Identification of elevated transcripts in a Trichoderma reesei strain expressing a chimeric transcription activator using suppression subtractive hybridization

Regulating Strategies for Producing Carbohydrate Active Enzymes by Filamentous Fungal Cell Factories

Filamentous fungi are important eukaryotic organisms crucial in substrate degradation and carbon cycle on the earth and have been harnessed as cell factories for the production of proteins and other high value-added products in recent decades. As cell factories, filamentous fungi play a crucial role in industrial protein production as both native hosts and heterologous hosts. In this review, the regulation strategies of carbohydrate active enzyme expression at both transcription level and protein level are introduced, and the transcription regulations are highlighted with induction mechanism, signaling pathway, and promoter and transcription factor regulation. Afterward, the regulation strategies in protein level including suitable posttranslational modification, protein secretion enhancement, and protease reduction are also presented. Finally, the challenges and perspectives in this field are discussed. In this way, a comprehensive knowledge regarding carbohydrate active enzyme production regulation at both transcriptional and protein levels is provided with the particular goal of aiding in the practical application of filamentous fungi for industrial protein production.

Engineering of filamentous fungi for efficient conversion of lignocellulose: Tools, recent advances and prospects

Filamentous fungi, as the main producers of lignocellulolytic enzymes in industry, need to be engineered to improve the economy of large-scale lignocellulose conversion. Investigation of the cellular processes involved in lignocellulolytic enzyme production, as well as optimization of enzyme mixtures for higher hydrolysis efficiency, have provided effective targets for the engineering of lignocellulolytic fungi. Recently, the development of efficient genetic manipulation systems in several lignocellulolytic fungi opens up the possibility of systems engineering of these strains. Here, we review the recent progresses made in the engineering of lignocellulolytic fungi and highlight the research gaps in this area.

Metabolic Engineering of Fungal Strains for Efficient Production of Cellulolytic Enzymes

Filamentous fungi are widely used for production of cellulase and other cellulolytic enzymes. Metabolic engineering of filamentous fungal strains has been applied to improve enzyme production, and rapid progress has been made in the recent years. In this chapter, genetic tools and methods to develop superior enzyme producers are summarized, which includes establishment of genetic modification systems, selection and redesign of promoters, and metabolic engineering using either native transcription factors or artificial ones. In addition, enhancement of cellulase production through morphology engineering was also discussed. Emerging tools including CRISPR-Cas9-based genome editing and synthetic biology are highlighted, which are speeding up mechanisms elucidation and strain development, and will further facilitate economic cellulolytic enzyme production.

Construction of enhanced transcriptional activators for improving cellulase production in Trichoderma reesei RUT C30

Enhancing cellulase production in Trichoderma reesei is of great interest for an economical biorefinery. Artificial transcription factors are a potentially powerful molecular strategy for improving cellulase production in T. reesei. In this study, enhanced transcriptional activators XYR1VP, ACE2VP, and ACE1VP were constructed by linking the C terminus of XYR1, ACE2, or ACE1 with an activation domain of herpes simplex virus protein VP16. T. reesei transformants T_XYR1VP, T_ACE2VP, and T_ACE1VP showed improved cellulase and/or xylanase production. T_XYR1VP has a cellulase-free phenotype but with significantly elevated xylanase production. Xylanase I and xylanase II activities [U/(mg biomass)] increased by 51% and 80%, respectively, in T_XYR1VP in comparison with parental strain RUT C30. The filter paper activity of T_ACE2VP in the Avicel-based medium increased by 52% compared to that of RUT C30. In the Avicel-based medium, T_ACE1VP manifested an 80% increase in FPase activity and a 50% increase in xylanase activity as compared to those of RUT C30. Additionally, when pretreated corn stover was hydrolyzed, crude enzymes produced from T_ACE1VP yielded a greater glucose release than did the enzymes produced by parental strain RUT C30.

Enhanced cellulase production in Trichoderma reesei RUT C30 via constitution of minimal transcriptional activators

Background

Cellulase can convert lignocellulosic feedstocks into fermentable sugars, which can be used for the industrial production of biofuels and chemicals. The high cost of cellulase production remains a challenge for lignocellulose breakdown. Trichoderma reesei RUT C30 serves as a well-known industrial workhorse for cellulase production. Therefore, the enhancement of cellulase production by T. reesei RUT C30 is of great importance.

Results

Two sets of novel minimal transcriptional activators (DBD_ace2-VP16 and DBD_cre1-VP16) were designed and expressed in T. reesei RUT C30. Expression of DBD_ace2-VP16 and DBD_cre1-VP16 improved cellulase production under induction (avicel or lactose) and repression (glucose) conditions, respectively. The strain T_MTA66 under avicel and T_MTA139 under glucose with the highest cellulase activities outperformed other transformants and the parental strain under the corresponding conditions. For T_MTA66 strains, the highest FPase activity was approximately 1.3-fold greater than that of the parental strain RUT C30 at 120 h of cultivation in a shake flask using avicel as the sole carbon source. The FPase activity (U/mg biomass) in T_MTA139 strains was approximately 26.5-fold higher than that of the parental strain RUT C30 at 72 h of cultivation in a shake flask using glucose as the sole carbon source. Furthermore, the crude enzymes produced in the 7-L fermenter from T_MTA66 and T_MTA139 supplemented with commercial β-glucosidase hydrolyzed pretreated corn stover effectively.

Conclusions

These results show that replacing natural transcription factors with minimal transcriptional activators is a powerful strategy to enhance cellulase production in T. reesei. Our current study also offers an alternative genetic engineering strategy for the enhanced production of industrial products by other fungi.

Electronic supplementary material

The online version of this article (10.1186/s12934-018-0926-7) contains supplementary material, which is available to authorized users.

Constitutive cellulase production from glucose using the recombinant Trichoderma reesei strain overexpressing an artificial transcription activator

The high cost of cellulase production presents biggest challenge in biomass deconstruction. Cellulase production by Trichoderma reesei using low cost carbon source is of great interest. In this study, an artificial transcription activator containing the Cre1 binding domain linked to the Xyr1 effector and binding domains was designed and constitutively overexpressed in T. reesei RUT C30. The recombinant strain T. reesei zxy-2 displayed constitutive cellulase production using glucose as a sole carbon source, and the production titer was 12.75-fold of that observed with T. reesei RUT C30 in shake flask culture. Moreover, FPase and xylanase titers of 2.63 and 108.72IU/mL, respectively, were achieved using glucose as sole carbon source within 48h in a 7-L fermenter by batch fermentation using T. reesei zxy-2. The crude enzyme obtained was used to hydrolyze alkali pretreated corn stover, and a high glucose yield of 99.18% was achieved.

Development of highly efficient, low-cost lignocellulolytic enzyme systems in the post-genomic era

The current high cost of lignocellulolytic enzymes is a major bottleneck in the economic bioconversion of lignocellulosic biomass to fuels and chemicals. Fungal lignocellulolytic enzyme systems are secreted at high levels, making them the most promising starting points for further development of highly efficient lignocellulolytic enzyme systems. In this paper, recent advances in improvement of fungal lignocellulolytic enzyme systems are reviewed, with an emphasis on the achievements made using genomic approaches. A general strategy for lignocellulolytic enzyme system development is proposed, including the improvement of the hydrolysis efficiencies and productivities of current enzyme systems. The applications of genomic, transcriptomic and proteomic analysis methods in examining the composition of native enzyme systems, discovery of novel enzymes and synergistic proteins from natural sources, and understanding of regulatory mechanisms for lignocellulolytic enzyme biosynthesis are summarized. By combining systems biology and synthetic biology tools, engineered fungal strains are expected to produce high levels of optimized lignocellulolytic enzyme systems.