Improved α-amylase production by Aspergillus oryzae after a double deletion of genes involved in carbon catabolite repression

Construction of an Aspergillus oryzae △nptB△pyrG Host for Homologous Expression of Lipase and Catalytic Property Characterization of Recombinant Lipase

Aspergillus oryzae is an ideal cell factory for protein expression with powerful protein processing and secretion capabilities. The current study aimed to explore the homologous expression of A. oryzae lipase AOL (GenBank: KP975533) by constructing an auxotrophic A. oryzae △pyrG△nptB and subsequently characterizing the immobilization and catalytic properties of recombinant lipase. Initially, the pyrG gene knocked out in wild-type A. oryzae by homologous recombination, followed by the creation of a uridine/uracil auxotroph transformation. Through this system, the protease gene nptB was precisely knocked out, leading to a substantial decrease in extracellular (39.04%) and intracellular (90.07%) protease activity. The A. oryzae △nptB△pyrG strain was used as host for homologous expression of lipase AOL. After transformation of linearized lipase-expression cassette, the engineered A. oryzae AOL-8 was screened out with the lipase gene copy number of 14, exhibiting extracellular and intracellular lipase activities of 1.75 U/mL and 46.4 U/g, respectively. Subsequently, the production and immobilization of the recombinant lipase, via physical adsorption on macroporous resin XRZ04B, were achieved through submerged fermentation of the AOL-8 strain. The results of esterification catalytic properties of immobilized recombinant lipase indicated that the lipase exhibited optimal catalytic activity with lauric acid and methanol as substrates, a reaction temperature of 35 °C, and n-hexane as the preferred solvent medium; its highest conversion rate can reach at 72.3%.

Glucose-induced endocytic degradation of the maltose transporter MalP is mediated through ubiquitination by the HECT-ubiquitin ligase HulA and its adaptor CreD in Aspergillus oryzae

In the filamentous fungus Aspergillus oryzae, large amounts of amylolytic enzymes are inducibly produced by isomaltose, which is converted from maltose incorporated via the maltose transporter MalP. In contrast, the preferred sugar glucose strongly represses the expression of both amylolytic and malP genes through carbon catabolite repression. Simultaneously, the addition of glucose triggers the endocytic degradation of MalP on the plasma membrane. In budding yeast, the signal-dependent ubiquitin modification of plasma membrane transporters leads to selective endocytosis into the vacuole for degradation. In addition, during glucose-induced MalP degradation, the homologous of E6AP C-terminus-type E3 ubiquitin ligase (HulA) is responsible for the ubiquitin modification of MalP, and the arrestin-like protein CreD is required for HulA targeting. Although CreD-mediated MalP internalization occurs in response to glucose, the mechanism by which CreD regulates HulA-dependent MalP ubiquitination remains unclear. In this study, we demonstrated that three (P/L)PxY motifs present in the CreD protein are essential for functioning as HulA adaptors so that HulA can recognize MalP in response to glucose stimulation, enabling MalP internalization. Furthermore, four lysine residues (three highly conserved among Aspergillus species and yeast and one conserved among Aspergillus species) of CreD were found to be necessary for its ubiquitination, resulting in efficient glucose-induced MalP endocytosis. The results of this study pave the way for elucidating the regulatory mechanism of MalP endocytic degradation through ubiquitination by the HulA-CreD complex at the molecular level.

Transcriptional and post-transcriptional regulation of genes encoding secretory proteins in Aspergillus oryzae

Aspergillus oryzae, also known as the yellow koji mold, produces various hydrolytic enzymes that are widely used in different industries. Its high capacity to produce secretory proteins makes this filamentous fungus a suitable host for heterologous protein production. Amylolytic gene promoter is widely used to express heterologous genes in A. oryzae. The expression of this promoter is strictly regulated by several transcription factors, whose activation involves various factors. Furthermore, the expression levels of amylolytic and heterologous genes are post-transcriptionally regulated by mRNA degradation mechanisms in response to aberrant transcriptional termination or endoplasmic reticulum stress. This review discusses the transcriptional and post-transcriptional regulatory mechanisms controlling the expression of genes encoding secretory proteins in A. oryzae.

Plant polysaccharide degradation-related enzymes in Aspergillus oryzae

Plants synthesize large amounts of stored and structural polysaccharides. Aspergillus oryzae is used in traditional Japanese fermentation and produces many types of plant polysaccharide degradation-related enzymes. The carbohydrate-active enzymes of A. oryzae are important in the fermentation process and biotechnological applications. Because plant polysaccharides have a complex structure, cooperative and synergistic actions of enzymes are crucial for the degradation of plant polysaccharides. For example, the cooperative action of isoprimeverose-producing oligoxyloglucan hydrolase, β-galactosidase, and α-xylosidase is important for the degradation of xyloglucan, and A. oryzae coordinates these enzymes at the expression level. In this review, I focus on the plant polysaccharide degradation-related enzymes identified in A. oryzae.

Copper ion (Cu2+) is involved in the transcription of the tyrosinase-encoding melB gene of Aspergillus oryzae in solid-state culture

In Aspergillus oryzae, the tyrosinase-encoding gene melB causes undesirable browning of sake and sake lees. This gene is known to be expressed specifically in solid-state culture; however, its expression mechanisms remain unknown. Here, we evaluated the possible factors affecting the transcription of melB and found that the copper ion (Cu2+) significantly enhanced the transcription level of melB in solid-state culture.

A CRISPR/Cas9-based multicopy integration system for protein production in Aspergillus niger

The filamentous fungus Aspergillus niger is well known for its high protein secretion capacity and a preferred host for homologous and heterologous protein production. To improve the protein production capacity of A. niger even further, a set of dedicated protein production strains was made containing up to 10 glucoamylase landing sites (GLSs) at predetermined sites in the genome. These GLSs replace genes encoding enzymes abundantly present or encoding unwanted functions. Each GLS contains the promotor and terminator region of the glucoamylase gene (glaA), one of the highest expressed genes in A. niger. Integrating multiple gene copies, often realized by random integration, is known to boost protein production yields. In our approach the GLSs allow for rapid targeted gene replacement using CRISPR/Cas9-mediated genome editing. By introducing the same or different unique DNA sequences (dubbed KORE sequences) in each GLS and designing Cas9-compatible single guide RNAs, one is able to select at which GLS integration of a target gene occurs. In this way a set of identical strains with different copy numbers of the gene of interest can be easily and rapidly made to compare protein production levels. As an illustration of its potential, we successfully used the expression platform to generate multicopy A. niger strains producing the Penicillium expansum PatE::6xHis protein catalysing the final step in patulin biosynthesis. The A. niger strain expressing 10 copies of the patE::6xHis expression cassette produced about 70 μg·mL-1 PatE protein in the culture medium with a purity just under 90%.

Physiological ER stress caused by amylase production induces regulated Ire1-dependent mRNA decay in Aspergillus oryzae

Regulated Ire1-dependent decay (RIDD) is a feedback mechanism in which the endoribonuclease Ire1 cleaves endoplasmic reticulum (ER)-localized mRNAs encoding secretory and membrane proteins in eukaryotic cells under ER stress. RIDD is artificially induced by chemicals that generate ER stress; however, its importance under physiological conditions remains unclear. Here, we demonstrate the occurrence of RIDD in filamentous fungus using Aspergillus oryzae as a model, which secretes copious amounts of amylases. α-Amylase mRNA was rapidly degraded by IreA, an Ire1 ortholog, depending on its ER-associated translation when mycelia were treated with dithiothreitol, an ER-stress inducer. The mRNA encoding maltose permease MalP, a prerequisite for the induction of amylolytic genes, was also identified as an RIDD target. Importantly, RIDD of malP mRNA is triggered by inducing amylase production without any artificial ER stress inducer. Our data provide the evidence that RIDD occurs in eukaryotic microorganisms under physiological ER stress.

cAMP signaling factors regulate carbon catabolite repression of hemicellulase genes in Aspergillus nidulans

Carbon catabolite repression (CCR) enables preferential utilization of easily metabolizable carbon sources, implying the presence of mechanisms to ensure discriminatory gene repression depending on the ambient carbon sources. However, the mechanisms for such hierarchical repression are not precisely understood. In this report, we examined how deletion of pkaA and ganB, which encode cAMP signaling factors, and creA, which encodes a well-characterized repressor of CCR, affects CCR of hemicellulase genes in the filamentous fungus Aspergillus nidulans. β-Xylanase production increased not only in ΔcreA but also in ΔpkaA and ΔganB, with the highest level observed in their double deletants, irrespective of the presence or absence of d-glucose. Expression of the β-xylanase genes in the presence of d-glucose was de-repressed in all the deletion mutants, with significantly higher tolerance against d-glucose repression in ΔpkaA and ΔganB than in ΔcreA. In the presence of galactomannan and d-glucose, partial de-repression of β-mannanase production was detected in ΔcreA, but not in ΔpkaA and ΔganB. The double deletion of creA/pkaA and creA/ganB led to earlier production. Release from d-glucose repression of the β-mannanase genes was partial in the single deletants, while nearly full de-repression was observed in ΔcreAΔpkaA and ΔcreAΔganB. The contribution of PkaA and GanB to CCR by d-xylose of the β-mannanase genes was very minor compared to that of CreA. Consequently, the present study revealed that cAMP signaling plays a major role in CCR of hemicellulase gene expression in a manner that is clearly independent from CreA.

RNA-Seq Based Transcriptome Analysis of Aspergillus oryzae DSM 1863 Grown on Glucose, Acetate and an Aqueous Condensate from the Fast Pyrolysis of Wheat Straw

Due to its acetate content, the pyrolytic aqueous condensate (PAC) formed during the fast pyrolysis of wheat straw could provide an inexpensive substrate for microbial fermentation. However, PAC also contains several inhibitors that make its detoxification inevitable. In our study, we examined the transcriptional response of Aspergillus oryzae to cultivation on 20% detoxified PAC, pure acetate and glucose using RNA-seq analysis. Functional enrichment analysis of 3463 significantly differentially expressed (log2FC >2 & FDR < 0.05) genes revealed similar metabolic tendencies for both acetate and PAC, as upregulated genes in these cultures were mainly associated with ribosomes and RNA processing, whereas transmembrane transport was downregulated. Unsurprisingly, metabolic pathway analysis revealed that glycolysis/gluconeogenesis and starch and sucrose metabolism were upregulated for glucose, whereas glyoxylate and the tricarboxylic acid (TCA) cycle were important carbon utilization pathways for acetate and PAC, respectively. Moreover, genes involved in the biosynthesis of various amino acids such as arginine, serine, cysteine and tryptophan showed higher expression in the acetate-containing cultures. Direct comparison of the transcriptome profiles of acetate and PAC revealed that pyruvate metabolism was the only significantly different metabolic pathway and was overexpressed in the PAC cultures. Upregulated genes included those for methylglyoxal degradation and alcohol dehydrogenases, which thus represent potential targets for the further improvement of fungal PAC tolerance.

Effect of Glucose on Endo-xylanase and β-xylosidase Production by Fungi Isolated in Indonesia

Xylanases are widely produced by fungi, and the production of polysaccharide-degrading enzymes, in general, are usually subjected to carbon catabolite repression. In this work, the ability of several Indonesian indigenous fungi to produce endo-xylanase and β-xylosidase and their responses to glucose as a repressor were determined. Ten fungi were grown in a liquid medium supplemented with glucose as the repressor (0, 1%, 3%, and 5%), and the endo-xylanase and β-xylosidase productions were assayed. Aspergillus aculeatus FIG1 and A. oryzae KKB4 produced 3.85 and 0.70 U/mL of endo-xylanase, respectively, compared with other strains (0.22 U/mL or less). Trichoderma asperellum PK1J2, T. virens MLT2J2, A. aculeatus FIG1, T. asperellum MLT5J1, A. oryzae KKB4, and T. asperellum MLT3J2 produced 0.021–0.065 U/mL of β-xylosidase, whereas the other strains produced 0.013 U/mL or less of β-xylosidase. Adding 1% glucose to the growth medium can partially repress endo-xylanase production in A. aculeatus FIG1, T. asperellum PK1J2, and T. virens MLT4J1 and completely repress other strains. By adding 1% glucose, strains FIG1, PK1J2, and MLT4J1 suffered almost complete repression of β-xylosidase production, although such strains exhibited partial repression of endo-xylanase production. β-Xylosidase produced by the other strains showed complete repression by adding 1% glucose, except for A. aculeatus FIG1, A. tamarii FNCC 6151, and T. asperellum MLT1J1, which showed partial repression. Therefore, adding 3% glucose to the growth medium can result in complete repression of endo-xylanase and β-xylosidase productions in all strains examined.

The signaling pathways involved in metabolic regulation and stress responses of the yeast-like fungi Aureobasidium spp

Aureobasidium spp. can use a wide range of substrates and are widely distributed in different environments, suggesting that they can sense and response to various extracellular signals and be adapted to different environments. It is true that their pullulan, lipid and liamocin biosynthesis and cell growth are regulated by the cAMP-PKA signaling pathway; Polymalate (PMA) and pullulan biosynthesis is controlled by the Ca²⁺ and TORC1 signaling pathways; the HOG1 signaling pathway determines high osmotic tolerance and high pullulan and liamocin biosynthesis; the Snf1/Mig1 pathway controls glucose repression on pullulan and liamocin biosynthesis; DHN-melanin biosynthesis and stress resistance are regulated by the CWI signaling pathway and TORC1 signaling pathway. In addition, the HSF1 pathway may control cell growth of some novel strains of A. melanogenum at 37 °C. However, the detailed molecular mechanisms of high temperature growth and thermotolerance of some novel strains of A. melanogenum and glucose derepression in A. melanogenum TN3-1 are still unclear.

Synergistic effects of multiple enzymes from industrial Aspergillus niger strain O1 on starch saccharification

BACKGROUND

Starch is one of the most important renewable polysaccharides in nature for production of bio-ethanol. The starch saccharification step facilitates the depolymerization of starch to yield glucose for biofuels production. The filamentous fungus Aspergillus niger (A. niger) is the most used microbial cell factory for production of the commercial glucoamylase. However, the role of each component in glucoamylases cocktail of A. niger O1 for starch saccharification remains unclear except glucoamylase.

RESULTS

In this study, we identified the key enzymes contributing to the starch saccharification process are glucoamylase, α-amylase and acid α-amylase out of 29 glycoside hydrolases from the 6-day fermentation products of A. niger O1. Through the synergistic study of the multienzymes for the starch saccharification in vitro, we found that increasing the amount of α-amylase by 5-10 times enhanced the efficiency of starch saccharification by 14.2-23.2%. Overexpression of acid α-amylase in strain O1 in vivo increased the total glucoamylase activity of O1 cultures by 15.0%.

CONCLUSIONS

Our study clarifies the synergistic effects among the components of glucoamylases cocktail, and provides an effective approach to optimize the profile of saccharifying enzymes of strain O1 for improving the total glucoamylase activity.

Crucial role of the intracellular α-glucosidase MalT in the activation of the transcription factor AmyR essential for amylolytic gene expression in Aspergillus oryzae

We examined the role of the intracellular α-glucosidase gene malT, which is part of the maltose-utilizing cluster (MAL cluster) together with malR and malP, in amylolytic gene expression in Aspergillus oryzae. malT disruption severely affected fungal growth on medium containing maltose or starch. Furthermore, the transcription level of the α-amylase gene was significantly reduced by malT disruption. Given that the transcription factor AmyR responsible for amylolytic gene expression is activated by isomaltose converted from maltose incorporated into the cells, MalT may have transglycosylation activity that converts maltose to isomaltose. Indeed, transglycosylated products such as isomaltose/maltotriose and panose were generated from the substrate maltose by MalT purified from a malT-overexpressing strain. The results of this study, taken together, suggests that MalT plays a pivotal role in AmyR activation via its transglycosylation activity that converts maltose to the physiological inducer isomaltose.

Expression profiles of amylolytic genes in AmyR and CreA transcription factor deletion mutants of the black koji mold Aspergillus luchuensis

The black koji mold, Aspergillus luchuensis, which belongs to Aspergillus section Nigri, is used for the production of traditional Japanese spirits (shochu) mainly in the southern districts of Japan. This mold is known to produce amylolytic enzymes essential for shochu production; however, mechanisms regulating amylolytic gene expression in A. luchuensis have not been studied in as much detail as those in the yellow koji mold, Aspergillus oryzae. Here, we examined the gene expression profiles of deletion mutants of transcription factors orthologous to A. oryzae AmyR and CreA in A. luchuensis. A. luchuensis produces acid-unstable (AmyA) and acid-stable (AsaA) α-amylases. AmyA production and amyA gene expression were not influenced by amyR or creA deletion, indicating that amyA was constitutively expressed. In contrast, asaA gene expression was significantly down- and upregulated upon deletion of amyR and creA, respectively. Furthermore, the glaA and agdA genes (encoding glucoamylase and α-glucosidase, respectively) showed expression profiles similar to those of asaA. Thus, genes that play pivotal roles in starch saccharification, asaA, glaA, and agdA, were found to be regulated by AmyR and CreA. Moreover, despite previous reports on AsaA being only produced in solid-state culture, deletion of the ortholog of A. oryzae flbC, which is involved in the expression of the solid-state culture-specific genes, did not affect AsaA α-amylase activity, suggesting that FlbC was not associated with asaA expression.

Induction and Repression of Hydrolase Genes in Aspergillus oryzae

The filamentous fungus Aspergillus oryzae, also known as yellow koji mold, produces high levels of hydrolases such as amylolytic and proteolytic enzymes. This property of producing large amounts of hydrolases is one of the reasons why A. oryzae has been used in the production of traditional Japanese fermented foods and beverages. A wide variety of hydrolases produced by A. oryzae have been used in the food industry. The expression of hydrolase genes is induced by the presence of certain substrates, and various transcription factors that regulate such expression have been identified. In contrast, in the presence of glucose, the expression of the glycosyl hydrolase gene is generally repressed by carbon catabolite repression (CCR), which is mediated by the transcription factor CreA and ubiquitination/deubiquitination factors. In this review, we present the current knowledge on the regulation of hydrolase gene expression, including CCR, in A. oryzae.

Accelerated glucose metabolism in hyphae-dispersed Aspergillus oryzae is suitable for biological production

Recently, a hyphae-dispersed type of filamentous fungus Aspergillus oryzae was constructed via genetic engineering, and industrial applications are expected due to the ease of handling and to the level of protein production properties. In this study, we constructed cellulase-expressing strains using wild-type and hyphae-dispersed strains to investigate the correlation between protein productivity and metabolism. Compared with the original strain, the hyphae-dispersed cellulase-expressing strain showed elevated cellulase activity, rapid glucose consumption, increased mycelial dry weight, an increased expression of cellulase genes, and activated respiration activity. Comparative metabolomic analysis showed fewer metabolites in the glycolysis and TCA cycles in the dispersed strains than in the original strains. These results indicate that the flux of carbohydrate metabolism in the hyphae-dispersed strains is smoother than that in the original strains. Such efficient metabolic flux would contribute to efficient energy conversion and to sufficient energy supply to anabolisms, such as mycelial growth and protein production. Our findings suggest that the hyphae-dispersed strains could be a useful host not only for protein production but also for the biological production of various chemicals such as organic acids.

Deletion of Aspergillus nidulans cpsA/rseA induces increased extracellular hydrolase production in solid-state culture partly through the high osmolarity glycerol pathway

Koji molds, such as Aspergillus oryzae and Aspergillus sojae, are used in the food industry in East Asia and have been explored for the large-scale production of extracellular hydrolases. We previously found that the deletion of a gene encoding a putative GT2 glycosyltransferase increased production of extracellular hydrolases in A. sojae. The gene was named rseA (regulator of the secretory enzyme A). We predicted that intracellular signaling pathways were involved in the increased production of hydrolases in the ΔrseA mutant of A. sojae. However, little has been reported on molecular biological knowledge about A. sojae. Hence, Aspergillus nidulans, a typical model organism used in molecular biology, was employed for the functional characterization of rseA in this study. Deletion of the rseA ortholog in A. nidulans induced increased extracellular production of hydrolases under the solid-state cultivation condition, similar to that in A. sojae. The involvement of the cell wall integrity pathway and the high osmolarity glycerol pathway in ΔrseA was further investigated. The results indicated that the HOG pathway played an important role in the increased extracellular production of hydrolases caused by the deletion of the rseA gene. rseA ortholog in A. nidulans was identical to cpsA, which was reported to function as a regulator of mycotoxin production, morphogenesis, and cell wall biosynthesis. However, this is the first study reporting that rseA/cpsA regulates extracellular hydrolase production in A. nidulans.

Advances in Genetic Engineering Technology and Its Application in the Industrial Fungus Aspergillus oryzae

The filamentous fungus Aspergillus oryzae is an important strain in the traditional fermentation and food processing industries and is often used in the production of soy sauce, soybean paste, and liquor-making. In addition, A. oryzae has a strong capacity to secrete large amounts of hydrolytic enzymes; therefore, it has also been used in the enzyme industry as a cell factory for the production of numerous native and heterologous enzymes. However, the production and secretion of foreign proteins by A. oryzae are often limited by numerous bottlenecks that occur during transcription, translation, protein folding, translocation, degradation, transport, secretion, etc. The existence of these problems makes it difficult to achieve the desired target in the production of foreign proteins by A. oryzae. In recent years, with the decipherment of the whole genome sequence, basic research and genetic engineering technologies related to the production and utilization of A. oryzae have been well developed, such as the improvement of homologous recombination efficiency, application of selectable marker genes, development of large chromosome deletion technology, utilization of hyphal fusion techniques, and application of CRISPR/Cas9 genome editing systems. The development and establishment of these genetic engineering technologies provided a great deal of technical support for the industrial production and application of A. oryzae. This paper reviews the advances in basic research and genetic engineering technologies of the fermentation strain A. oryzae mentioned above to open up more effective ways and research space for the breeding of A. oryzae production strains in the future.

Carbon Catabolite Repression Governs Diverse Physiological Processes and Development in Aspergillus nidulans

Carbon catabolite repression (CCR) is a common phenomenon of microorganisms that enable efficient utilization of carbon nutrients, critical for the fitness of microorganisms in the wild and for pathogenic species to cause infection. In most filamentous fungal species, the conserved transcription factor CreA/Cre1 mediates CCR. Previous studies demonstrated a primary function for CreA/Cre1 in carbon metabolism; however, the phenotype of creA/cre1 mutants indicated broader roles. The global function and regulatory mechanism of this wide-domain transcription factor has remained elusive. Here, we applied two powerful genomics methods (transcriptome sequencing and chromatin immunoprecipitation sequencing) to delineate the direct and indirect roles of Aspergillus nidulans CreA across diverse physiological processes, including secondary metabolism, iron homeostasis, oxidative stress response, development, N-glycan biosynthesis, unfolded protein response, and nutrient and ion transport. The results indicate intricate connections between the regulation of carbon metabolism and diverse cellular functions. Moreover, our work also provides key mechanistic insights into CreA regulation and identifies CreA as a master regulator controlling many transcription factors of different regulatory networks. The discoveries for this highly conserved transcriptional regulator in a model fungus have important implications for CCR in related pathogenic and industrial species. IMPORTANCE The ability to scavenge and use a wide range of nutrients for growth is crucial for microorganisms' survival in the wild. Carbon catabolite repression (CCR) is a transcriptional regulatory phenomenon of both bacteria and fungi to coordinate the expression of genes required for preferential utilization of carbon sources. Since carbon metabolism is essential for growth, CCR is central to the fitness of microorganisms. In filamentous fungi, CCR is mediated by the conserved transcription factor CreA/Cre1, whose function in carbon metabolism has been well established. However, the global roles and regulatory mechanism of CreA/Cre1 are poorly defined. This study uncovers the direct and indirect functions of CreA in the model organism Aspergillus nidulans over diverse physiological processes and development and provides mechanistic insights into how CreA controls different regulatory networks. The work also reveals an interesting functional divergence between filamentous fungal and yeast CreA/Cre1 orthologues.

Identification and distinct regulation of three di/tripeptide transporters in Aspergillus oryzae

The uptake of di/tripeptides is mediated by the proton-dependent oligopeptide transporter (POT) family. In this study, 3 POT family transporters, designated PotA, PotB, and PotC were identified in Aspergillus oryzae. Growth comparison of deletion mutants of these transporter genes suggested that PotB and PotC are responsible for di/tripeptide uptake. PotA, which had the highest sequence similarity to yeast POT (Ptr2), contributed little to the uptake. Nitrogen starvation induced potB and potC expression, but not potA expression. When 3 dipeptides were provided as nitrogen sources, the expression profiles of these genes were different. PrtR, a transcription factor that regulates proteolytic genes, was involved in regulation of potA and potB but not in potC expression. Only potC expression levels were dramatically reduced by disruption of ubrA, an orthologue of yeast ubiquitin ligase UBR1 responsible for PTR2 expression. Expression of individual POT genes is apparently controlled by different regulatory mechanisms.

Carbon Catabolite Repression Gene AoCreA Regulates Morphological Development and Ochratoxin A Biosynthesis Responding to Carbon Sources in Aspergillus ochraceus

Carbon is one of the most important nutrients for the development and secondary metabolism in fungi. CreA is the major transcriptional factor mediating carbon catabolite repression, which is employed in the utilization of carbon sources. Aspergillus ochraceus contaminates various food and feed containing different carbon sources by producing ochratoxin A (OTA). However, little is known about the function of AoCreA in regulating the morphology and OTA production of A. ochraceus. To give an insight into the mechanism of the carbon sources regulating development of A. ochraceus and OTA production, we have identified AoCreA in A. ochraceus. The homologous recombination strategy was used to generate the AoCreA deletion mutant (ΔAoCreA). We have investigated the morphology and OTA production of the wild type (WT) and ΔAoCreA of A. ochraceus with media containing different carbon sources (glucose, fructose, maltose, D-xylose, D-mannose, acetate, D-galactose, D-mannitol and lactose). ΔAoCreA showed a significant growth and conidiation defect on all media as compared with WT. Glucose and maltose were the most inducing media for OTA production by A. ochraceus, followed by sucrose and the nutrient-rich Yeast Extract Sucrose (YES) and Potato Dextrose Agar (PDA). The deletion of AoCreA led to a drastic reduction of OTA production on all kinds of media except PDA, which was supported by the expression profile of OTA biosynthetic genes. Furthermore, infection studies of ΔAoCreA on oats and pears showed the involvement of AoCreA in the pathogenicity of A. ochraceus. Thus, these results suggest that AoCreA regulates morphological development and OTA biosynthesis in response to carbon sources in A. ochraceus.

Studies of Cellulose and Starch Utilization and the Regulatory Mechanisms of Related Enzymes in Fungi

Polysaccharides are biopolymers made up of a large number of monosaccharides joined together by glycosidic bonds. Polysaccharides are widely distributed in nature: Some, such as peptidoglycan and cellulose, are the components that make up the cell walls of bacteria and plants, and some, such as starch and glycogen, are used as carbohydrate storage in plants and animals. Fungi exist in a variety of natural environments and can exploit a wide range of carbon sources. They play a crucial role in the global carbon cycle because of their ability to break down plant biomass, which is composed primarily of cell wall polysaccharides, including cellulose, hemicellulose, and pectin. Fungi produce a variety of enzymes that in combination degrade cell wall polysaccharides into different monosaccharides. Starch, the main component of grain, is also a polysaccharide that can be broken down into monosaccharides by fungi. These monosaccharides can be used for energy or as precursors for the biosynthesis of biomolecules through a series of enzymatic reactions. Industrial fermentation by microbes has been widely used to produce traditional foods, beverages, and biofuels from starch and to a lesser extent plant biomass. This review focuses on the degradation and utilization of plant homopolysaccharides, cellulose and starch; summarizes the activities of the enzymes involved and the regulation of the induction of the enzymes in well-studied filamentous fungi.

Use of the kojA promoter, involved in kojic acid biosynthesis, for polyketide production in Aspergillus oryzae: implications for long-term production

BACKGROUND

Aspergillus oryzae, a useful industrial filamentous fungus, produces limited varieties of secondary metabolites, such as kojic acid. Thus, for the production of valuable secondary metabolites by genetic engineering, the species is considered a clean host, enabling easy purification from cultured cells. A. oryzae has been evaluated for secondary metabolite production utilizing strong constitutive promoters of genes responsible for primary metabolism. However, secondary metabolites are typically produced by residual nutrition after microbial cells grow to the stationary phase and primary metabolism slows. We focused on a promoter of the secondary metabolism gene kojA, a component of the kojic acid biosynthetic gene cluster, for the production of other secondary metabolites by A. oryzae.

RESULTS

A kojA disruptant that does not produce kojic acid was utilized as a host strain for production. Using this host strain, a mutant that expressed a polyketide synthase gene involved in polyketide secondary metabolite production under the kojA gene promoter was constructed. Then, polyketide production and polyketide synthase gene expression were observed every 24 h in liquid culture. From days 0 to 10 of culture, the polyketide was continuously produced, and the synthase gene expression was maintained. Therefore, the kojA promoter was activated, and it enabled the continuous production of polyketide for 10 days.

CONCLUSIONS

The combined use of the kojA gene promoter and a kojA disruptant proved useful for the continuous production of a polyketide secondary metabolite in A. oryzae. These findings suggest that this combination can be applied to other secondary metabolites for long-term production.

Regulatory mechanisms for amylolytic gene expression in the koji mold Aspergillus oryzae

The koji mold Aspergillus oryzae has been used in traditional Japanese food and beverage fermentation for over a thousand years. Amylolytic enzymes are important in sake fermentation, wherein production is induced by starch or malto-oligosaccharides. This inducible production requires at least two transcription activators, AmyR and MalR. Among amylolytic enzymes, glucoamylase GlaB is produced exclusively in solid-state culture and plays a critical role in sake fermentation owing to its contribution to glucose generation from starch. A recent study demonstrated that glaB gene expression is regulated by a novel transcription factor, FlbC, in addition to AmyR in solid-state culture. Amylolytic enzyme production is generally repressed by glucose due to carbon catabolite repression (CCR), which is mediated by the transcription factor CreA. Modifying CCR machinery, including CreA, can improve amylolytic enzyme production. This review focuses on the role of transcription factors in regulating A. oryzae amylolytic gene expression.

CreA-independent carbon catabolite repression of cellulase genes by trimeric G-protein and protein kinase A in Aspergillus nidulans

Cellulase production in filamentous fungi is repressed by various carbon sources. In our preliminary survey in Aspergillus nidulans, degree of de-repression differed depending on carbon sources in a mutant of creA, encoding the transcriptional repressor for carbon catabolite repression (CCR). To further understand mechanisms of CCR of cellulase production, we compared the effects of creA deletion with deletion of protein kinase A (pkaA) and G (ganB) genes, which constitute a nutrient sensing and signaling pathway. In plate culture with carboxymethyl cellulose and D-glucose, deletion of pkaA and ganB, but not creA, led to significant de-repression of cellulase production. In submerged culture with cellobiose and D-glucose or 2-deoxyglucose, both creA or pkaA single deletion led to partial de-repression of cellulase genes with the highest level by their double deletion, while ganB deletion caused de-repression comparable to that of the creA/pkaA double deletion. With ball-milled cellulose and D-glucose, partial de-repression was detected by deletion of creA but not of pkaA or ganB. The creA/pkaA or creA/ganB double deletion led to earlier expression than the creA deletion. Furthermore, the effect of each deletion with D-xylose or L-arabinose as the repressing carbon source was significantly different from that with D-glucose, D-fructose, and D-mannose. Consequently, this study revealed that PkaA and GanB participate in CreA-independent CCR and that contribution of CreA, PkaA, and GanB in CCR differs depending on the inducers, repressing carbon sources, and culture conditions (plate or submerged). Further study of CreA-independent mechanisms is needed to fully understand CCR in filamentous fungi.

A unique Zn(II)2-Cys6-type protein, KpeA, is involved in secondary metabolism and conidiation in Aspergillus oryzae

Aspergillus oryzae is an important microorganism in the bio- and food industries; therefore, understanding the mechanism underlying its secondary metabolism regulation is important for ensuring its safe use. Here, we screened a novel Zn(II)₂-Cys₆-type protein-encoding gene, AO090003001186, designated as kpeA (kojic acid production enhancement A), from an A. oryzae disruption mutant library of transcriptional regulators. kpeA is highly conserved among filamentous fungi and encodes a protein with Zn(II)₂-Cys₆ motif located in the middle of the sequence. Phylogenetic analysis revealed that KpeA was classified into a distal group compared to other fungal Zn(II)₂-Cys₆-type transcriptional regulators. A Cys to Ala substitution mutant of KpeA showed identical phenotype to the kpeA disruption strain, confirming that KpeA is novel type Zn(II)₂-Cys₆ binding protein. Colonies of the kpeA disruption strain (ΔkpeA) had longer aerial hyphae and showed decreased conidia production. Microscopic analysis suggested that the reduced vesicle size and conidial head formation in ΔkpeA strain account for the decreased conidia production. Transcriptional levels of brlA and downstream abaA and wetA were decreased in ΔkpeA strain. Moreover, ΔkpeA strain produced 6-fold more kojic acid than the control strains, and the expression of kojR and kojA was increased in ΔkpeA strain. Therefore, KpeA is a novel Zn(II)₂-Cys₆-type protein likely involved in conidiation and kojic acid production at the transcriptional level.

Engineered microbial host selection for value-added bioproducts from lignocellulose

Lignocellulose is a rich and sustainable globally available carbon source and is considered a prominent alternative raw material for producing biofuels and valuable chemical compounds. Enzymatic hydrolysis is one of the crucial steps of lignocellulose degradation. Cellulolytic and hemicellulolytic enzyme mixes produced by different microorganisms including filamentous fungi, yeasts and bacteria, are used to degrade the biomass to liberate monosaccharides and other compounds for fermentation or conversion to value-added products. During biomass pretreatment and degradation, toxic compounds are produced, and undesirable carbon catabolic repression (CCR) can occur. In order to solve this problem, microbial metabolic pathways and transcription factors involved have been investigated along with the application of protein engineering to optimize the biorefinery platform. Engineered Microorganisms have been used to produce specific enzymes to breakdown biomass polymers and metabolize sugars to produce ethanol as well other biochemical compounds. Protein engineering strategies have been used for modifying lignocellulolytic enzymes to overcome enzymatic limitations and improving both their production and functionality. Furthermore, promoters and transcription factors, which are key proteins in this process, are modified to promote microbial gene expression that allows a maximum performance of the hydrolytic enzymes for lignocellulosic degradation. The present review will present a critical discussion and highlight the aspects of the use of microorganisms to convert lignocellulose into value-added bioproduct as well combat the bottlenecks to make the biorefinery platform from lignocellulose attractive to the market.

Cold hydrolysis of cassava pulp and its use in simultaneous saccharification and fermentation (SSF) process for ethanol fermentation

The present study investigated cold hydrolysis of cassava pulp (CP) and the use of cold hydrolysis with simultaneous saccharification and fermentation (SSF) for ethanol production. Cold hydrolysis of 100 g-CP/L at 50 °C for 2 h, followed by at 30 °C for 72 h resulted in the production of 71.5 ± 1.8 g/L of reducing sugar, with a yield of 0.72 g/g-CP. A mathematical model describing the cold hydrolysis process was subsequently developed. The model proved to be applicable for other cold hydrolysis systems with satisfactory results. The sequential process of cold hydrolysis at 50 °C for 2 h, followed by SSF at 30 °C for 72 h gave 27.4 g-ethanol/L, with a productivity of 0.37 g/(L h) and a fermentation efficiency of 57.58%. Based on the results, a bioconversion process for CP to ethanol was proposed. In this process, 1 kg of ethanol could be produced from 3.65 kg of CP without any nutrient supplementation.

Random mutagenesis of super Koji (Aspergillus oryzae): improvement in production and thermal stability of α-amylases for maltose syrup production

Background

Alpha-amylases hydrolyze 1,4 α-glycosidic bonds of starch and produce malto-oligosaccharides. It is an important enzyme generally applied in textile, food and brewing industries. Enhancement in thermal stability and productivity of enzymes are the two most sought after properties for industrial use. The Aspergillus oryzae (Koji) has Generally Recognized as Safe (GRAS) status and safe for use in food industry. Hence, Koji strain’s development for the screening of potent mutants, hyper producer of thermostable α-amylases, with desired attributes is the need of the time.

Results

A process has been developed to improve super Koji (A. oryzae cmc1) strain through γ-rays treatment. The doses i.e. 0.60, 0.80, 1.00, 1.20 & 1.40 KGy gave more than 3.0 log kill. Initially, 52 Koji mutants resistant to 1% (w/v) Triton X-100 were selected. 2^nd screening was based on α-amylases hyper production and 23 mutants were sorted out by measuring clearing zones index (CI). Afterwards nine potent mutants, resistant to 2-deoxy D-glucose, were screened based on CI. These were further analyzed for thermal stability and productivity of α-amylase under submerged conditions. The mutants’ M-80(10), M-100(6) & M-120(5) gave about four fold increases in α-amylases productivity. The half life of M-100(6) α-amylase at 55 °C was 52 min and was highest among the mutants. Liquid Chromatography-Mass Spectrometry (LC-MS) analysis confirmed that mutants did not produce aflatoxins. Field Emission Scanning Electron Microscopy (FESEM) of Koji mycelia depicted that exposure to gamma rays increased rigidity of the mycelium. The potent Koji mutant M-100(6) was grown on soluble starch in 10L fermenter and produced 40.0 IU ml^-1 of α-amylases with specific activity of 2461 IU mg^-1 protein. Growth kinetic parameters were: μ = Specific growth rate= 0.069 h^-1, t_d = Biomass doubling time= 10.0 h, Y_p/x = Product yield coefficient with respect to cell mass = 482 U g^-1; q_p= Specific rate of product formation= 33.29 U g^-1 h^-1.

Conclusion

It was concluded that the developed five step screening process has great potential to generate potent mutants for the hyper production of thermostable enzymes through γ-rays mediated physical mutagenesis. The developed thermostable α-amylases of super Koji mutantM-100(6) has immense potential for application in saccharification process for maltose syrup production. Moreover, the developed five step strain’s development process may be used for the simultaneous improvement in productivity and thermal stability of other microbial enzymes.

Electronic supplementary material

The online version of this article (10.1186/s12866-018-1345-y) contains supplementary material, which is available to authorized users.

Nuclear export-dependent degradation of the carbon catabolite repressor CreA is regulated by a region located near the C-terminus in Aspergillus oryzae

Carbon catabolite repression (CCR) is regulated by the C₂ H₂ -type transcription factor CreA/Cre1 in filamentous fungi including Aspergillus oryzae. We investigated the stability and subcellular localization of CreA in A. oryzae. The abundance of FLAG-tagged CreA (FLAG-CreA) was dramatically reduced after incubation in maltose and xylose, which stimulated the export of CreA from the nucleus to the cytoplasm. Mutation of a putative nuclear export signal resulted in nuclear retention and significant stabilization of CreA. These results suggest that CreA is rapidly degraded in the cytoplasm after export from the nucleus. The FLAG-CreA protein level was reduced by disruption of creB and creC, which encode the deubiquitinating enzyme complex involved in CCR. In contrast, FLAG-CreA stability was not affected by disruption of creD which encodes an arrestin-like protein required for CCR relief. Deletion of the last 40 C-terminal amino acids resulted in remarkable stabilization and increased abundance of FLAG-CreA, whereas deletion of the last 20 C-terminal amino acids had no apparent effect on CreA stability. This result suggests that the 20 amino acid region located between positions 390 and 409 of CreA is critical for the rapid degradation of CreA.

Modulating Transcriptional Regulation of Plant Biomass Degrading Enzyme Networks for Rational Design of Industrial Fungal Strains

Filamentous fungi are the most important microorganisms for the industrial production of plant polysaccharide degrading enzymes due to their unique ability to secrete these proteins efficiently. These carbohydrate active enzymes (CAZymes) are utilized industrially for the hydrolysis of plant biomass for the subsequent production of biofuels and high-value biochemicals. The expression of the genes encoding plant biomass degrading enzymes is tightly controlled. Naturally, large amounts of CAZymes are produced and secreted only in the presence of the plant polysaccharide they specifically act on. The signal to produce is conveyed via so-called inducer molecules which are di- or mono-saccharides (or derivatives thereof) released from the specific plant polysaccharides. The presence of the inducer results in the activation of a substrate-specific transcription factor (TF), which is required not only for the controlled expression of the genes encoding the CAZymes, but often also for the regulation of the expression of the genes encoding sugar transporters and catabolic pathway enzymes needed to utilize the released monosaccharide. Over the years, several substrate-specific TFs involved in the degradation of cellulose, hemicellulose, pectin, starch and inulin have been identified in several fungal species and systems biology approaches have made it possible to uncover the enzyme networks controlled by these TFs. The requirement for specific inducers for TF activation and subsequently the expression of particular enzyme networks determines the choice of feedstock to produce enzyme cocktails for industrial use. It also results in batch-to-batch variation in the composition and amounts of enzymes due to variations in sugar composition and polysaccharide decorations of the feedstock which hampers the use of cheap feedstocks for constant quality of enzyme cocktails. It is therefore of industrial interest to produce specific enzyme cocktails constitutively and independently of inducers. In this review, we focus on the methods to modulate TF activities for inducer-independent production of CAZymes and highlight various approaches that are used to construct strains displaying constitutive expression of plant biomass degrading enzyme networks. These approaches and combinations thereof are also used to construct strains displaying increased expression of CAZymes under inducing conditions, and make it possible to design strains in which different enzyme mixtures are simultaneously produced independently of the carbon source.

Carbon Catabolite Repression in Filamentous Fungi

Carbon Catabolite Repression (CCR) has fascinated scientists and researchers around the globe for the past few decades. This important mechanism allows preferential utilization of an energy-efficient and readily available carbon source over relatively less easily accessible carbon sources. This mechanism helps microorganisms to obtain maximum amount of glucose in order to keep pace with their metabolism. Microorganisms assimilate glucose and highly favorable sugars before switching to less-favored sources of carbon such as organic acids and alcohols. In CCR of filamentous fungi, CreA acts as a transcription factor, which is regulated to some extent by ubiquitination. CreD-HulA ubiquitination ligase complex helps in CreA ubiquitination, while CreB-CreC deubiquitination (DUB) complex removes ubiquitin from CreA, which causes its activation. CCR of fungi also involves some very crucial elements such as Hexokinases, cAMP, Protein Kinase (PKA), Ras proteins, G protein-coupled receptor (GPCR), Adenylate cyclase, RcoA and SnfA. Thorough study of molecular mechanism of CCR is important for understanding growth, conidiation, virulence and survival of filamentous fungi. This review is a comprehensive revision of the regulation of CCR in filamentous fungi as well as an updated summary of key regulators, regulation of different CCR-dependent mechanisms and its impact on various physical characteristics of filamentous fungi.

Improved α-Amylase Production by Dephosphorylation Mutation of CreD, an Arrestin-Like Protein Required for Glucose-Induced Endocytosis of Maltose Permease and Carbon Catabolite Derepression in Aspergillus oryzae

Aspergillusoryzae produces copious amount of amylolytic enzymes, and MalP, a major maltose permease, is required for the expression of amylase-encoding genes. The expression of these genes is strongly repressed by carbon catabolite repression (CCR) in the presence of glucose. MalP is transported from the plasma membrane to the vacuole by endocytosis, which requires the homolog of E6-AP carboxyl terminus ubiquitin ligase HulA, an ortholog of yeast Rsp5. In yeast, arrestin-like proteins mediate endocytosis as adaptors of Rsp5 and transporters. In the present study, we examined the involvement of CreD, an arrestin-like protein, in glucose-induced MalP endocytosis and CCR of amylase-encoding genes. Deletion of creD inhibited the glucose-induced endocytosis of MalP, and CreD showed physical interaction with HulA. Phosphorylation of CreD was detected by Western blotting, and two serine residues were determined as the putative phosphorylation sites. However, the phosphorylation state of the serine residues did not regulate MalP endocytosis and its interaction with HulA. Although α-amylase production was significantly repressed by creD deletion, both phosphorylation and dephosphorylation mimics of CreD had a negligible effect on α-amylase activity. Interestingly, dephosphorylation of CreD was required for CCR relief of amylase genes that was triggered by disruption of the deubiquitinating enzyme-encoding gene creB The α-amylase activity of the creB mutant was 1.6-fold higher than that of the wild type, and the dephosphorylation mimic of CreD further improved the α-amylase activity by 2.6-fold. These results indicate that a combination of the dephosphorylation mutation of CreD and creB disruption increased the production of amylolytic enzymes in A. oryzaeIMPORTANCE In eukaryotes, glucose induces carbon catabolite repression (CCR) and proteolytic degradation of plasma membrane transporters via endocytosis. Glucose-induced endocytosis of transporters is mediated by their ubiquitination, and arrestin-like proteins act as adaptors of transporters and ubiquitin ligases. In this study, we showed that CreD, an arrestin-like protein, is involved in glucose-induced endocytosis of maltose permease and carbon catabolite derepression of amylase gene expression in Aspergillusoryzae Dephosphorylation of CreD was required for CCR relief triggered by the disruption of creB, which encodes a deubiquitinating enzyme; a combination of the phosphorylation-defective mutation of CreD and creB disruption dramatically improved α-amylase production. This study shows the dual function of an arrestin-like protein and provides a novel approach for improving the production of amylolytic enzymes in A. oryzae.

Cell wall α-1,3-glucan prevents α-amylase adsorption onto fungal cell in submerged culture of Aspergillus oryzae

We have previously reported that α-amylase (Taka-amylase A, TAA) activity disappears in the later stage of submerged Aspergillus oryzae culture as a result of TAA adsorption onto the cell wall. Chitin, one of the major components of the cell wall, was identified as a potential factor that facilitates TAA adsorption. However, TAA adsorption only occurred in the later stage of cultivation, although chitin was assumed to be sufficiently abundant in the cell wall regardless of the submerged culture period. This suggested the presence a factor that inhibits TAA adsorption to the cell wall in the early stage of cultivation. In the current study, we identified α-1,3-glucan as a potential inhibiting factor for TAA adsorption. We constructed single, double, and triple disruption mutants of three α-1,3-glucan synthase genes (agsA, agsB, and agsC) in A. oryzae. Growth characteristics and cell wall component analysis of these disruption strains showed that AgsB plays a major role in α-1,3-glucan synthesis. In the ΔagsB mutant, TAA was adsorbed onto the mycelium in all stages of cultivation (early and later), and the ΔagsB mutant cell walls had a significantly high capacity for TAA adsorption. Moreover, the α-1,3-glucan content of the cell wall prepared from the wild-type strain in the later stage of cultivation was markedly reduced compared with that in the early stage. These results suggest that α-1,3-glucan is a potential inhibiting factor for TAA adsorption onto the cell wall component, chitin, in the early stage of submerged culture in A. oryzae.

The CreB deubiquitinating enzyme does not directly target the CreA repressor protein in Aspergillus nidulans

Ubiquitination/deubiquitination pathways are now recognized as key components of gene regulatory mechanisms in eukaryotes. The major transcriptional repressor for carbon catabolite repression in Aspergillus nidulans is CreA, and mutational analysis led to the suggestion that a regulatory ubiquitination/deubiquitination pathway is involved. A key unanswered question is if and how this pathway, comprising CreB (deubiquitinating enzyme) and HulA (ubiquitin ligase) and other proteins, is involved in the regulatory mechanism. Previously, missense alleles of creA and creB were analysed for genetic interactions, and here we extended this to complete loss-of-function alleles of creA and creB, and compared morphological and biochemical phenotypes, which confirmed genetic interaction between the genes. We investigated whether CreA, or a protein in a complex with it, is a direct target of the CreB deubiquitination enzyme, using co-purifications of CreA and CreB, first using strains that overexpress the proteins and then using strains that express the proteins from their native promoters. The Phos-tag system was used to show that CreA is a phosphorylated protein, but no ubiquitination was detected using anti-ubiquitin antibodies and Western analysis. These findings were confirmed using mass spectrometry, which confirmed that CreA was differentially phosphorylated but not ubiquitinated. Thus, CreA is not a direct target of CreB, and nor are proteins that form part of a stable complex with CreA a target of CreB. These results open up new questions regarding the molecular mechanism of CreA repressing activity, and how the ubiquitination pathway involving CreB interacts with this regulatory network.

The C2H2-type transcription factor, FlbC, is involved in the transcriptional regulation of Aspergillus oryzae glucoamylase and protease genes specifically expressed in solid-state culture

Aspergillus oryzae produces a large amount of secreted proteins in solid-state culture, and some proteins such as glucoamylase (GlaB) and acid protease (PepA) are specifically produced in solid-state culture, but rarely in submerged culture. From the disruption mutant library of A. oryzae transcriptional regulators, we successfully identified a disruption mutant showing an extremely low production level of GlaB but a normal level of α-amylase production. This strain was a disruption mutant of the C2H2-type transcription factor, FlbC, which is reported to be involved in the regulation of conidiospore development. Disruption mutants of other upstream regulators comprising a conidiation regulatory network had no apparent effect on GlaB production in solid-state culture. In addition to GlaB, the production of acid protease in solid-state culture was also markedly decreased by flbC disruption. Northern blot analyses revealed that transcripts of glaB and pepA were significantly decreased in the flbC disruption strain. These results suggested that FlbC is involved in the transcriptional regulation of genes specifically expressed under solid-state cultivation conditions, possibly independent of the conidiation regulatory network.

Endocytosis of a maltose permease is induced when amylolytic enzyme production is repressed in Aspergillus oryzae

In the filamentous fungus Aspergillus oryzae, amylolytic enzyme production is induced by the presence of maltose. Previously, we identified a putative maltose permease (MalP) gene in the maltose-utilizing cluster of A. oryzae. malP disruption causes a significant decrease in α-amylase activity and maltose consumption, indicating that MalP is a maltose transporter required for amylolytic enzyme production in A. oryzae. Although the expression of amylase genes and malP is repressed by the presence of glucose, the effect of glucose on the abundance of functional MalP is unknown. In this study, we examined the effect of glucose and other carbon sources on the subcellular localization of green fluorescence protein (GFP)-tagged MalP. After glucose addition, GFP-MalP at the plasma membrane was internalized and delivered to the vacuole. This glucose-induced internalization of GFP-MalP was inhibited by treatment with latrunculin B, an inhibitor of actin polymerization. Furthermore, GFP-MalP internalization was inhibited by repressing the HECT ubiquitin ligase HulA (ortholog of yeast Rsp5). These results suggest that MalP is transported to the vacuole by endocytosis in the presence of glucose. Besides glucose, mannose and 2-deoxyglucose also induced the endocytosis of GFP-MalP and amylolytic enzyme production was inhibited by the addition of these sugars. However, neither the subcellular localization of GFP-MalP nor amylolytic enzyme production was influenced by the addition of xylose or 3-O-methylglucose. These results imply that MalP endocytosis is induced when amylolytic enzyme production is repressed.

Distinct mechanism of activation of two transcription factors, AmyR and MalR, involved in amylolytic enzyme production in Aspergillus oryzae

The production of amylolytic enzymes in Aspergillus oryzae is induced in the presence of starch or maltose, and two Zn2Cys6-type transcription factors, AmyR and MalR, are involved in this regulation. AmyR directly regulates the expression of amylase genes, and MalR controls the expression of maltose-utilizing (MAL) cluster genes. Deletion of malR gene resulted in poor growth on starch medium and reduction in α-amylase production level. To elucidate the activation mechanisms of these two transcription factors in amylase production, the expression profiles of amylases and MAL cluster genes under carbon catabolite derepression condition and subcellular localization of these transcription factors fused with a green fluorescent protein (GFP) were examined. Glucose, maltose, and isomaltose induced the expression of amylase genes, and GFP-AmyR was translocated from the cytoplasm to nucleus after the addition of these sugars. Rapid induction of amylase gene expression and nuclear localization of GFP-AmyR by isomaltose suggested that this sugar was the strongest inducer for AmyR activation. In contrast, GFP-MalR was constitutively localized in the nucleus and the expression of MAL cluster genes was induced by maltose, but not by glucose or isomaltose. In the presence of maltose, the expression of amylase genes was preceded by MAL cluster gene expression. Furthermore, deletion of the malR gene resulted in a significant decrease in the α-amylase activity induced by maltose, but had apparently no effect on the expression of α-amylase genes in the presence of isomaltose. These results suggested that activation of AmyR and MalR is regulated in a different manner, and the preceding activation of MalR is essential for the utilization of maltose as an inducer for AmyR activation.

L-lactic acid production from starch by simultaneous saccharification and fermentation in a genetically engineered Aspergillus oryzae pure culture

Lactic acid is a commodity chemical that can be produced biologically. Lactic acid-producing Aspergillus oryzae strains were constructed by genetic engineering. The A. oryzae LDH strain with the bovine L-lactate dehydrogenase gene produced 38 g/L of lactate from 100g/L of glucose. Disruption of the wild-type lactate dehydrogenase gene in A. oryzae LDH improved lactate production. The resulting strain A. oryzae LDHΔ871 produced 49 g/L of lactate from 100g/L of glucose. Because A. oryzae strains innately secrete amylases, A. oryzae LDHΔ871 produced approximately 30 g/L of lactate from various starches, dextrin, or maltose (all at 100 g/L). To our knowledge, this is the first report describing the simultaneous saccharification and fermentation of lactate from starch using a pure culture of transgenic A. oryzae. Our results indicate that A. oryzae could be a promising host for the bioproduction of useful compounds such as lactic acid.