresponding proteins, rhamnogalacturonan hydrolases A and B, are 78 and 72% identical, respectively, with the A. aculeatus enzyme. In A. niger cultures which were shifted from growth on sucrose to growth on apple pectin as a carbon source, the expression of the rhamnogalacturonan hydrolase A gene (rhgA) was transiently induced after 3 h of growth on apple pectin. The rhamnogalacturonan hydrolase B gene was not induced by apple pectin, but the rhgB gene was derepressed after 18 h of growth on either apple pectin or sucrose. Gene fusions of the A. niger rhgA and rhgB coding regions with the strong and inducible Aspergillus awamori exlA promoter were used to obtain high-producing A. awamori transformants which were then used for the purification of the two A. niger rhamnogalacturonan hydrolases. High-performance anion-exchange chromatography of oligomeric degradation products showed that optimal degradation of an isolated highly branched pectin fraction by A. niger rhamnogalacturonan hydrolases A and B occurred at pH 3.6 and 4.1, respectively. The specific activities of rhamnogalacturonan hydrolases A and B were then 0.9 and 0.4 U/mg, respectively, which is significantly lower than the specific activity of A. aculeatus rhamnogalacturonan hydrolase (2.5 U/mg at an optimal pH of 4.5). Compared to the A enzymes, the A. niger B enzyme appears to have a different substrate specificity, since additional oligomers are formed.

Selective isolation of Aspergillus niger mutants with enhanced glucose oxidase production

After mutagenization and selection, mutant Aspergillus niger strains resistant to certain agents were obtained. Seven of the mutants showed increased extracellular glucose oxidase (GOD), the level for individual cases ranged widely from 8.8 to over 138.5% in comparison with the parental strain. Studies of the relationship between method of selection and frequency of mutation showed that the highest frequency of positive mutations (15.8% and 17.3%) was obtained from mutants resistant to ethidium bromide (1 mmol l-1 and sodium gluconate (45%), respectively. The time course of growth and enzyme production by the most active mutant AM-11 showed intra- and extracellular GOD activities to have increased about 2.2- and 2.4-fold, respectively, compared with the parental strain.

Characterization and overexpression of the Aspergillus niger gene encoding the cAMP-dependent protein kinase catalytic subunit

The gene pkaC encoding the catalytic subunit of cAMP-dependent protein kinase has been isolated from the industrially important filamentous fungus Aspergillus niger. A probe for screening A. niger phage libraries was generated by a polymerase chain reaction using degenerate primers. cDNA and genomic DNA clones were isolated and sequenced. An open reading frame of 1440 bp, interrupted by three short introns, encodes a polypeptide of 480 amino acids with a calculated molecular mass of 53813 Da. The cAMP-dependent protein kinase catalytic subunit (PKA-C) from A. niger has a 126 amino acid extension at the N-terminus compared to the PKA-C of higher eukaryotes that-except for the first 15 amino acids, which are homologous to the Magnaporthe grisea PKA-C-shows no significant similarity to the N-terminal extension of PKA-C of other lower eukaryotes. The catalytic core of PKA-C of A. niger shows extensive homology with the PKA-C isolated from all other eukaryotes. Low-stringency hybridization did not reveal any other pkaC homologue in A. niger. The cloned pkaC was used for transformation of A. niger, leading to increased levels of pkaC mRNA and PKA-C activity. Transformants overexpressing pkaC were phenotypically different with respect to growth, showing a more compact colony morphology, accompanied by a more dense sporulation, especially on media containing trehalose and glycerol. A number of transformants also showed a strongly reduced or complete absence of sporulation. This phenotype was quickly lost upon propagation of the strains.

A new glucose oxidase from Aspergillus niger: characterization and regulation studies of enzyme and gene

A new glucose oxidase from Aspergillus niger was isolated and characterized. The enzyme showed different kinetic and stability characteristics when compared to a commercially available batch of A. niger glucose oxidase. The gene encoding the new glucose oxidase was isolated and DNA sequence analysis of the coding region showed 80% identity to the sequence of a glucose oxidase gene previously published. However, the similarity of the non-coding sequences up- and downstream of the open reading frame was much less, showing only 66% and 50% identity respectively. Despite the low degree of similarity between the promotor region of the new gene and the previously published one, the new glucose oxidase was likewise induced by calcium carbonate. In addition, we showed that this induction occurred on the transcriptional level. Observations concerning the effect of gluconolactone and the levels of glucose-6 phosphate isomerase upon calcium carbonate induction suggested that the enhancement of glucose oxidase biosynthesis by calcium carbonate was accompanied by a metabolic shift from glycolysis to the pentose phosphate pathway.

Optimization of glucose oxidase production by Aspergillus niger using genetic- and process-engineering techniques

Wild-type Aspergillus niger NRRL-3 was transformed with multiple copies of the glucose oxidase structural gene (god). The gene was placed under the control of the gpdA promoter of A. nidulans. For more efficient secretion the alpha-amylase signal peptide from A. oryzae was inserted in front of god. Compared to the wild type, the recombinant strain NRRL-3 (GOD3-18) produced up to four times more extracellular glucose oxidase under identical culture conditions. Addition of yeast extract (2 gl-1) to a mineral salts medium containing only glucose as carbon source increased volumetric and specific extracellular glucose oxidase activities by 130% and 50% respectively. With the same medium composition and inoculum size, volumetric and specific extracellular glucose oxidase activities increased more than ten times in bioreactor cultivations compared to shake-flask cultures.

Regulation of the xylanase-encoding xlnA gene of Aspergillus tubigensis

A gene encoding an endo-1,4-beta-xylanase from Aspergillus tubigensis was cloned by oligonucleotide screening using oligonucleotides derived from amino acid sequence data obtained from the purified protein. The isolated gene was functional as it could be expressed in the very closely related fungus Aspergillus niger. The xylanase encoded by this gene is synthesized as a protein of 211 amino acids. After cleavage of the presumed prepropeptide this results in a mature protein of 184 amino acids with a molecular weight of 19 kDa and an isoelectric point of 3.6. The regulatory region of the xlnA gene was studied with respect to the response to xylan induction and carbon catabolite repression. By deletion analysis of the 5' upstream region of the gene a 158 bp region involved in the xylan specific induction was identified. To study this regulatory element a reporter system for transcriptional activating sequences was developed that is based on the A. niger glucose oxidase-encoding gene. From the results with this reporter system it is concluded that this 158 bp fragment not only contains the information required for induction of transcription but that it also plays a role in carbon catabolite repression of the xlnA gene. The region directly upstream of this fragment contains four potential CREA target sites; deletion of this region leads to an increase in the level of transcription. These results suggest that carbon catabolite repression of the xlnA gene is controlled at two levels, directly by repression of xlnA gene transcription and indirectly by repression of the expression of a transcriptional activator. This type of mechanism would be similar to the double lock mechanism for the regulation of gene expression of alcA in Aspergillus nidulans. The reporter system was also used to study the regulation of expression via the functions located on this fragment in A. niger and in A. nidulans. Essentially the same pattern of regulation was found in both of these hosts. Therefore, regulation of xylanase gene expression is basically conserved in all three aspergilli.