Improving heterologous gene expression in Aspergillus niger by introducing multiple copies of protein-binding sequence containing CCAAT to the promoter

Enhanced expression of xylanase in Aspergillus niger enabling a two-step enzymatic pathway for extracting β-glucan from oat bran

The high cost and process complexity limit the enzymatic extraction of β-glucan. In this study, β-glucan was extracted from oat bran in a two-step enzymatic pathway using a recombinant strain of Aspergillus niger AG11 overexpressing the endogenous xylanase (xynA) and amylolytic enzyme. First, co-optimization of promoter and signal peptide and a fusion of glucoamylase (glaA) fragment were integrated into the β-glucosidase (bgl) locus to improve xynA expression. Then, the optimized expression cassette was simultaneously integrated into bgl, α-amylase amyA, and acid α-amylase ammA loci, yielding the Rbya with 3,650-fold and 31.2% increase in xynA and amylolytic enzyme activity than the wild-type strain, respectively. Finally, Rbya's supernatants at 72 h (rich in xynA and amylolytic enzyme) and 10 d (rich in proteases) were used to decompose xylan/starch and proteins in oat bran, respectively, to obtain 85.1% pure β-glucan. Rbya could be a robust candidate for the cost-effective extraction of β-glucan.

CRISPR/Cas Genome Editing in Filamentous Fungi

The review describes the CRISPR/CAS system and its adaptation for the genome editing in filamentous fungi commonly used for production of enzyme complexes, enzymes, secondary metabolites, and other compounds used in industrial biotechnology and agriculture. In the second part of this review, examples of the CRISPR/CAS technology application for improving properties of the industrial strains of fungi from the Trichoderma, Aspergillus, Penicillium, and other genera are presented. Particular attention is given to the efficiency of genome editing, as well as system optimization for specific industrial producers.

Aspergillus: A Powerful Protein Production Platform

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

Developing Aspergillus niger as a cell factory for food enzyme production

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

Engineering the cbh1 Promoter of Trichoderma reesei for Enhanced Protein Production by Replacing the Binding Sites of a Transcription Repressor ACE1 to Those of the Activators

The strong and inducible cbh1 promoter is most widely used to express heterologous proteins, useful in food and feed industries, in Trichoderma reesei. Enhancing its ability to direct transcription provides a general strategy to improve protein production in T. reesei. The cbh1 promoter was engineered by replacing eight binding sites of the transcription repressor ACE1 to those of the activators ACE2, Hap2/3/5, and Xyr1. While changing ACE1 to Hap2/3/5-binding sites completely abolished the transcription ability, replacements with ACE2- and Xyr1-binding sites (designated cbh1pA and cbh1pX promoters, respectively) largely improved the promoter transcription efficiency, as reflected by expression of a reporter gene DsRed. The cbh1pA and cbh1pX promoters were applied to improve secretory expression of a codon-optimized mannanase from Aspergillus niger to 3.6- and 5.0-fold higher, respectively, which has high application potential in feed industry.

Advances in targeting and heterologous expression of genes involved in the synthesis of fungal secondary metabolites

Gene targeting involves integration of foreign DNA into the fungal genome by several strategies including Agrobacterium tumefaciens-mediated transformation (ATMT).

Heterologous gene expression in filamentous fungi

Filamentous fungi are critical to production of many commercial enzymes and organic compounds. Fungal-based systems have several advantages over bacterial-based systems for protein production because high-level secretion of enzymes is a common trait of their decomposer lifestyle. Furthermore, in the large-scale production of recombinant proteins of eukaryotic origin, the filamentous fungi become the vehicle of choice due to critical processes shared in gene expression with other eukaryotic organisms. The complexity and relative dearth of understanding of the physiology of filamentous fungi, compared to bacteria, have hindered rapid development of these organisms as highly efficient factories for the production of heterologous proteins. In this review, we highlight several of the known benefits and challenges in using filamentous fungi (particularly Aspergillus spp., Trichoderma reesei, and Neurospora crassa) for the production of proteins, especially heterologous, nonfungal enzymes. We review various techniques commonly employed in recombinant protein production in the filamentous fungi, including transformation methods, selection of gene regulatory elements such as promoters, protein secretion factors such as the signal peptide, and optimization of coding sequence. We provide insights into current models of host genomic defenses such as repeat-induced point mutation and quelling. Furthermore, we examine the regulatory effects of transcript sequences, including introns and untranslated regions, pre-mRNA (messenger RNA) processing, transcript transport, and mRNA stability. We anticipate that this review will become a resource for researchers who aim at advancing the use of these fascinating organisms as protein production factories, for both academic and industrial purposes, and also for scientists with general interest in the biology of the filamentous fungi.

A new high-performance heterologous fungal expression system based on regulatory elements from the Aspergillus terreus terrein gene cluster

Recently, the Aspergillus terreus terrein gene cluster was identified and selected for development of a new heterologous expression system. The cluster encodes the specific transcription factor TerR that is indispensable for terrein cluster induction. To identify TerR binding sites, different recombinant versions of the TerR DNA-binding domain were analyzed for specific motif recognition. The high affinity consensus motif TCGGHHWYHCGGH was identified from genes required for terrein production and binding site mutations confirmed their essential contribution to gene expression in A. terreus. A combination of TerR with its terA target promoter was tested as recombinant expression system in the heterologous host Aspergillus niger. TerR mediated target promoter activation was directly dependent on its transcription level. Therefore, terR was expressed under control of the regulatable amylase promoter PamyB and the resulting activation of the terA target promoter was compared with activation levels obtained from direct expression of reporters from the strong gpdA control promoter. Here, the coupled system outcompeted the direct expression system. When the coupled system was used for heterologous polyketide synthase expression high metabolite levels were produced. Additionally, expression of the Aspergillus nidulans polyketide synthase gene orsA revealed lecanoric acid rather than orsellinic acid as major polyketide synthase product. Domain swapping experiments assigned this depside formation from orsellinic acid to the OrsA thioesterase domain. These experiments confirm the suitability of the expression system especially for high-level metabolite production in heterologous hosts.

Characterization of the 5' flanking region of lipase gene from Penicillium expansum and its application in molecular breeding

A major challenge for further promotion of lipase productivity in Penicillium expansum PE-12 is to find a suitable promoter that can function efficiently in this industrial strain. In this study, the 5' flanking region of P. expansum lipase (Ppel) containing a putative novel promoter sequence was characterized by fusing to β-glucuronidase (GUS) and subsequently introducing into P. expansum. As a result, all the transformants showed blue color quickly after incubation in GUS detection buffer, suggesting a strong promoter activity of this fragment. Glucose repression was identified for the promoter, whereas olive oil acted as a positive regulator. Facilitated by this novel promoter, P. expansum PE-12 was genetically modified, with an improved lipase yield, via a recombinant plasmid with P. expansum lipase gene (PEL) under the control of Ppel promoter and TtrpC terminator. The highest lipase yield among the modified strains could attain 2,100 U/mL, which is more than twofold of the previous industrial strain (900 U/mL). The engineered strain through molecular breeding method as well as this new promoter has great value in lipase industry.

Improved heterologous gene expression in Trichoderma reesei by cellobiohydrolase I gene (cbh1) promoter optimization

To improve heterologous gene expression in Trichoderma reesei, a set of optimal artificial cellobiohydrolase I gene (cbh1) promoters was obtained. The region from -677 to -724 with three potential glucose repressor binding sites was deleted. Then the region from -620 to -820 of the modified cbh1 promoter, including the CCAAT box and the Ace2 binding site, was repeatedly inserted into the modified cbh1 promoter, obtaining promoters with copy numbers 2, 4, and 6. The results showed that the glucose repression effects were abolished and the expression level of the glucuronidase (gus) reporter gene regulated by these multi-copy promoters was markedly enhanced as the copy number increased simultaneously. The data showed the great promise of using the promoter artificial modification strategy to increase heterologous gene expression in filamentous fungi and provided a set of optional high-expression vectors for gene function investigation and strain modification.