Heterologous gene expression in Aspergillus niger: a glucoamylase-porcine pancreatic prophospholipase A2 fusion protein is secreted and processed to yield mature enzyme

Expression of microbial lipase in filamentous fungus Aspergillus niger: a review

Lipase has high economic importance and is widely used in biodiesel, food, detergents, cosmetics, and pharmaceutical industries. The rapid development of synthetic biology and system biology has not only paved the way for comprehensively understanding the efficient operation mechanism of Aspergillus niger cell factories but also introduced a new technological system for creating and optimizing high-efficiency A. niger cell factories. In this review, all relevant data on microbial lipase enzyme sources and general properties are gathered and updated. The relationship between A. niger strain morphology and protein production is discussed. The safety of A. niger strain is investigated to ensure product safety. The biotechnologies and factors influencing lipase expression in A. niger are summarized. This review focuses on various strategies to improve lipase expression in A. niger. The summary of these methods and the application of the gene editing technology CRISPR/Cas9 system can further improve the efficiency of constructing the engineered lipase-producing A. niger.

Establishment, optimization, and application of genetic technology in Aspergillus spp

Aspergillus is widely distributed in nature and occupies a crucial ecological niche, which has complex and diverse metabolic pathways and can produce a variety of metabolites. With the deepening of genomics exploration, more Aspergillus genomic informations have been elucidated, which not only help us understand the basic mechanism of various life activities, but also further realize the ideal functional transformation. Available genetic engineering tools include homologous recombinant systems, specific nuclease based systems, and RNA techniques, combined with transformation methods, and screening based on selective labeling. Precise editing of target genes can not only prevent and control the production of mycotoxin pollutants, but also realize the construction of economical and efficient fungal cell factories. This paper reviewed the establishment and optimization process of genome technologies, hoping to provide the theoretical basis of experiments, and summarized the recent progress and application in genetic technology, analyzes the challenges and the possibility of future development with regard to Aspergillus.

Design of Artificial Enzymes Bearing Several Active Centers: New Trends, Opportunities and Problems

Harnessing enzymes which possess several catalytic activities is a topic where intense research has been carried out, mainly coupled with the development of cascade reactions. This review tries to cover the different possibilities to reach this goal: enzymes with promiscuous activities, fusion enzymes, enzymes + metal catalysts (including metal nanoparticles or site-directed attached organometallic catalyst), enzymes bearing non-canonical amino acids + metal catalysts, design of enzymes bearing a second biological but artificial active center (plurizymes) by coupling enzyme modelling and directed mutagenesis and plurizymes that have been site directed modified in both or in just one active center with an irreversible inhibitor attached to an organometallic catalyst. Some examples of cascade reactions catalyzed by the enzymes bearing several catalytic activities are also described. Finally, some foreseen problems of the use of these multi-activity enzymes are described (mainly related to the balance of the catalytic activities, necessary in many instances, or the different operational stabilities of the different catalytic activities). The design of new multi-activity enzymes (e.g., plurizymes or modified plurizymes) seems to be a topic with unarguable interest, as this may link biological and non-biological activities to establish new combo-catalysis routes.

Developing Aspergillus niger as a cell factory for food enzyme production

Aspergillus niger has become one of the most important hosts for food enzyme production due to its unique food safety characteristics and excellent protein secretion systems. A series of food enzymes such as glucoamylase have been commercially produced by A. niger strains, making this species a suitable platform for the engineered of strains with improved enzyme production. However, difficulties in genetic manipulations and shortage of expression strategies limit the progress in this regard. Moreover, several mycotoxins have recently been detected in some A. niger strains, which raises the necessity for a regulatory approval process for food enzyme production. With robust strains, processing engineering strategies are also needed for producing the enzymes on a large scale, which is also challenging for A. niger, since its culture is aerobic, and non-Newtonian fluid properties are developed during submerged culture, making mixing and aeration very energy-intensive. In this article, the progress and challenges of developing A. niger for the production of food enzymes are reviewed, including its genetic manipulations, strategies for more efficient production of food enzymes, and elimination of mycotoxins for product safety.

Engineering of a Microbial Cell Factory for the Extracellular Production of Catalytically Active Phospholipase A2 of Streptomyces violaceoruber

Phospholipase A2 (PLA2) from Streptomyces violaceoruber is a lipolytic enzyme used in a wide range of industrial applications including production of lysolecithins and enzymatic degumming of edible oils. We have therefore investigated expression and secretion of PLA2 in two workhorse microbes, Pichia pastoris and Escherichia coli. The PLA2 was produced to an activity of 0.517 ± 0.012 U/ml in the culture broth of the recombinant P. pastoris. On the other hand, recombinant E. coli BL21 star (DE3), overexpressing the authentic PLA2 (P-PLA2), showed activity of 17.0 ± 1.3 U/ml in the intracellular fraction and 21.7 ± 0.7 U/ml in the culture broth. The extracellular PLA2 activity obtained with the recombinant E. coli system was 3.2-fold higher than the corresponding value reached in a previous study, which employed recombinant E. coli BL21 (DE3) overexpressing codon-optimized PLA2. Finally, we observed that the extracellular PLA2 from the recombinant E. coli P-PLA2 culture was able to hydrolyze 31.1 g/l of crude soybean lecithin, an industrial substrate, to a conversion yield of approximately 95%. The newly developed E. coli-based PLA2 expression system led to extracellular production of PLA2 to a productivity of 678 U/l·h, corresponding to 157-fold higher than that obtained with the P. pastoris-based system. This study will contribute to the extracellular production of a catalytically active PLA2.

Improved Homologous Expression of the Acidic Lipase from Aspergillus niger

In this study, the acidic lipase from Aspergillus niger (ANL) was homologously expressed in A. niger. The expression of ANL was significantly improved by the expression of the native ANL with the introns, the addition of the Kozak sequence and the optimization of the signal sequences. When the cDNA sequence of ANL fused with the glaA signal was expressed under the gpdA promoter in A. niger, no lipase activity could be detected. We then tried to improve the expression by using the full-length ANL gene containing three introns, and the lipase activity in the supernatant reached 75.80 U/ml, probably as a result of a more stable mRNA structure. The expression was further improved to 100.60 U/ml by introducing a Kozak sequence around the start codon due to a higher translation efficiency. Finally, the effects of three signal sequences including the cbhI signal, the ANL signal and the glaA signal on the lipase expression were evaluated. The transformant with the cbhI signal showed the highest lipase activity (314.67 U/ml), which was 1.90-fold and 3.13-fold higher than those with the ANL signal and the glaA signal, respectively. The acidic lipase was characterized and its highest activity was detected at pH 3.0 and a temperature of 45°C. These results provided promising strategies for the production of the acidic lipase from A. niger.

How a fungus shapes biotechnology: 100 years of Aspergillus niger research

In 1917, a food chemist named James Currie made a promising discovery: any strain of the filamentous mould Aspergillus niger would produce high concentrations of citric acid when grown in sugar medium. This tricarboxylic acid, which we now know is an intermediate of the Krebs cycle, had previously been extracted from citrus fruits for applications in food and beverage production. Two years after Currie’s discovery, industrial-level production using A. niger began, the biochemical fermentation industry started to flourish, and industrial biotechnology was born. A century later, citric acid production using this mould is a multi-billion dollar industry, with A. niger additionally producing a diverse range of proteins, enzymes and secondary metabolites. In this review, we assess main developments in the field of A. niger biology over the last 100 years and highlight scientific breakthroughs and discoveries which were influential for both basic and applied fungal research in and outside the A. niger community. We give special focus to two developments of the last decade: systems biology and genome editing. We also summarize the current international A. niger research community, and end by speculating on the future of fundamental research on this fascinating fungus and its exploitation in industrial biotechnology.

Drugs and drug intermediates from fungi: Striving for greener processes

There is an ever-increasing demand of newer and improved drugs from biological sources to cater to the bio-pharmaceutical sector. Among various other resources, fungal species have an immense contribution owing to their potential to carry out the bio-transformations and drug synthesis in diverse conditions and in an eco-friendly manner. Advancement in the biotechnological processes has accelerated the process. Genome sequence information of various fungal species has opened newer avenues for improved and faster drug targeting and designing. The review highlights the production of pharmaceutical drugs and drug intermediates like antibiotics, anti-cancer, anti-cholesterol, anti-diabetic, immunosuppressant, anti-anxiety, anti-virals and many other drugs from fungus. Many of these have been commercialized and there are many more which are either in research or in clinical trial phase. There is a need to exploit and explore the vast biota of fungi in the hope of discovering untapped therapeutic uses of the earth's countless species of fungus.

Extracellular production of phospholipase A2 from Streptomyces violaceoruber by recombinant Escherichia coli

Phospholipase A(2) (PLA(2)) from Streptomyces violaceoruber was successfully produced extracellularly in an active form by using a recombinant strain of Escherichia coli. The PLA(2) gene, which was artificially synthesized with optimized codons for E. coli and fused with pelB signal sequence, was expressed in E. coli using pET system. Most of the enzyme activity was detected in the culture supernatant with negligible activity in the cells. The recombinant enzyme was purified to homogeneity from the culture supernatant simply by ammonium sulfate precipitation and an anion exchange chromatography. The purified enzyme showed a specific activity comparable to that of the authentic enzyme. The recombinant enzyme had the same N-terminal amino acid sequence to that of the mature protein, indicating the correct removal of the signal peptide. An inactive PLA(2) with a mutation at the catalytic center was also secreted to the culture medium, suggesting that the observed secretion was not dependent on enzymatic activity. A simple screening method for the PLA(2)-producing colonies was established by detecting clear zone formation around the colonies on agar media containing lecithin. This is the first example of direct extracellular production of active PLA(2) by recombinant E. coli.

Production, purification, and characterization of human alpha1 proteinase inhibitor from Aspergillus niger

Human alpha one proteinase inhibitor (alpha1-PI) was cloned and expressed in Aspergillus niger, filamentious fungus that can grow in defined media and can perform glycosylation. Submerged culture conditions were established using starch as carbon source, 30% dissolved oxygen concentration, pH 7.0 and 28 degrees C. Eight milligrams per liter of active alpha1-PI were secreted to the growth media in about 40 h. Controlling the protein proteolysis was found to be an important factor in the production. The effects of various carbon sources, pH and temperature on the production and stability of the protein were tested and the product was purified and characterized. Two molecular weights variants of the recombinant alpha1-PI were produced by the fungus; the difference is attributed to the glycosylated part of the molecule. The two glycoproteins were treated with PNGAse F and the released glycans were analyzed by HPAEC, MALDI/TOF-MS, NSI-MS(n), and GC-MS. The MALDI and NSI- full MS spectra of permethylated N-glycans revealed that the N-glycans of both variants contain a series of high-mannose type glycans with 5-20 hexose units. Monosaccharide analysis showed that these were composed of N-acetylglucos-amine, mannose, and galactose. Linkage analysis revealed that the galactosyl component was in the furanoic conformation, which was attaching in a terminal non-reducing position. The Galactofuranose-containing high-mannnose type N-glycans are typical structures, which recently have been found as part of several glycoproteins produced by Aspergillus niger.

Effect of culture temperature on the heterologous expression of Pleurotus eryngii versatile peroxidase in Aspergillus hosts

Production of recombinant versatile peroxidase in Aspergillus hosts was optimized through the modification of temperature during bioreactor cultivations. To further this purpose, the cDNA encoding a versatile peroxidase of Pleurotus eryngii was expressed under control of the alcohol dehydrogenase (alcA) promoter of Aspergillus nidulans. A dependence of recombinant peroxidase production on cultivation temperature was found. Lowering the culture temperature from 28 to 19 degrees C enhanced the level of active peroxidase 5.8-fold and reduced the effective proteolytic activity twofold. Thus, a maximum peroxidase activity of 466 U L(-1) was reached. The same optimization scheme was applied to a recombinant Aspergillus niger that bore the alcohol dehydrogenase regulator (alcR), enabling transformation with the peroxidase cDNA under the same alcA promoter. However, with this strain, the peroxidase activity was not improved, while the effective proteolytic activity was increased between 3- and 11-fold compared to that obtained with A. nidulans.

Expression of human alpha1-proteinase inhibitor in Aspergillus niger

Background

Human α₁-proteinase inhibitor (α₁-PI), also known as antitrypsin, is the most abundant serine protease inhibitor (serpin) in plasma. Its deficiency is associated with development of progressive, ultimately fatal emphysema. Currently in the United States, α₁-PI is available for replacement therapy as an FDA licensed plasma-derived (pd) product. However, the plasma source itself is limited; moreover, even with efficient viral inactivation steps used in manufacture of plasma products, the risk of contamination from emerging viruses may still exist. Therefore, recombinant α₁-PI (r-α₁-PI) could provide an attractive alternative. Although r-α₁-PI has been produced in several hosts, protein stability in vitro and rapid clearance from the circulation have been major issues, primarily due to absent or altered glycosylation.

Results

We have explored the possibility of expressing the gene for human α₁-PI in the filamentous fungus Aspergillus niger (A. niger), a system reported to be capable of providing more "mammalian-like" glycosylation patterns to secretable proteins than commonly used yeast hosts. Our expression strategy was based on fusion of α₁-PI with a strongly expressed, secreted leader protein (glucoamylase G2), separated by dibasic processing site (N-V-I-S-K-R) that provides in vivo cleavage. SDS-PAGE, Western blot, ELISA, and α₁-PI activity assays enabled us to select the transformant(s) secreting a biologically active glycosylated r-α₁-PI with yields of up to 12 mg/L. Matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) analysis further confirmed that molecular mass of the r-α₁-PI was similar to that of the pd-α₁-PI. In vitro stability of the r-α₁-PI from A. niger was tested in comparison with pd-α₁-PI reference and non-glycosylated human r-α₁-PI from E. coli.

Conclusion

We examined the suitability of the filamentous fungus A. niger for the expression of the human gene for α₁-PI, a medium size glycoprotein of high therapeutic value. The heterologous expression of the human gene for α₁-PI in A. niger was successfully achieved to produce the secreted mature human r-α₁-PI in A. niger as a biologically active glycosylated protein with improved stability and with yields of up to 12 mg/L in shake-flask growth.

Phospholipases and their industrial applications

Phospholipids are present in all living organisms. They are a major component of all biological membranes, along with glycolipids and cholesterol. Enzymes aimed at modifying phospholipids, namely, phospholipases, are consequently widespread in nature, playing very diverse roles from aggression in snake venom to signal transduction and digestion in humans. In this review, we give a general overview of phospholipases A1, A2, C and D from a sequence and structural perspective and their industrial application. The use of phospholipases in industrial processes has grown hand-in-hand with our ability to clone and express the genes in microbial hosts with commercially attractive amounts. Further, the use in industrial processes is increasing by optimizing the enzymes by protein engineering. Here, we give a perspective on the work done to date to express phospholipases in heterologous hosts and the efforts to optimize them by protein engineering. We will draw attention to the industrial processes where phospholipases play a key role and show how the use of a phospholipase for oil degumming leads to substantial environmental benefits. This illustrates a very general trend: the use of enzymes as an alternative to chemical processes to make products often provides a cleaner solution for the industrial processes. In a world with great demands on non-polluting, energy saving technical solutions--white biotechnology is a strong alternative.

Chymostatin can combine with pepstatin to eliminate extracellular protease activity in cultures of Aspergillus niger NRRL-3

Aspergillus strains are being considered as potential hosts for recombinant heterologous protein production because of their excellent extracellular enzyme production characteristics. However, Aspergillus proteases are problematic in that they modify and degrade the heterologous proteins in the extracellular medium. In previous studies we observed that media adjustments and maintenance of a filamentous morphology greatly reduced protease activity and that a low concentration of the aspartic protease inhibitor pepstatin inhibited the latter protease activity to the extent of approximately 90%. In this paper we report that when the serine protease inhibitor chymostatin is used in combination with pepstatin 99-100% of total protease activity in Aspergillus cultures is inhibited. In protease assays a concentration of 30 microM chymostatin combined with 0.075 microM pepstatin was required for maximum inhibition. Inhibitor concentrations of chymostatin and pepstatin of 120 and 0.3 microM, respectively, when added to Aspergillus cultures, has no significant effect on biomass production, glucose utilization or culture pH pattern. The potential of using these protease inhibitors in cultures of recombinant Aspergillus strains producing heterologous proteins will now be investigated to determine if the previously observed recombinant protein denaturing effects of Aspergillus proteases can be negated.

Generation, annotation, and analysis of an extensive Aspergillus niger EST collection

BACKGROUND

Aspergillus niger, a saprophyte commonly found on decaying vegetation, is widely used and studied for industrial purposes. Despite its place as one of the most important organisms for commercial applications, the lack of available information about its genetic makeup limits research with this filamentous fungus.

RESULTS

We present here the analysis of 12,820 expressed sequence tags (ESTs) generated from A. niger cultured under seven different growth conditions. These ESTs identify about 5,108 genes of which 44.5% code for proteins sharing similarity (E < or = 1e(-5)) with GenBank entries of known function, 38% code for proteins that only share similarity with GenBank entries of unknown function and 17.5% encode proteins that do not have a GenBank homolog. Using the Gene Ontology hierarchy, we present a first classification of the A. niger proteins encoded by these genes and compare its protein repertoire with other well-studied fungal species. We have established a searchable web-based database that includes the EST and derived contig sequences and their annotation. Details about this project and access to the annotated A. niger database are available.

CONCLUSION

This EST collection and its annotation provide a significant resource for fundamental and applied research with A. niger. The gene set identified in this manuscript will be highly useful in the annotation of the genome sequence of A. niger, the genes described in the manuscript, especially those encoding hydrolytic enzymes will provide a valuable source for researchers interested in enzyme properties and applications.

Design and production in Aspergillus niger of a chimeric protein associating a fungal feruloyl esterase and a clostridial dockerin domain

A chimeric enzyme associating feruloyl esterase A (FAEA) from Aspergillus niger and dockerin from Clostridium thermocellum was produced in A. niger. A completely truncated form was produced when the dockerin domain was located downstream of the FAEA (FAEA-Doc), whereas no chimeric protein was produced when the bacterial dockerin domain was located upstream of the FAEA (Doc-FAEA). Northern blot analysis showed similar transcript levels for the two constructs, indicating a posttranscriptional bottleneck for Doc-FAEA production. The sequence encoding the first 514 amino acids from A. niger glucoamylase and a dibasic proteolytic processing site (kex-2) were fused upstream of the Doc-FAEA sequence. By using this fusion strategy, the esterase activity found in the extracellular medium was 20-fold-higher than that of the wild-type reference strain, and the production yield was estimated to be about 100 mg of chimeric protein/liter. Intracellular and extracellular production was analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, dockerin-cohesin interaction assays, and Western blotting. Labeled cohesins detected an intact extracellular Doc-FAEA of about 43 kDa and a cleaved-off dockerin domain of about 8 kDa. In addition, an intracellular 120-kDa protein was recognized by using labeled cohesins and antibodies raised against FAEA. This protein corresponded to the unprocessed Doc-FAEA form fused to glucoamylase. In conclusion, these results indicated that translational fusion to glucoamylase improved the secretion efficiency of a chimeric Doc-FAEA protein and allowed production of the first functional fungal enzyme joined to a bacterial dockerin.

The role of the Aspergillus niger furin-type protease gene in processing of fungal proproteins and fusion proteins

We have characterized growth and protein processing characteristics of Aspergillus niger strains carrying a disrupted allele of the previously cloned and characterized kexB gene [Appl. Environ. Microbiol. 66 (2000) 363] encoding a furin-type endoprotease. Deletion of the single-copy gene confirms it to be non-essential but disruptant strains exhibit a morphologically distinct phenotype characterized by hyperbranching. Processing of homologous pro-proteins and fusion proteins comprised of a heterologous protein fused down-stream of glucoamylase and separated at the fusion junction by an endoproteolytic cleavage site was compared in wildtype and mutant strains of A. niger. We show that maturation of the native glucoamylase requires KexB, whereas maturation of aspergillopepsin does not. The processing of fusion proteins carrying Lys-Arg requires KexB, although alternative endoproteases are capable of cleaving protein fusions at sites adjacent to Lys-Arg.

Expression of a synthetic copy of the bovine chymosin gene in Aspergillus awamori from constitutive and pH-regulated promoters and secretion using two different pre-pro sequences

A copy of the bovine chymosin gene (chy) with a codon usage optimized for its expression in Aspergillus awamori was constructed starting from synthetic oligonucleotides. To study the ability of this filamentous fungus to secrete bovine prochymosin, two plasmids were constructed in which the transcriptional, translational, and secretory control regions of the A. nidulans gpdA gene and pepB genes were coupled to either preprochymosin or prochymosin genes. Secretion of a protein enzymatically and immunologically indistinguishable from bovine chymosin was achieved in A. awamori transformants with each of these constructions. In all cases, the primary translation product (40.5 kDa) was self-processed to a mature chymosin polypeptide having a molecular weight of 35.6 kDa. Immunological assays indicated that most of the chymosin was secreted to the extracellular medium. Hybridization analysis of genomic DNA from chymosin transformants showed chromosomal integration of prochymosin sequences and, in some transformants, multiple copies of the expression cassettes were observed. Expression from the gpdA promoter was constitutive, whereas expression from the pepB promoter was strongly influenced by pH. A very high expression from the pepB promoter was observed during the growth phase. The A. awamori pepB gene terminator was more favorable for chymosin production than the S. cerevisiae CYC1 terminator.

Glucoamylase::green fluorescent protein fusions to monitor protein secretion in Aspergillus niger

A glucoamylase::green fluorescent protein fusion (GLA::sGFP) was constructed which allows the green fluorescent protein to be used as an in vivo reporter of protein secretion in Aspergillus niger. Two secretory fusions were designed for secretion of GLA::sGFP which employed slightly different lengths of the glucoamylase protein (GLA499 and GLA514). Expression of GLA::sGFP revealed that fluorescence was localized in the hyphal cell walls and septa, and that fluorescence was most intense at hyphal apices. Extracellular GLA::sGFP was detectable by Western blotting only in the supernatant of young cultures grown in soya milk medium. In older cultures, acidification of the medium and induction of proteases were probably responsible for the loss of extracellular and cell wall fluorescence and the inability to detect GLA::sGFP by Western analysis. A strain containing the GLA::sGFP construct was subjected to UV mutagenesis and survivors screened for mutations in the general secretory pathway. Three mutants were isolated that were unable to form a halo on either starch or gelatin medium. All three mutants grew poorly compared to the parental strain. Fluorescence microscopy revealed that for two of the mutants, GLA::sGFP accumulated intracellularly with no evidence of wall fluorescence, whereas for the third mutant, wall fluorescence was observed with no evidence of intracellular accumulation. These results indicate that the GLA::sGFP fusion constructs can be used as convenient fluorescent markers to study the dynamics of protein secretion in vivo and as a tool in the isolation of mutants in the general secretory pathway.

Filamentous fungi as production organisms for glycoproteins of bio-medical interest

Filamentous fungi are commonly used in the fermentation industry for large scale production of glycoproteins. Several of these proteins can be produced in concentrations up to 20-40 g per litre. The production of heterologous glycoproteins is at least one or two orders of magnitude lower but research is in progress to increase the production levels. In the past years the structure of protein-linked carbohydrates of a number of fungal proteins has been elucidated, showing the presence of oligo-mannosidic and high-mannose chains, sometimes with typical fungal modifications. A start has been made to engineer the glycosylation pathway in filamentous fungi to obtain strains that show a more mammalian-like type of glycosylation. This mini review aims to cover the current knowledge of glycosylation in filamentous fungi, and to show the possibilities to produce glycoproteins with these organisms with a more mammalian-like type of glycosylation for research purposes or pharmaceutical applications.

The molecular biology of secreted enzyme production by fungi

Enzymes from filamentous fungi are already widely exploited, but new applications for known enzymes and new enzymic activities continue to be found. In addition, enzymes from less amenable non-fungal sources require heterologous production and fungi are being used as the production hosts. In each case there is a need to improve production and to ensure quality of product. While conventional, mutagenesis-based, strain improvement methods will continue to be applied to enzyme production from filamentous fungi the application of recombinant DNA techniques is beginning to reveal important information on the molecular basis of fungal enzyme production and this knowledge is now being applied both in the laboratory and commercially. We review the current state of knowledge on the molecular basis of enzyme production by filamentous fungi. We focus on transcriptional and post-transcriptional regulation of protein production, the transit of proteins through the secretory pathway and the structure of the proteins produced including glycosylation.

Glucoamylase gene fusions alleviate limitations for protein production in Aspergillus awamori at the transcriptional and (post) translational levels

In this study we have analyzed the effects of a glucoamylase gene fusion on the mRNA levels and protein levels for the human interleukin-6 gene (hil6) and the guar alpha-galactosidase gene (aglA). Previously it was shown that production of nonfused alpha-galactosidase and hIL-6 in Aspergillus awamori was limited at transcriptional and (post)translational levels, respectively (R. J. Gouka, P. J. Punt, J. G. M. Hessing, and C. A. M. J. J. van den Hondel, Appl. Environ. Microbiol. 62:1951-1957, 1996). Vectors were constructed which contained either the hil6 or aglA gene fused to the Aspergillus niger glucoamylase gene (glaA) under control of the efficient 1,4-beta-endoxylanase A promoter and transcription terminator. For comparison, the vectors were integrated in a single copy at the pyrG locus of A. awamori. A glaA fusion to the 5' end of the hil6 gene resulted in a large increase in hIL-6 yield, whereas with a glaA fusion to the 3' end of the hil6 gene, almost no protein was produced. Nevertheless, the steady-state mRNA levels of both fusions were very similar and not clearly increased compared to those of a strain expressing nonfused hIL-6. Fusions of glaA to the 5' end of the wild-type guar aglA gene resulted in truncated mRNA lacking almost 900 bases (> 80%) of the aglA sequence. When the coding sequence of the wild-type aglA gene was replaced by a synthetic aglA gene with optimized Saccharomyces cerevisiae codon usage, full-length mRNA was obtained. Compared to a nonfused synthetic aglA gene, a glaA fusion with the synthetic aglA gene resulted in a 25-fold increase in the mRNA level and, as a consequence, a similar increase in the alpha-galactosidase protein level. The truncated transcripts derived from the wild-type aglA gene were further analyzed by nuclear run-on transcription assays. These experiments indicated that transcription elongation in the nucleus proceeded at least 400 bases downstream of the site where the truncation was determined, indicating that transcription elongation or premature termination was not the reason for the generation of truncated mRNAs. As the truncated mRNA also contained a poly(A) tail, truncation most likely occurs by incorrect processing of the aglA mRNA in the nucleus.

Secretion of active human mucus proteinase inhibitor by Aspergillus niger after KEX2-like processing of a glucoamylase-inhibitor fusion protein

We report the production of human mucus proteinase inhibitor (MPI) by the filamentous fungus Aspergillus niger to test the ability of this host organism to secrete low molecular weight, highly disulfide-bonded proteins in biologically active conformation. Fungal transformants have been obtained with an expression cassette consisting of a chimeric gene founded on a mpi cDNA, encoding mature MPI, fused in frame to sequences encoding A. niger glucoamylase (glaA), separated by a KEX2-like processing sequence. Expression of the glucoamylase fusion gene in these transformants resulted in secretion of MPI into the growth medium with yields up to 3 mg 1-1. N-terminal sequence analysis of the purified inhibitor confirmed that the glucoamylase-MPI fusion protein was correctly processed to mature MPI by a KEX2-type endopeptidase present in A. niger. Furthermore, recombinant MPI retains full inhibitory activity against chymotrypsin and leukocyte elastase indicating that the protein was folded properly.

Analysis of heterologous protein production in defined recombinant Aspergillus awamori strains

A study was carried out to obtain more insight into the parameters that determine the secretion of heterologous proteins from filamentous fungi. A strategy was chosen in which the mRNA levels and protein levels of a number of heterologous genes of different origins were compared. All genes were under control of the Aspergillus awamori 1,4-beta-endoxylanase A (exlA) expression signals and were integrated in a single copy at the A. awamori pyrG locus. A Northern (RNA) analysis showed that large differences occurred in the steady-state mRNA levels obtained with the various genes; those levels varied from high values for genes of fungal origin (A. awamori 1,4-beta-endoxylanase A, Aspergillus niger glucoamylase, and Thermomyces lanuginosa lipase) to low values for genes of nonfungal origin (human interleukin 6 and Cyamopsis tetragonoloba [guar] alpha-galactosidase). With the C. tetragonoloba alpha-galactosidase wild-type gene full-length mRNA was even undetectable. Surprisingly, small amounts of full-length mRNA could be detected when a C. tetragonoloba alpha-galactosidase gene with an optimized Saccharomyces cerevisiae codon preference was expressed. In all cases except human interleukin 6, the protein levels corresponded to the amounts expected on basis of the mRNA levels. For human interleukin 6, very low protein levels were observed, whereas relatively high steady-state mRNA levels were obtained. Our data suggest that intracellular protein degradation is the most likely explanation for the low levels of secreted human interleukin 6.

Expression in Aspergillus niger of the starch-binding domain of glucoamylase. Comparison with the proteolytically produced starch-binding domain

Glucoamylase 1 from Aspergillus niger is an economically important enzyme in many industrial processes. It hydrolyses granular starch and comprises two distinct domains, a catalytic and a starch-binding domain (SBD). We have transformed A. niger with an expression vector for the secretion of SBD for physico-chemical studies. This was achieved by introducing into the glucoamylase gene a short sequence encoding an endoproteolytic cleavage recognition site such that free SBD was secreted at yields up to 200 mg/l. Free SBD was also obtained by proteolytic digestion of full-length glucoamylase 1. Electrospray mass spectroscopy was used to determine the carbohydrate content of both SBDs. It revealed that the engineered one is more glycosylated: an average of three mannose residues compared to one for the proteolytically derived SBD. Sequencing results also suggest partial glycosylation for the three Thr residues involved (510, 511, 513). It is probable that the engineered SBD represents the true glycosylation level of the SBD in native glucoamylase. Binding of beta-cyclodextrin to the SBD was investigated. It was found that the stoichiometry and the spectral perturbation of Trp residues were identical for both SBDs, but the engineered SBD bound less strongly to the ligand. At high concentrations of beta-cyclodextrin relative to the estimated Kd values, the maximum absorbance changes were identical. The observed difference at low beta-cyclodextrin levels was probably due to the higher level of glycosylation of the expressed SBD. We conclude that the proteolytically derived and expressed starch binding domains both bind 2 mol beta-cyclodextrin/mol protein, but that the pattern of glycosylation and strength of binding are different.

Nitrogen, carbon, and pH regulation of extracellular acidic proteases of Aspergillus niger

Aspergillus niger secretes a number of enzymes, including proteases, into its culture fluid. The regulation of the two major acidic extracellular proteases, pepA and pepB, was investigated using Northern analyses. Our data suggest that the regulation of pepA and pepB expression occurs predominantly at the level of mRNA content and that, while they are regulated in a similar manner, differences are also clear in their expression. Both genes were found to be under complex regulatory control. The expression of the two genes could be turned off by the presence of good nitrogen or carbon sources in the media, and external protein sources did not induce expression of either gene under conditions of carbon and nitrogen repression. The pH of the medium also played a major role in their regulation as the expression of both genes was completely turned off under alkaline conditions, even when grown in media lacking good nitrogen and carbon sources but containing proteins. We isolated clones containing 5' non-coding sequences of the pepA gene from a lambda genomic library with a pepA specific probe. Analysis and comparison of the promoter sequences of the pepA and pepB genes revealed that both contain several putative AREA- and CREA-binding sites and they also share an 18-bp-long sequence which is 83% identical in these two genes.

Cloning and characterization of the pepE gene of Aspergillus niger encoding a new aspartic protease and regulation of pepE and pepC

We have cloned the pepE gene of Aspergillus niger, encoding an aspartic protease (PEPE), by screening a lambda genomic DNA library with a heterologous probe, the Neurospora crassa gene coding for a vacuolar proteinase. Sequencing of pepE genomic and cDNA clones revealed that the gene contains three introns, which are 91, 56 and 58-bp long. The deduced protein consists of 398 amino acids, has a putative signal sequence to allow transport into the endoplasmic reticulum and probably undergoes a second proteolytic processing step at its N terminus to yield the mature enzyme. The putative mature part of PEPE has extensive homology with other aspartic proteinases such as pepsins, cathepsins and, in particular, with proteinase A of Saccharomyces cerevisiae and pepsin 1 of Candida albicans. Northern blot analyses revealed that cells contain an abundant pepE transcript whose amount does not change upon carbon or nitrogen limitation, the presence of proteins in the medium or changes in the pH of the medium. We also show that pepC, the A. niger homologue of yeast protease B, is also expressed constitutively under these conditions.

Strategies for improving heterologous protein production from filamentous fungi

Despite the naturally high capacity for protein secretion by many species of filamentous fungi, secreted yields of many heterologous proteins have been comparatively low. The strategies for yield improvement have included the use of strong homologous promoters, increased gene copy number, gene fusions with a gene encoding a naturally well-secreted protein, protease-deficient host strains and screening for high yields following random mutagenesis. Such approaches have been effective with some target heterologous proteins but not others. Approaches used in heterologous protein production from filamentous fungi are discussed and a perspective on emerging strategies is presented.

Secretion of the Hormoconis resinae glucoamylase P enzyme from Trichoderma reesei directed by the natural and the cbh1 gene secretion signal

Secretion of the Hormoconis resinae glucoamylase P (GAMP) enzyme from Trichoderma reesei using either the natural N-terminal extension of the premature glucoamylase P or the cellobiohydrolase I (CBHI) signal peptide was examined. The expression conditions for the heterologous glucoamylase P (gamP) gene in T. reesei were standardized by targeting one copy of a plasmid fragment, containing the gamP gene, to the cbh1 locus of the host. The results showed that the transient N-terminal extension of the premature GAMP acts as an efficient secretion signal in T. reesei and leads to a higher yield of extracellular glucoamylase activity than does the signal peptide of CBHI.

The yeast Kluyveromyces lactis as an efficient host for heterologous gene expression

Several different yeast species have been developed into systems for efficient heterologous gene expression. In this paper we review foreign gene expression in the dairy yeast Kluyveromyces lactis. This yeast presents several advantageous properties in comparison to other yeast species. These include its impressive secretory capacities, its excellent fermentation characteristics on large scale, its food grade status and the availability of both episomal and integrative expression vectors. Moreover, in contrast to the methylotrophic yeasts that are frequently used for the expression of foreign genes, K. lactis does not require explosion-proof fermentation equipment. Here, we present an overview of the available tools for heterologous gene expression in K. lactis (available promoters, vector systems, etc). Also, the production of prochymosin, human serum albumin and pancreatic phospholipase by K. lactis is discussed in more detail, and used to rate the achievements of K. lactis with respect to other micro-organisms in which these proteins have been produced.