Analysis of heterologous protein production in defined recombinant Aspergillus awamori strains

An overview on current molecular tools for heterologous gene expression in Trichoderma

Fungi of the genus Trichoderma are routinely used as biocontrol agents and for the production of industrial enzymes. Trichoderma spp. are interesting hosts for heterologous gene expression because their saprotrophic and mycoparasitic lifestyles enable them to thrive on a large number of nutrient sources and some members of this genus are generally recognized as safe (GRAS status). In this review, we summarize and discuss several aspects involved in heterologous gene expression in Trichoderma, including transformation methods, genome editing strategies, native and synthetic expression systems and implications of protein secretion. This review focuses on the industrial workhorse Trichoderma reesei because this fungus is the best-studied member of this genus for protein expression and secretion. However, the discussed strategies and tools can be expected to be transferable to other Trichoderma species.

Aspergillus: A Powerful Protein Production Platform

Aspergilli have been widely used in the production of organic acids, enzymes, and secondary metabolites for almost a century. Today, several GRAS (generally recognized as safe) Aspergillus species hold a central role in the field of industrial biotechnology with multiple profitable applications. Since the 1990s, research has focused on the use of Aspergillus species in the development of cell factories for the production of recombinant proteins mainly due to their natively high secretion capacity. Advances in the Aspergillus-specific molecular toolkit and combination of several engineering strategies (e.g., protease-deficient strains and fusions to carrier proteins) resulted in strains able to generate high titers of recombinant fungal proteins. However, the production of non-fungal proteins appears to still be inefficient due to bottlenecks in fungal expression and secretion machinery. After a brief overview of the different heterologous expression systems currently available, this review focuses on the filamentous fungi belonging to the genus Aspergillus and their use in recombinant protein production. We describe key steps in protein synthesis and secretion that may limit production efficiency in Aspergillus systems and present genetic engineering approaches and bioprocessing strategies that have been adopted in order to improve recombinant protein titers and expand the potential of Aspergilli as competitive production platforms.

Beneficial influences of dietary Aspergillus awamori fermented soybean meal on oxidative homoeostasis and inflammatory response in turbot (Scophthalmus maximus L.)

High levels of soybean meal (SBM) in aquafeed leads to detrimental inflammatory response and oxidative stress in fish. In the present study, fermentation with Aspergillus awamori was conducted to explore the potential effects on improving the nutritional quality of soybean meal and the health status of turbot. A 63-day feeding trial (initial weight 8.53 ± 0.11 g) was carried out to evaluate the utilization of fermented soybean meal (FSM) by juvenile turbot. 0% (FM, control), 30% (S30, F30), 45% (S45, F45), and 60% (S60, F60) of fish meal were replaced with SBM or FSM, respectively. As the results showed, fermentation significantly reduced the contents of anti-nutritional factors in SBM, including raffinose (-98.8%), glycinin (-98.5%), β-conglycinin (-97.4%), trypsin inhibitors (-80%) and stachyose (-80%). A depression of fish growth performance and activities of superoxide dismutase and lysozyme were observed in S45 and S60 groups, while these inferiorities were only observed in F60 group. Meanwhile, fermentation also improved the heights of enterocytes and microvillus significantly in the F45 and F60 groups compared with those in SBM. An induced expression of anti-inflammatory cytokine transforming growth factor-β and depression of pro-inflammatory cytokines tumor necrosis factor-α and interleukin-1β in the distal intestine were observed in the F45 and F60 groups. Taken together, this study indicated that fermentation with Aspergillus awamori could improve the replacement level with soybean meal from 30% to 45% in turbot.

Effects of codon optimization on the mRNA levels of heterologous genes in filamentous fungi

Filamentous fungi, particularly Aspergillus species, have recently attracted attention as host organisms for recombinant protein production. Because the secretory yields of heterologous proteins are generally low compared with those of homologous proteins or proteins from closely related fungal species, several strategies to produce substantial amounts of recombinant proteins have been conducted. Codon optimization is a powerful tool for improving the production levels of heterologous proteins. Although codon optimization is generally believed to improve the translation efficiency of heterologous genes without affecting their mRNA levels, several studies have indicated that codon optimization causes an increase in the steady-state mRNA levels of heterologous genes in filamentous fungi. However, the mechanism that determines the low mRNA levels when native heterologous genes are expressed was poorly understood. We recently showed that the transcripts of heterologous genes are polyadenylated prematurely within the coding region and that the heterologous gene transcripts can be stabilized significantly by codon optimization, which is probably attributable to the prevention of premature polyadenylation in Aspergillus oryzae. In this review, we describe the detailed mechanism of premature polyadenylation and the rapid degradation of mRNA transcripts derived from heterologous genes in filamentous fungi.

Combi-lipase for heterogeneous substrates: a new approach for hydrolysis of soybean oil using mixtures of biocatalysts

The concept of thecombi-lipasebiocatalyst has been proposed. It is based on the combination of different lipases as biocatalysts in reactions using heterogeneous substrates.

Cloning and Expression of Gumboro VP2 Antigen in Aspergillus niger

BACKGROUND

Infectious Bursal Disease Virus (IBDV) causes a highly immunosuppressive disease in chickens and is a pathogen of major economic importance to the poultry industry worldwide. The VP2 protein is the major host-protective immunogen of IBDV and has been considered as a potential subunit vaccine against the disease. VP2 coding sequence was cloned in an inducible fungal vector and the protein was expressed in Aspergillus niger (A. niger).

METHODS

Aiming at a high level of expression, a multicopy AMA1-pyrG-based episomal construct driven by a strong inducible promoter, glaA, was prepared and used in transformation of A. niger pyrG-protoplasts. SDS-PAGE and western blot analysis was carried out to confirm the expression of the protein.

RESULTS

A number of pyrG (+) positive transformants were isolated and the presence of expression cassette was confirmed. Western blot analysis of one of these recombinant strains using monospecific anti-VP2 antibodies demonstrated the successful expression of the protein. The recombinant protein was also detected by serum obtained from immunized chicken.

CONCLUSION

In the present study, we have generated a recombinant A. niger strain expressing VP2 protein intracellulary. This recombinant strain of A. niger may have potential applications in oral vaccination against IBDV in poultry industry.

Codon optimization increases steady-state mRNA levels in Aspergillus oryzae heterologous gene expression

We investigated the effect of codon optimization on the expression levels of heterologous proteins in Aspergillus oryzae, using the mite allergen Der f 7 as a model protein. A codon-optimized Der f 7 gene was synthesized according to the frequency of codon usage in A. oryzae by recursive PCR. Both native and optimized Der f 7 genes were expressed under the control of a high-level-expression promoter with their own signal peptides or in a fusion construct with A. oryzae glucoamylase (GlaA). Codon optimization markedly increased protein and mRNA production levels in both nonfused and GlaA-fused Der f 7 constructs. For constructs with native codons, analysis by 3' rapid amplification of cDNA ends revealed that poly(A) tracts tended to be added within the coding region, producing aberrant mRNAs that lack a termination codon. Insertion of a termination codon between the carrier GlaA and native Der f 7 proteins in the GlaA fusion construct resulted in increases in mRNA and secreted-carrier-GlaA levels. These results suggested that mRNAs without a termination codon as a result of premature polyadenylation are degraded, possibly through the nonstop mRNA decay pathway. We suggest that codon optimization in A. oryzae results in elimination of cryptic polyadenylation signals in native Der f 7, thereby circumventing the production of truncated transcripts and resulting in an increase in steady-state mRNA levels.

Cloning and characterization of the glyceraldehyde-3-phosphate dehydrogenase gene and the use of its promoter for expression in Myrothecium gramineum, a novel expression host

At our laboratory, research has focused on the development of Myrothecium gramineum as a novel expression host. The glyceraldehyde-3-phosphate dehydrogenase (gpd)-promoter of M. gramineum was isolated and characterized (Genbank accession number EF486690). In order to prove its functionality and to explore the potential of M. gramineum as a novel fungal expression host, use of this gpd-promoter for the expression of a fungal alpha-amylase was investigated. Myrothecium gramineum was transformed with pGPDlpAmyAO, containing the gpd-promoter followed by the amy3 encoding sequence of Aspergillus oryzae. Study of the amylase production indicated that the promoter can be successfully used for the expression of heterologous proteins in M. gramineum. To the best of our knowledge, this is the first time a homologous expression system has been described for M. gramineum.

Plasmids for expression of heterologous proteins in Rhizopus oryzae

Rhizopus oryzae has long been used for enzyme production (e.g., glucoamylase and lipase), organic acid synthesis, and various fermented food applications. In this work, we describe a set of plasmid-based expression vectors that can be used for the production of heterologous proteins in R. oryzae. Three plasmid vectors have been created using either the glucoamylase A (amyA), pyruvate decarboxylase (pdcA), or phosphoglycerate kinase (pgk1) promoters to drive expression of heterologous proteins. All three plasmids use the pdcA terminator for transcription termination, the pyrG gene for restoration of uracil prototrophy, and an ampicillin resistance gene and origin of replication for maintenance in Escherichia coli. We have expressed green fluorescent protein (GFP) and compared transcription and protein accumulation for each of the expression vectors. Accumulation of GFP transcript and protein was directly correlated with the choice of promoter with pdcA > amyA > pgk1. Transcript level appears to parallel GFP protein accumulation. Plasmid copy number had little impact on transcription or protein accumulation. These vectors should be useful for overexpression of heterologous proteins and potentially, metabolic engineering of Rhizopus strains.

High expression of a synthetic gene encoding potato alpha-glucan phosphorylase in Aspergillus niger

We describe the successful heterologous expression of the Solanum tuberosum alpha-glucan phosphorylase (GP) gene in Aspergillus niger. Special attention was paid to the influence of different codon usage and A+T content in the coding region on GP protein expression. Use of A. niger-preferred codon usage and lower A+T content in a synthetic gene (GP-syn) resulted in a significant improvement in the level of the GP mRNA and a dramatic increase in the quantity of GP protein produced such that it accounted for approximately 10% of the total soluble protein. We suggest that redesigning the primary DNA sequence encoding a desired protein product can be an extremely effective method for improving heterologous protein production in filamentous fungi.

Production of bifunctional proteins by Aspergillus awamori: Llama variable heavy chain antibody fragment (VHH) R9 coupled to Arthromyces ramosus peroxidase (ARP)

The Arthromyces ramosus peroxidase gene (arp) was genetically fused to either the 5'- or 3'-terminal ends of the gene encoding llama variable heavy chain antibody fragment V(HH) R9, resulting in the fusion expression cassettes ARP-R9 or R9-ARP. Aspergillus awamori transformants were obtained which produced up to 30 mgl(-1) fusion protein in the culture medium. Both fusion proteins showed peroxidase activity in an ABTS activity test. Considerable amounts of fusion protein were detected intracellularly, suggesting that the fungus encounters problems in secreting these kind of proteins. ELISA experiments showed that ARP-R9 was less able to bind its antigen, the azo-dye RR6, as compared to R9-ARP. Furthermore, in contrast to R9-ARP, ARP-R9 bound to RR6 did not show peroxidase activity anymore. These results indicate that fusion of ARP to the C-terminus of the antibody fragment V(HH) R9 (R9-ARP) is the preferred orientation.

Expression and production of llama variable heavy-chain antibody fragments (VHHs) by Aspergillus awamori

We report the expression and production of llama variable heavy-chain antibody fragments (V(HH)s) by Aspergillus awamori. Fragments encoding V(HH)s were cloned in a suitable Aspergillus expression vector and transformants secreting V(HH) fragments were analysed for integrated gene copy-numbers, mRNA levels and protein production. Functional V(HH)s were detected in the culture medium, indicating the feasibility of producing this type of protein in a fungal expression system. Secreted V(HH)s were subjected to (extracellular) degradation, which could be partially prevented by the addition of BSA to the culture medium.

Improvement of foreign-protein production in Aspergillus niger var. awamori by constitutive induction of the unfolded-protein response

Unfolded-protein response (UPR) denotes the upregulation of endoplasmic reticulum (ER)-resident chaperone and foldase genes and numerous other genes involved in secretory functions during the accumulation of unfolded proteins into the ER. Overexpression of individual foldases and chaperones has been used in attempts to improve protein production in different production systems. We describe here a novel strategy to improve foreign-protein production. We show that the constitutive induction of the UPR pathway in Aspergillus niger var. awamori can be achieved by expressing the activated form of the transcription factor hacA. This induction enhances the production of Trametes versicolor laccase by up to sevenfold and of bovine preprochymosin by up to 2.8-fold in this biotechnically important fungus. The regulatory range of UPR was studied by analyzing the mRNA levels of novel A. niger var. awamori genes involved in different secretory functions. This revealed both similarities and differences to corresponding studies in Saccharomyces cerevisiae.

High-yield production of a bacterial xylanase in the filamentous fungus Trichoderma reesei requires a carrier polypeptide with an intact domain structure

A bacterial xylanase gene, Nonomuraea flexuosa xyn11A, was expressed in the filamentous fungus Trichoderma reesei from the strong cellobiohydrolase 1 promoter as fusions to a variety of carrier polypeptides. By using single-copy isogenic transformants, it was shown that production of this xylanase was clearly increased (up to 820 mg/liter) when it was produced as a fusion protein with a carrier polypeptide having an intact domain structure compared to the production (150 to 300 mg/liter) of fusions to the signal sequence alone or to carriers having incomplete domain structures. The carriers tested were the T. reesei mannanase I (Man5A, or MANI) core-hinge and a fragment thereof and the cellulose binding domain of T. reesei cellobiohydrolase II (Cel6A, or CBHII) with and without the hinge region(s) and a fragment thereof. The flexible hinge region was shown to have a positive effect on both the production of Xyn11A and the efficiency of cleavage of the fusion polypeptide. The recombinant Xyn11A produced had properties similar to those of the native xylanase. It constituted 6 to 10% of the total proteins secreted by the transformants. About three times more of the Man5A core-hinge carrier polypeptide than of the recombinant Xyn11A was observed. Even in the best Xyn11A producers, the levels of the fusion mRNAs were only approximately 10% of the level of cel7A (cbh1) mRNA in the untransformed host strain.

Agrobacterium tumefaciens mediated transformation of the oomycete plant pathogen Phytophthora infestans

SUMMARY Agrobacterium tumefaciens is widely used for plant DNA transformation and, more recently, has also been used to transform yeast and filamentous fungi. Here we present a protocol for Agrobacterium-mediated DNA transformation of the oomycete Phytophthora infestans, the causal agent of potato late blight. Binary T-DNA vectors containing neomycin phosphotransferase (npt) and beta-glucuronidase (gus) fused to oomycete transcriptional regulatory sequences were constructed. Seven days of co-cultivation followed by transfer to a selective medium containing cefotaxim to kill Agrobacterium and geneticin to select for transformants, resulted in geneticin resistant colonies. Under optimal conditions with Agrobacterium supplemented with a ternary plasmid carrying a constitutive virG gene and in the presence of acetosyringone as inducer, up to 30 transformants per 10(7) zoospores could be obtained. The majority of these transformants contained a single T-DNA copy randomly integrated at a chromosomal locus. Using a similar protocol, geneticin resistant transformants of two other oomycetes species were obtained, Phytophthora palmivora and Pythium ultimum.

Evaluation of Aspergillus niger as host for virus-like particle production, using the hepatitis B surface antigen as a model

The filamentous fungus Aspergillus niger was transformed with the hepatitis B virus S gene encoding the major viral envelope protein under control of the constitutive A. nidulans glyceraldehyde-3-phosphate dehydrogenase ( gpdA) promoter. Approximately seven copies of the expression cassette were integrated on the genome, resulting in high-level transcription of the S gene. Production of the 24-kDa S protein and a 48-kDa S protein dimer in the membrane-associated protein fraction of the recombinant A. niger strain was shown through Western analysis. Electron microscopy of partially purified recombinant S protein revealed the formation of spherical pseudoviral particles with a diameter of 22 nm. The production level of hepatitis B pseudoviral particles was estimated to be 0.4 mg/l culture, which compares favourably with the reported levels initially obtained in yeast, indicating the potential of the Aspergillus expression system as an alternative, cost-effective vaccine production system.

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.

Efficient production of Arthromyces ramosus peroxidase by Aspergillus awamori

The heterologous production of Arthromyces ramosus peroxidase (ARP) was analysed in the filamentous fungus Aspergillus awamori under control of the inducible endoxylanase promoter. Secretion of active ARP was achieved up to 800 mg l(-1) in shake flask cultures. Western blot analysis showed that an rARP product of the correct molecular weight was produced. In contrast to several other studies about heterologous production of heme containing peroxidases, our results suggest that in A. awamori no heme limitation exists during overproduction of ARP.

Recent studies of protein secretion by filamentous fungi

Filamentous fungi have been widely exploited for the homologous and heterologous protein production, because of the high capacity of their protein secretion machinery. However, the production of heterologous proteins is often limited while the production of homologous proteins can be very high. Various researches have reported the methods for overcoming this problem and some techniques, such as the fusion gene system, improve the production of heterologous proteins. Recently, the molecular biological study of solid-state culture attracts the attention, because the long history of biological studies has shown that the productivity of protein in the solid-state culture frequently exceeds the productivity of protein in the submerged culture. The recent researches of solid-state culture have revealed the new aspects of protein production in filamentous fungi. Solid-state specific gene expression was observed in the glaB and pepA genes of Aspergillus oryzae. A GC-box and HSE element of the glaB promoter region affected solid-state specific gene expression of this gene. Solid-state culture-specific release of enzymes from the cell wall was also observed in the production of beta-glucosidases in Aspergillus kawachii. Extracellular soluble polysaccharide (ESP) from A. kawachii was concerned with the location of beta-glucosidases. Moreover, ESP and the cell wall fraction of A. kawachii were shown to be involved in the stability of beta-glucosidases. The knowledge of the molecular biology of solid-state culture should provide new approaches for the production of both homologous and heterologous proteins in both submerged culture and solid-state culture of filamentous fungi.

Filamentous fungi as cell factories for heterologous protein production

Filamentous fungi have been used as sources of metabolites and enzymes for centuries. For about two decades, molecular genetic tools have enabled us to use these organisms to express extra copies of both endogenous and exogenous genes. This review of current practice reveals that molecular tools have enabled several new developments. But it has been process development that has driven the final breakthrough to achieving commercially relevant quantities of protein. Recent research into gene expression in filamentous fungi has explored their wealth of genetic diversity with a view to exploiting them as expression hosts and as a source of new genes. Inevitably, the progress in the 'genomics' technology will further develop high-throughput technologies for these organisms.

Effect of introns and AT-rich sequences on expression of the bacterial hygromycin B resistance gene in the basidiomycete Schizophyllum commune

Previously, it was shown that introns are required for efficient mRNA accumulation in Schizophyllum commune and that the presence of AT-rich sequences in the coding region of genes can result in truncation of transcripts in this homobasidiomycete. Here we show that intron-dependent mRNA accumulation and truncation of transcripts are two independent events that both affect expression of the bacterial hygromycin B resistance gene in S. commune.

Codon optimization of xylanase gene xynB from the thermophilic bacterium Dictyoglomus thermophilum for expression in the filamentous fungus Trichoderma reesei

The catalytic domain of the xynB (xylanase) gene from the thermophilic bacterium Dictyoglomus thermophilum was reconstructed by PCR to match the codon preference of Trichoderma reesei. The 0.6-kb DNA fragment encoding the enzyme was first amplified by primer extension with a mixture of eight overlapping oligonucleotides, followed by PCR with outside primers containing restriction enzyme sites for directional cloning into Escherichia coli and T. reesei vectors. The synthetic gene was expressed in both organisms, producing a clearing halo around transformant colonies in plate assay utilizing an overlay of oat spelts xylan. Effective transcription of xyn B in T. reesei was obtained after changing 20 codons.

Growth-rate-independent production of recombinant glucoamylase by Fusarium venenatum JeRS 325

Most recombinant proteins generated in filamentous fungi are produced in fed-batch cultures, in which specific growth rate normally decreases progressively with time. Because of this, such cultures are more suited to the production of products that are produced efficiently at low-growth rates (e.g., penicillin) than to products which are produced more efficiently at high-growth rates (e. g., glucoamylase). Fusarium venenatum A3/5 has been transformed (JeRS 325) to produce Aspergillus niger glucoamylase (GAM) under the control of the Fusarium oxysporum trypsin-like protease promoter. No glucoamylase was detected in the culture supernatant during exponential growth of F. venenatum JeRS 325 in batch culture. In glucose-limited chemostat cultures, GAM concentration increased with decrease in dilution rate, but the specific production rate of GAM (g GAM [g biomass](-1) h(-1)) remained approximately constant over the dilution-rate range 0.05 h to 0.19 h(-1), i.e., the recombinant protein was produced in a growth-rate-independent manner. The specific production rate decreased at dilution rates of 0.04 h(-1) and below. Specific production rates of 5.8 mg and 4.0 mg GAM [g biomass](-1) h(-1) were observed in glucose-limited chemostat cultures in the presence and absence of 1 g mycological peptone L(-1). Compared to production in batch culture, and for the same final volume of medium, there was no increase in glucoamylase production when cultures were grown in fed-batch culture. The results suggested that a chemostat operated at a slow dilution rate would be the most productive culture system for enzyme production under this trypsin-like promoter.

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.

Characterization of a foldase, protein disulfide isomerase A, in the protein secretory pathway of Aspergillus niger

Protein disulfide isomerase (PDI) is important in assisting the folding and maturation of secretory proteins in eukaryotes. A gene, pdiA, encoding PDIA was previously isolated from Aspergillus niger, and we report its functional characterization here. Functional analysis of PDIA showed that it catalyzes the refolding of denatured and reduced RNase A. pdiA also complemented PDI function in a Saccharomyces cerevisiae Deltapdi1 mutant in a yeast-based killer toxin assay. Levels of pdiA mRNA and PDIA protein were raised by the accumulation of unfolded proteins in the endoplasmic reticulum. This response of pdiA mRNA levels was slower and lower in magnitude than that of A. niger bipA, suggesting that the induction of pdiA is not part of the primary stress response. An increased level of pdiA transcripts was also observed in two A. niger strains overproducing a heterologous protein, hen egg white lysozyme (HEWL). Although overexpression of PDI has been successful in increasing yields of some heterologous proteins in S. cerevisiae, overexpression of PDIA did not increase secreted yields of HEWL in A. niger, suggesting that PDIA itself is not limiting for secretion of this protein. Downregulation of pdiA by antisense mRNA reduced the levels of microsomal PDIA activity by up to 50%, lowered the level of PDIA as judged by Western blots, and lowered the secreted levels of glucoamylase by 60 to 70%.

Effect of branch frequency in Aspergillus oryzae on protein secretion and culture viscosity

Highly branched mutants of two strains of Aspergillus oryzae (IFO4177, which produces alpha-amylase, and a transformant of IFO4177 [AMG#13], which produces heterologous glucoamylase in addition to alpha-amylase) were generated by UV or nitrous acid mutagenesis. Four mutants of the parental strain (IFO4177), which were 10 to 50% more branched than the parental strain, were studied in stirred batch culture and no differences were observed in either the amount or the rate of enzyme production. Five mutants of the transformed parental strain (AMG#13), which were 20 to 58% more branched than the parental strain, were studied in either batch, fed-batch or continuous culture. In batch culture, three of the mutants produced more glucoamylase than the transformed parental strain, although only two mutants produced more glucoamylase and alpha-amylase combined. No increase in enzyme production was observed in either chemostat or fed-batch culture. Cultures of highly branched mutants were less viscous than those of the parental and transformed parental strains. A linear relationship was found between the degree of branching (measured as hyphal growth unit length) and culture viscosity (measured as the torque exerted on the rheometer impeller) for these strains. DOT-controlled fed-batch cultures (in which the medium feed rate was determined by the DOT) were thus inoculated with either the transformed parent or highly branched mutants of the transformed parent to determine whether the reduced viscosity would improve aeration and give higher enzyme yields. The average rate of medium addition was higher for the two highly branched mutants (ca. 8.3 g medium h(-1)) than for the parental strain (5.7 g medium h(-1)). Specific enzyme production in the DOT controlled fed-batch cultures was similar for all three strains (approx. 0.24 g alpha-amylase and glucoamylase [g of biomass](-1)), but one of the highly branched mutants made more total enzyme (24.3 +/- 0.2 g alpha-amylase and glucoamylase) than the parental strain (21.7 +/- 0.4 g alpha-amylase and glucoamylase).

Transformation of Aspergillus awamori by Agrobacterium tumefaciens-mediated homologous recombination

Agrobacterium tumefaciens is known to transfer part of its tumor-inducing (Ti) plasmid to the filamentous fungus Aspergillus awamori by illegitimate recombination with the fungal genome. Here, we show that when this Ti DNA shares homology with the A. awamori genome, integration can also occur by homologous recombination. On the basis of this finding, we have developed an efficient method for constructing recombinant mold strains free from bacterial DNA by A. tumefaciens-mediated transformation. Multiple copies of a gene can be integrated rapidly at a predetermined locus in the genome, yielding transformants free of bacterial antibiotic resistance genes or other foreign DNA. Recombinant A. awamori strains were constructed containing up to nine copies of a Fusarium solani pisi cutinase expression cassette integrated in tandem at the pyrG locus. This allowed us to study how mRNA and protein levels are affected by gene copy number, without the influence of chromosomal environmental effects. Cutinase mRNA and protein were maximal with four gene copies, indicating a limitation at the transcriptional level. This transformation system will potentially stimulate market acceptance of derived products by avoiding introduction of bacterial and other foreign DNA into the fungi.

Expression and secretion of defined cutinase variants by Aspergillus awamori

Several cutinase variants derived by molecular modelling and site-directed mutagenesis of a cutinase gene from Fusarium solani pisi are poorly secreted by Saccharomyces cerevisiae. The majority of these variants are successfully produced by the filamentous fungus Aspergillus awamori. However, the L51S and T179Y mutations caused reductions in the levels of extracellular production of two cutinase variants by A. awamori. Metabolic labelling studies were performed to analyze the bottleneck in enzyme production by the fungus in detail. These studies showed that because of the single L51S substitution, rapid extracellular degradation of cutinase occurred. The T179Y substitution did not result in enhanced sensitivity towards extracellular proteases. Presumably, the delay in the extracellular accumulation of this cutinase variant is caused by the enhanced hydrophobicity of the molecule. Overexpression of the A. awamori gene encoding the chaperone BiP in the cutinase-producing A. awamori strains had no significant effect on the secretion efficiency of the cutinases. A cutinase variant with the amino acid changes G28A, A85F, V184I, A185L, and L189F that was known to aggregate in the endoplasmic reticulum of S. cerevisiae, resulting in low extracellular protein levels, was successfully produced by A. awamori. An initial bottleneck in secretion occurred before or during translocation into the endoplasmic reticulum but was rapidly overcome by the fungus.

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.

Multicellular ascomycetous fungal genomes contain more than 8000 genes

Fungi comprise a large monophyletic group of uni- and multicellular eukaryotic organisms in which many species are of economic or medical importance. Fungal genomes are variable in size (13-42 Mb), and multicellular species support true spatial and temporal cell-type-specific regulation of gene expression. In a 38.8-kb Aspergillus nidulans contiguous genomic DNA region, a transposable element and 12 potential genes were identified, 7 similar to genes in other organisms. This observation is consistent with the prediction that multicellular ascomycetous fungi harbor 8000-9000 genes in a 36-Mb average genome. Thus, the genomic DNA sequence of filamentous fungi will provide substantial amounts of genetic and functional information that is not available in yeast, for the human and other metazoan minimal gene complement.

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.