Silencing of the aspergillopepsin B (pepB) gene of Aspergillus awamori by antisense RNA expression or protease removal by gene disruption results in a large increase in thaumatin production

Exploration of the Strategy for Improving the Expression of Heterologous Sweet Protein Monellin in Aspergillus niger

Aspergillus niger is a primary cell factory for food-grade protein (enzyme) production due to its strong protein secretion capacity and unique safety characteristics. The bottleneck issue for the current A. niger expression system is the difference in expression yield of heterologous proteins of non-fungal origin compared to those of fungal origin, which is about three orders of magnitude. The sweet protein monellin, derived from West African plants, has the potential to become a food-grade sweetener due to its high sweetness and the benefit of not containing sugar itself, but it is extremely difficult to establish a research model for heterologous expression in A. niger, owing to extremely low expression, a small molecular weight, and being undetectable with conventional protein electrophoresis. HiBiT-Tag was fused with low-expressing monellin in this work to create a research model for heterologous protein expression in A. niger at ultra-low levels. We increased monellin expression by increasing the monellin copy number, fusing monellin with the endogenous highly expressed glycosylase glaA, and eliminating extracellular protease degradation, among other strategies. In addition, we investigated the effects of overexpression of molecular chaperones, inhibiting the ERAD pathway, and enhancing the synthesis of phosphatidylinositol, phosphatidylcholine, and diglycerides in the biomembrane system. Using medium optimization, we finally obtained 0.284 mg/L of monellin in the supernatant of the shake flask. This is the first time recombinant monellin has been expressed in A. niger, with the goal of investigating ways to improve the secretory expression of heterologous proteins at ultra-low levels, which can serve as a model for the expression of other heterologous proteins in A. niger.

Study of protease activity from Aspergillus awamori INCQS2B.361U2/1 extracellular fraction and modification of culture medium composition to isolate a novel aspartic protease

Aspergillus awamori was cultivated in a modified Breccia medium, and the extracellular fraction was obtained, which presented 260 ± 15 µg of protein/mg and specific protease activity of 3.87 ± 0.52 mM.min^-1.mg of protein^-1 using N_α-p-tosyl-L-arginine methyl ester hydrochloride (_L-TAME) as substrate. This fraction showed major proteins about 104 and 44 kDa and maximal protease activity at pH 5.5, 6.5, and 9.0, suggesting that A. awamori secretes acidic, neutral, and alkaline proteases with expressive thermal stability, however, aspartic protease was the most important activity. When yeast extract was supplemented to a modified Breccia medium, A. awamori protein secretion and protease activity were maximal and the affinity chromatography on pepstatin-agarose was employed to isolate the aspartic protease activity, which was called ASPA, with approximately 75 kDa. ASPA maximal activity was obtained at pH 4.5 and 6.5, and 50 °C. Pepstatin inhibited about 80% of ASPA activity, with IC₅₀ and Ki values of 0.154 and 0.072 μM, respectively. ASPA cleaved protein and peptides substrates with the highest activity against gelatin (95 U/mg) and good peptidase activity with K_M 0.0589 mM and V_max 1.909 mM.min^-1.mg protein^-1, using _L-TAME as substrate. A. awamori extracellular fraction is a source of proteases with important activity, and the supplementation of modified Breccia medium increased the aspartic protease production. This enzyme presented different biochemical characteristics from the previously reported A. awamori aspartic proteases. Therefore, ASPA is an excellent candidate for biotechnological application due to its important activity and thermostability.

Proteolytic analysis of Trichoderma reesei in celluase-inducing condition reveals a role for trichodermapepsin (TrAsP) in cellulase production

Filamentous fungi produce a variety of proteases with significant biotechnological potential and show diverse substrate specificities. Proteolytic analysis of the industrial enzyme producer Trichoderma reesei has been sparse. Therefore, we determined the substrate specificity of T. reesei secretome and its main protease Trichodermapepsin (TrAsP) up to P1 position using FRETS-25Xaa-libraries. The role of TrAsP was analyzed using T. reesei QM9414 and the deletant QM∆trasp in Avicel. We observed higher activities of CMCase, Avicelase, and Xylanase in QM∆t rasp compared to that of QM9414. Saccharification rate of cellulosic biomass also increased when using secretome of QM∆trasp but the effect was not significant due to the absence of difference in BGL activity compared to QM9414. Higher TrAsP was produced when monosaccharides were used as a carbon source compared to cellulase inducers such as Avicel and α-sophorose. These results elucidate the relationship between TrAsP and cellulase production in T. reesei and suggest a physiological role for TrAsP.

Forced Recycling of an AMA1-Based Genome-Editing Plasmid Allows for Efficient Multiple Gene Deletion/Integration in the Industrial Filamentous Fungus Aspergillus oryzae

Filamentous fungi are used for food fermentation and industrial production of recombinant proteins. They also serve as a source of secondary metabolites and are recently expected as hosts for heterologous production of useful secondary metabolites. Multiple-step genetic engineering is required to enhance industrial production involving these fungi, but traditional sequential modification of multiple genes using a limited number of selection markers is laborious. Moreover, efficient genetic engineering techniques for industrial strains have not yet been established. We have previously developed a clustered regulatory interspaced short palindromic repeats (CRISPR)/Cas9-based mutagenesis technique for the industrial filamentous fungus Aspergillus oryzae, enabling mutation efficiency of 10 to 20%. Here, we improved the CRISPR/Cas9 approach by including an AMA1-based autonomously replicating plasmid harboring the drug resistance marker ptrA By using the improved mutagenesis technique, we successfully modified A. oryzae wild and industrial strains, with a mutation efficiency of 50 to 100%. Conditional expression of the Aoace2 gene from the AMA1-based plasmid severely inhibited fungal growth. This enabled forced recycling of the plasmid, allowing repeated genome editing. Further, double mutant strains were successfully obtained with high efficiency by expressing two guide RNA molecules from the genome-editing plasmid. Cotransformation of fungal cells with the genome-editing plasmid together with a circular donor DNA enabled marker-free multiplex gene deletion/integration in A. oryzae The presented repeatable marker-free genetic engineering approach for mutagenesis and gene deletion/integration will allow for efficient modification of multiple genes in industrial fungal strains, increasing their applicability.IMPORTANCE Multiple gene modifications of specific fungal strains are required for achieving industrial-scale production of enzymes and secondary metabolites. In the present study, we developed an efficient multiple genetic engineering technique for the filamentous fungus Aspergillus oryzae The approach is based on a clustered regulatory interspaced short palindromic repeats (CRISPR)/Cas9 system and recycling of an AMA1-based autonomous replicating plasmid. Because the plasmid harbors a drug resistance marker (ptrA), the approach does not require the construction of auxotrophic industrial strains prior to genome editing and allows for forced recycling of the gene-editing plasmid. The established plasmid-recycling technique involves an Aoace2-conditional expression cassette, whose induction severely impairs fungal growth. We used the developed genetic engineering techniques for highly efficient marker-free multiple gene deletion/integration in A. oryzae The genome-editing approaches established in the present study, which enable unlimited repeatable genetic engineering, will facilitate multiple gene modification of industrially important fungal strains.

Antisense RNA: the new favorite in genetic research

Antisense RNA molecule represents a unique type of DNA transcript that comprises 19-23 nucleotides and is complementary to mRNA. Antisense RNAs play the crucial role in regulating gene expression at multiple levels, such as at replication, transcription, and translation. In addition, artificial antisense RNAs can effectively regulate the expression of related genes in host cells. With the development of antisense RNA, investigating the functions of antisense RNAs has emerged as a hot research field. This review summarizes our current understanding of antisense RNAs, particularly of the formation of antisense RNAs and their mechanism of regulating the expression of their target genes. In addition, we detail the effects and applications of antisense RNAs in antivirus and anticancer treatments and in regulating the expression of related genes in plants and microorganisms. This review is intended to highlight the key role of antisense RNA in genetic research and guide new investigators to the study of antisense RNAs.

Engineered Microorganisms for the Production of Food Additives Approved by the European Union-A Systematic Analysis

In the 1950s, the idea of a single harmonized list of food additives for the European Union arose. Already in 1962, the E-classification system, a robust food safety system intended to protect consumers from possible food-related risks, was introduced. Initially, it was restricted to colorants, but at later stages also preservatives, antioxidants, emulsifiers, stabilizers, thickeners, gelling agents, sweeteners, and flavorings were included. Currently, the list of substances authorized by the European Food Safety Authority (EFSA) (referred to as "E numbers") comprises 316 natural or artificial substances including small organic molecules, metals, salts, but also more complex compounds such as plant extracts and polymers. Low overall concentrations of such compounds in natural producers due to inherent regulation mechanisms or production processes based on non-regenerative carbon sources led to an increasing interest in establishing more reliable and sustainable production platforms. In this context, microorganisms have received significant attention as alternative sources providing access to these compounds. Scientific advancements in the fields of molecular biology and genetic engineering opened the door toward using engineered microorganisms for overproduction of metabolites of their carbon metabolism such as carboxylic acids and amino acids. In addition, entire pathways, e.g., of plant origin, were functionally introduced into microorganisms, which holds the promise to get access to an even broader range of accessible products. The aim of this review article is to give a systematic overview on current efforts during construction and application of microbial cell factories for the production of food additives listed in the EU "E numbers" catalog. The review is focused on metabolic engineering strategies of industrially relevant production hosts also discussing current bottlenecks in the underlying metabolic pathways and how they can be addressed in the future.

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.

Enabling low cost biopharmaceuticals: high level interferon alpha-2b production in Trichoderma reesei

BACKGROUND

The filamentous fungus Trichoderma reesei has tremendous capability to secrete over 100 g/L of proteins and therefore it would make an excellent host system for production of high levels of therapeutic proteins at low cost. We have developed T. reesei strains suitable for production of therapeutic proteins by reducing the secreted protease activity. Protease activity has been the major hindrance to achieving high production levels. We have constructed a series of interferon alpha-2b (IFNα-2b) production strains with 9 protease deletions to gain knowledge for further strain development.

RESULTS

We have identified two protease deletions that dramatically improved the production levels. Deletion of the subtilisin protease slp7 and the metalloprotease amp2 has enabled production levels of IFNα-2b up to 2.1 and 2.4 g/L, respectively. With addition of soybean trypsin protease inhibitor the level of production improved to 4.5 g/L, with an additional 1.8 g/L still bound to the secretion carrier protein.

CONCLUSIONS

High levels of IFNα-2b were produced using T. reesei strains with reduced protease secretion. Further strain development can be done to improve the production system by reducing protease activity and improving carrier protein cleavage.

A long natural-antisense RNA is accumulated in the conidia of Aspergillus oryzae

Analysis of expressed sequence tag libraries from various culture conditions revealed the existence of conidia-specific transcripts assembled to putative conidiation-specific reductase gene (csrA) in Aspergillus oryzae. However, the all transcripts were transcribed with opposite direction to the gene csrA. The sequence analysis of the transcript revealed that the RNA overlapped mRNA of csrA with 3'-end, and did not code protein longer than 60 amino acid residues. We designated the transcript Conidia Specific Long Natural-antisense RNA (CSLNR). The real-time PCR analysis demonstrated that the CSLNR is conidia-specific transcript, which cannot be transcribed in the absence of brlA, and the amount of CSLNR was much more than that of the transcript from csrA in conidia. Furthermore, the csrA deletion, also lacking coding region of CSLNR in A. oryzae reduced the number of conidia. Overexpression of CsrA demonstrated the inhibition of growth and conidiation, while CSLNR did not affect conidiation.

Microbial aspartic proteases: current and potential applications in industry

Aspartic proteases are a relatively small group of proteolytic enzymes that are active in acidic environments and are found across all forms of life. Certain microorganisms secrete such proteases as virulence agents and/or in order to break down proteins thereby liberating assimilable sources of nitrogen. Some of the earlier applications of these proteolytic enzymes are found in the manufacturing of cheese where they are used as milk-clotting agents. Over the last decade, they have received tremendous research interest because of their involvement in human diseases. Furthermore, there has also been a growing interest on these enzymes for their applications in several other industries. Recent research suggests in particular that they could be used in the wine industry to prevent the formation of protein haze while preserving the wines' organoleptic properties. In this mini-review, the properties and mechanisms of action of aspartic proteases are summarized. Thereafter, a brief overview of the industrial applications of this specific class of proteases is provided. The use of aspartic proteases as alternatives to clarifying agents in various beverage industries is mentioned, and the potential applications in the wine industry are thoroughly discussed.

Sweeteners

Polyols as sugar substitutes, intense sweeteners and some new carbohydrates are increasingly used in foods and beverages. Some sweeteners are produced by fermentation or using enzymatic conversion. Many studies for others have been published. This chapter reviews the most important sweeteners.

Engineering of protein folding and secretion-strategies to overcome bottlenecks for efficient production of recombinant proteins

SIGNIFICANCE

Recombinant protein production has developed into a huge market with enormous positive implications for human health and for the future direction of a biobased economy. Limitations in the economic and technical feasibility of production processes are often related to bottlenecks of in vivo protein folding.

RECENT ADVANCES

Based on cell biological knowledge, some major bottlenecks have been overcome by the overexpression of molecular chaperones and other folding related proteins, or by the deletion of deleterious pathways that may lead to misfolding, mistargeting, or degradation.

CRITICAL ISSUES

While important success could be achieved by this strategy, the list of reported unsuccessful cases is disappointingly long and obviously dependent on the recombinant protein to be produced. Singular engineering of protein folding steps may not lead to desired results if the pathway suffers from several limitations. In particular, the connection between folding quality control and proteolytic degradation needs further attention.

FUTURE DIRECTIONS

Based on recent understanding that multiple steps in the folding and secretion pathways limit productivity, synergistic combinations of the cell engineering approaches mentioned earlier need to be explored. In addition, systems biology-based whole cell analysis that also takes energy and redox metabolism into consideration will broaden the knowledge base for future rational engineering strategies.

Penicillium decumbens BrlA extensively regulates secondary metabolism and functionally associates with the expression of cellulase genes

Penicillium decumbens has been used in the industrial production of lignocellulolytic enzymes in China for more than 15 years. Conidiation is essential for most industrial fungi because conidia are used as starters in the first step of fermentation. To investigate the mechanism of conidiation in P. decumbens, we generated mutants defective in two central regulators of conidiation, FluG and BrlA. Deletion of fluG resulted in neither "fluffy" phenotype nor alteration in conidiation, indicating possible different upstream mechanisms activating brlA between P. decumbens and Aspergillus nidulans. Deletion of brlA completely blocked conidiation. Further investigation of brlA expression in different media (nutrient-rich or nutrient-poor) and different culture states (liquid or solid) showed that brlA expression is required but not sufficient for conidiation. The brlA deletion strain exhibited altered hyphal morphology with more branches. Genome-wide expression profiling identified BrlA-dependent genes in P. decumbens, including genes previously reported to be involved in conidiation as well as previously reported chitin synthase genes and acid protease gene (pepB). The expression levels of seven secondary metabolism gene clusters (from a total of 28 clusters) were drastically regulated in the brlA deletion strain, including a downregulated cluster putatively involved in the biosynthesis of the mycotoxins roquefortine C and meleagrin. In addition, the expression levels of most cellulase genes were upregulated in the brlA deletion strain detected by real-time quantitative PCR. The brlA deletion strain also exhibited an 89.1 % increase in cellulase activity compared with the wild-type strain. The results showed that BrlA in P. decumbens not only has a key role in regulating conidiation, but it also regulates secondary metabolism extensively as well as the expression of cellulase genes.

Activation of the Hog1p kinase in Isc1p-deficient yeast cells is associated with mitochondrial dysfunction, oxidative stress sensitivity and premature aging

The Saccharomyces cerevisiae Isc1p, an orthologue of mammalian neutral sphingomyelinase 2, plays a key role in mitochondrial function, oxidative stress resistance and chronological lifespan. Isc1p functions upstream of the ceramide-activated protein phosphatase Sit4p through the modulation of ceramide levels. Here, we show that both ceramide and loss of Isc1p lead to the activation of Hog1p, the MAPK of the high osmolarity glycerol (HOG) pathway that is functionally related to mammalian p38 and JNK. The hydrogen peroxide sensitivity and premature aging of isc1Δ cells was partially suppressed by HOG1 deletion. Notably, Hog1p activation mediated the mitochondrial dysfunction and catalase A deficiency associated with oxidative stress sensitivity and premature aging of isc1Δ cells. Downstream of Hog1p, Isc1p deficiency activated the cell wall integrity (CWI) pathway. Deletion of the SLT2 gene, which encodes for the MAPK of the CWI pathway, was lethal in isc1Δ cells and this mutant strain was hypersensitive to cell wall stress. However, the phenotypes of isc1Δ cells were not associated with cell wall defects. Our findings support a role for Hog1p in the regulation of mitochondrial function and suggest that constitutive activation of Hog1p is deleterious for isc1Δ cells under oxidative stress conditions and during chronological aging.

Aspergillus as a multi-purpose cell factory: current status and perspectives

Aspergilli have a long history in biotechnology as expression platforms for the production of food ingredients, pharmaceuticals and enzymes. The achievements made during the last years, however, have the potential to revolutionize Aspergillus biotechnology and to assure Aspergillus a dominant place among microbial cell factories. This mini-review will highlight most recent breakthroughs in fundamental and applied Aspergillus research with a focus on new molecular tools, techniques and products. New trends and concepts related to Aspergillus genomics and systems biology will be discussed as well as the challenges that have to be met to integrate omics data with metabolic engineering attempts.

The effect of environmental conditions on extracellular protease activity in controlled fermentations of Aspergillus niger

Proteolytic degradation by host proteases is one of the key issues in the application of filamentous fungi for non-fungal protein production. In this study the influence of several environmental factors on the production of extracellular proteases of Aspergillus niger was investigated systematically in controlled batch cultures. Of all factors investigated in a series of initial screening experiments, culture pH and nitrogen concentration in particular strongly affected extracellular protease activities. For instance, at a culture pH of 4, protease activity was higher than at pH 5, and protease activity increased with increasing concentrations of ammonium as nitrogen source. Interestingly, an interdependence was observed for several of the factors studied. These possible interaction effects were investigated further using a full factorial experimental design. Amongst others, the results showed a clear interaction effect between nitrogen source and nitrogen concentration. Based on the observed interactions, the selection of environmental factors to reduce protease activity is not straightforward, as unexpected antagonistic or synergistic effects occur. Furthermore, not only were the effects of the process parameters on maximum protease activity investigated, but five other protease-related phenotypes were studied as well, such as maximum specific protease activity and maximum protease productivity. There were significant differences in the effect of the environmental parameters on the various protease-related phenotypes. For instance, pH significantly affected final levels of protease activity, but not protease productivity. The results obtained in this study are important for the optimization of A. niger for protein production.

Monitoring global gene expression of proteases and improvement of human lysozyme production in the nptB gene disruptant of Aspergillus oryzae

Aspergillus oryzae has numerous protease genes that might cause proteolytic degradation of heterologously-produced proteins. The productivity of the heterologous protein can be improved by protease gene disruption, but it is difficult to select disruption targets efficiently. In this study, we monitored the expression of 132 protease genes by DNA microarray. A group of protease genes up-regulated during cultivation was identified by clustering analysis. In this protease group, the nptB gene encoding neutral protease II was included as well as the alpA, tppA, and pepA genes, disruption of which has improved human lysozyme (HLY) production. The nptB gene was disrupted to investigate its involvement in HLY production, and nptB disruptants showed an improvement in the production. These observations suggest that monitoring the expression of protease genes is an efficient strategy in screening potential disruption targets for heterologous protein production in A. oryzae.

Genetic engineering of filamentous fungi--progress, obstacles and future trends

Filamentous fungi are widely used in biotechnology as cell factories for the production of chemicals, pharmaceuticals and enzymes. In order to improve their productivities, genetic engineering strategies can be powerful approaches. Different transformation techniques as well as DNA- and RNA-based methods to rationally design metabolic fluxes have been developed for industrially important filamentous fungi. However, the lack of efficient genetic engineering approaches still forms an obstacle for a multitude of fungi producing new and commercially interesting metabolites. This review summarises the variety of options that have recently become available to introduce and control gene expression in filamentous fungi and discusses their advantages and disadvantages. Furthermore, important considerations that have to be taken into account to design the best engineering strategy will be discussed.

A comparison of methods for successful triggering of gene silencing in Coprinus cinereus

Post-transcriptional gene-silencing methods (PTGS), including RNAi, are becoming increasingly pervasive in functional genomics. To advance analysis of the recently sequenced Coprinus cinereus genome, a high throughput gene silencing method is essential. We have exploited the GFP reporter gene to evaluate and quantify efficacy of three different silencing strategies. Modular constructs that encompassed antisense, untranslatable sense, and RNAi-mediating hairpin sequences, were transformed into a GFP-expressing host strain. Transformants exhibiting strong downregulation and partial suppression of GFP were recovered with all three constructs. Analyses of protein and transcriptional nucleic acids revealed that the antisense and hairpin sequences yielded similar levels of GFP suppression, and were both more efficient than untranslatable sense sequences. Our antisense vectors will expedite functional characterisation of C. cinereus and the modular nature of the constructs should permit exploitation of directional cDNA libraries for high throughput screening.

Double disruption of the proteinase genes, tppA and pepE, increases the production level of human lysozyme by Aspergillus oryzae

In this study, we investigated the effects of proteinase gene disruption on heterologous protein production by Aspergillus oryzae. The human lysozyme (HLY) was selected for recombinant production as a model for the heterologous protein. A tandem HLY construct fused with alpha-amylase (AmyB) was expressed by A. oryzae in which the Kex2 cleavage site was inserted at the upstream of HLY. HLY was successfully processed from AmyB and produced in the medium. We performed a systematic disruption analysis of five proteinase genes (pepA, pepE, alpA, tppA, and palB) in the HLY-producing strain with the adeA selectable marker. Comparative analysis indicated that disruption of the tppA gene encoding a tripeptidyl peptidase resulted in the highest increase (36%) in the HLY production. We further deleted the tppA gene in the pepE or palB disruptant with another selectable marker, argB. Consequently, a double disruption of the tppA and pepE genes led to a 63% increase in the HLY production compared to the control strain. This is the first study to report that the double disruption of the tppA and pepE genes improved the production level of a heterologous protein by filamentous fungi.

Developments in biotechnological production of sweet proteins

Most proteins are tasteless and flavorless, while some proteins elicit a sweet-taste response on the human palate. Six proteins, thaumatin, monellin, mabinlin, brazzein, egg lysozyme, and neoculin (previously considered as curculin) have been identified as sweet-tasting proteins. However, no common features among them have been observed. Herein, recent advances in the research of sweet-tasting proteins and the production of such proteins by biotechnological approaches are reviewed. Information on the structure-sweetness relationship for these proteins would help not only in the clarification of the mechanism of interaction of sweet-tasting proteins with their receptors, but also in the design of more effective low-calorie sweeteners.

Crystal structure of scytalidoglutamic peptidase with its first potent inhibitor provides insights into substrate specificity and catalysis

Scytalidoglutamic peptidase (SGP) from Scytalidium lignicolum is the founding member of the newly discovered\ family of peptidases, G1, so far found exclusively in fungi. The crystal structure of SGP revealed a previously undescribed fold for peptidases and a unique catalytic dyad of residues Gln53 and Glu136. Surprisingly, the beta-sandwich structure of SGP is strikingly similar to members of the carbohydrate-binding concanavalin A-like lectins/glucanases superfamily. By analogy with the active sites of aspartic peptidases, a mechanism employing nucleophillic attack by a water molecule activated by the general base functionality of Glu136 has been proposed. Here, we report the first crystal structures of SGP in complex with two transition state peptide analogs designed to mimic the tetrahedral intermediate of the proteolytic reaction. Of these two analogs, the one containing a central S-hydroxyl group is a potent sub-nanomolar inhibitor of SGP. The inhibitor binds non-covalently to the concave surface of the upper beta-sheet and enables delineation of the S4 to S3' substrate specificity pockets of the enzyme. Structural differences in these pockets account for the unique substrate preferences of SGP among peptidases having an acidic pH optimum. Inhibitor binding is accompanied by a structuring of the region comprising residues Tyr71-Gly80 from being mostly disordered in the apoenzyme and leading to positioning of crucial active site residues for establishing enzyme-inhibitor contacts. In addition, conformational rearrangements are seen in a disulfide bridged surface loop (Cys141-Cys148), which moves inwards, partially closing the open substrate binding cleft of the native enzyme. The non-hydrolysable scissile bond analog of the inhibitor is located in the active site forming close contacts with Gln53 and Glu136. The nucleophilic water molecule is displaced and a unique mode of binding is observed with the S-OH of the inhibitor occupying the oxyanion binding site of the proposed tetrahedral intermediate. Details of the enzyme-inhibitor interactions and mechanistic interpretations are discussed.

Protein kinase C (PkcA) of Aspergillus nidulans is involved in penicillin production

The biosynthesis of the beta-lactam antibiotic penicillin in the filamentous fungus Aspergillus nidulans is catalyzed by three enzymes that are encoded by the acvA, ipnA, and aatA genes. A variety of cis-acting DNA elements and regulatory factors form a complex regulatory network controlling these beta-lactam biosynthesis genes. Regulators involved include the CCAAT-binding complex AnCF and AnBH1. AnBH1 acts as a repressor of the penicillin biosynthesis gene aatA. Until now, however, little information has been available on the signal transduction cascades leading to the transcription factors. Here we show that inhibition of protein kinase C (Pkc) activity in A. nidulans led to cytoplasmic localization of an AnBH1-enhanced green fluorescent protein (EGFP) fusion protein. Computer analysis of the genome and screening of an A. nidulans gene library revealed that the fungus possesses two putative Pkc-encoding genes, which we designated pkcA and pkcB. Only PkcA showed all the characteristic features of fungal Pkc's. Production of pkcA antisense RNA in A. nidulans led to reduced growth and conidiation in Aspergillus minimal medium, while in fermentation medium it led to enhanced expression of an aatAp-lacZ gene fusion, reduced pencillin production, and predominantly cytoplasmic localization of AnBH1. These data agree with the finding that inhibition of Pkc activity prevented nuclear localization of AnBH1-EGFP. As a result, repression of aatA expression was relieved. The involvement of Pkc in penicillin biosynthesis is also interesting in light of the fact that in the yeast Saccharomyces cerevisiae, Pkc plays a major role in maintaining cell integrity.

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.

Glutamic protease distribution is limited to filamentous fungi

Glutamic proteases are a distinct, and recently re-classified, group of peptidases that are thought to be found only in fungi. We have identified and analysed the distribution of over 20 putative glutamic proteases from all fungal species whose genomes have been sequenced so far. Although absent from the Saccharomycetales class, glutamic proteases appear to be present in all other ascomycetes species examined. A large number of coding regions for glutamic proteases were also found clustered together in the Phanerochaete chrysosporium genome, despite apparently being absent from three other species of Basidiomycota.

Disruption of the subtilase gene, albin1, in Ophiostoma piliferum

Wood sapstaining fungi produce multiple proteases that break down wood protein. Three groups of subtilases have been identified in sapstaining fungi; however, it is not known if these groups have distinct physiological roles (B. Hoffman and C. Breuil, Curr. Genet. 41:168-175, 2002). In this work we examined the role of the subtilase Albin1 from Ophiostoma piliferum. Reamplification of cDNA ends PCR was used to obtain the albin1 gene sequence. The encoded subtilase is probably extracellular and involved in nutrient acquisition. This gene was disrupted with an Agrobacterium tumefaciens-mediated transformation system. Two of the disruptants obtained had significantly lower levels of proteolytic activity, slower growth in bovine serum albumin, and significantly reduced growth on wood. Thus, albin1 plays an important role in O. piliferum's ability to acquire nitrogen from wood proteins.

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.