Redesigning the Aspergillus nidulans xylanase regulatory pathway to enhance cellulase production with xylose as the carbon and inducer source

Discovery of a novel translation-machinery-associated protein that positively correlates with cellulase production

BACKGROUND

The production of cellulases by filamentous fungi is a crucial aspect of sustainable bioproduction from renewable lignocellulosic biomass. Following the transcription of cellulase genes in the nucleus, a complex pathway involving translation, folding, and secretion is required to produce extracellular cellulases. Most studies about cellulase production have focused on examining transcriptional regulatory mechanisms and enhancement of enzyme gene levels; comparatively, little is known about protein translation and secretion for cellulase production.

RESULTS

A translation-machinery-associated (TMA) protein PoTma15 was identified in cellulosic Penicillium oxalicum. The PoTma15 is conserved in various filamentous fungi, but not in yeast, plants, or animals. All homologous proteins of PoTma15 have previously been uncharacterized. PoTma15 was initially thought to be one of the putative interactors of transcription factor PoXlnR, as it was preyed by tandem affinity purification (TAP) coupled with the mass spectrometry (TAP-MS) technique using PoXlnR as the bait. Subsequent research revealed that PoTma15 is associated with the translation machinery. The top three proteins associated with PoTma15 are orthologs of Saccharomyces cerevisiae translation-machinery-associated protein (Tma19), translation elongation factor eIF5A, and ribosomal protein S28, respectively. PoTma15 is widely distributed in fungal hyphae and positively correlates with the production of cellulases and extracellular proteins. Deleting the Potma15 gene (Δtma15) decreased cellulase production, while overexpressing the Potma15 gene (OEtma15) increased cellulase production. However, the Δtma15 mutant was not observed to have downregulated transcript levels of major (hemi)cellulase and amylase genes, compared to the P. oxalicum wild type (WT). The production of extracellular cellulases and extracellular proteins of the Δtma15 mutant was less affected by cycloheximide, an inhibitor of eukaryotic translation elongation, compared to the WT strain and OEtma15 mutant, suggesting a stronger resistance to the translation-inhibiting effects of cycloheximide in the Δtma15 mutant. The results demonstrate that PoTma15 is a translation-machinery-associated protein that affects translation elongation and, consequently, the production of enzyme proteins.

CONCLUSIONS

PoTma15 is the first TMA protein characterized in cellulosic filamentous fungi and the first TMA protein used in fungi to increase cellulase production. PoTma15's role in the production of cellulases and total extracellular proteins suggests that not only can it be used to widen the cellulase production pathway, but can even be engineered as a target to improve the production of other heterologous protein or bioproducts using filamentous fungi as cell factories in the future.

An overview on the current status and future prospects in Aspergillus cellulase production

Cellulase is a new research point besides glucoamylase, amylase, and protease in the enzyme industry. Cellulase can decompose lignocellulosic biomass into small-molecule sugars, which facilitates microbial utilization; thus, it has a vast market potential in the field of feed, food, energy, and chemistry. The Aspergillus was the first strain used in cellulase preparation because of its safety and non-toxicity, strong growth ability, and high enzyme yield. This review provides the latest research and advances on preparing cellulase from Aspergillus. The metabolic mechanisms of cellulase secretion by Aspergillus, the selection of fermentation substrates, the comparison of the fermentation modes, and the effect of fermentation conditions have been discussed in this review. Also, the subsequent separation and purification techniques of Aspergillus cellulase, including salting out, organic solvent precipitation, ultrafiltration, and chromatography, have been declared. Further, bottlenecks in Aspergillus cellulase preparation and corresponding feasible approaches, such as genetic engineering, mixed culture, and cellulase immobilization, have also been proposed in this review. This paper provides theoretical support for the efficient production and application of Aspergillus cellulase.

Production of L-Malic Acid by Metabolically Engineered Aspergillus nidulans Based on Efficient CRISPR-Cas9 and Cre-loxP Systems

Aspergillus nidulans has been more extensively characterized than other Aspergillus species considering its morphology, physiology, metabolic pathways, and genetic regulation. As it has a rapid growth rate accompanied by simple nutritional requirements and a high tolerance to extreme cultural conditions, A. nidulans is a promising microbial cell factory to biosynthesize various products in industry. However, it remains unclear for whether it is also a suitable host for synthesizing abundant L-malic acid. In this study, we developed a convenient and efficient double-gene-editing system in A. nidulans strain TN02A7 based on the CRISPR-Cas9 and Cre-loxP systems. Using this gene-editing system, we made a L-malic acid-producing strain, ZQ07, derived from TN02A7, by deleting or overexpressing five genes (encoding Pyc, pyruvate carboxylase; OahA, oxaloacetate acetylhydrolase; MdhC, malate dehydrogenase; DctA, C4-dicarboxylic acid transporter; and CexA, citric acid transporter). The L-malic acid yield in ZQ07 increased to approximately 9.6 times higher (up to 30.7 g/L titer) than that of the original unedited strain TN02A7, in which the production of L-malic acid was originally very low. The findings in this study not only demonstrate that A. nidulans could be used as a potential host for biosynthesizing organic acids, but also provide a highly efficient gene-editing strategy in filamentous fungi.

Engineering interventions in industrial filamentous fungal cell factories for biomass valorization

Filamentous fungi possess versatile capabilities for synthesizing a variety of valuable bio compounds, including enzymes, organic acids and small molecule secondary metabolites. The advancements of genetic and metabolic engineering techniques and the availability of sequenced genomes discovered their potential as expression hosts for recombinant protein production. Remarkably, plant-biomass degrading filamentous fungi show the unique capability to decompose lignocellulose, an extremely recalcitrant biopolymer. The basic biochemical approaches have motivated several industrial processes for lignocellulose biomass valorisation into fermentable sugars and other biochemical for biofuels, biomolecules, and biomaterials. The review gives insight into current trends in engineering filamentous fungi for enzymes, fuels, and chemicals from lignocellulose biomass. This review describes the variety of enzymes and compounds that filamentous fungi produce, engineering of filamentous fungi for biomass valorisation with a special focus on lignocellulolytic enzymes and other bulk chemicals.

Artificial AmyR::XlnR transcription factor induces α-amylase production in response to non-edible xylan-containing hemicellulosic biomass

Filamentous fungi belonging to the Aspergillus genus are one of the most favored microorganisms for industrial enzyme production because they can secrete large amounts of proteins into the culture medium. α-Amylase, an enzyme produced by Aspergillus species, is important for food and industrial applications. The production of α-amylase is induced by starch, mainly obtained from the edible biomass; however, the increasing demand for foods is limiting the application of the latter. Therefore, it is expected that using the non-edible biomass, such as rice straw, could improve the competition for industrial application starch containing resources. The transcription factor AmyR activates the transcription of amylolytic enzyme genes, while the transcription factor XlnR activates the transcription of xylanolytic enzyme genes in response to xylose. In this study, we aimed to construct an artificial AmyR::XlnR transcription factor (AXTF) by replacing the DNA-binding domain (1-159 amino acids) of XlnR with that (1-68 aa) of AmyR, which is capable of inducing amylolytic enzyme production in response to xylan-containing hemicellulosic biomass. The chimeric transcription factor AXTF was constructed and expressed using the gapA promoter in the amyR-deficient mutant strain SA1. When the AXTF strain was cultured in the minimal medium containing xylose as the carbon source, the amyB, amyF, agdB, and agdE transcription levels were 41.1-, 11.3-, 37.9-, and 23.7-fold higher, respectively, than those of the wild-type strain. The α-amylase and α-glucosidase activities in the culture supernatant of the AXTF strain grown with xylose for 48 h were 696.6 and 536.1 U/mL, respectively, while these activities were not detected in the culture supernatant of the wild-type and SA1 strains. When rice straw hydrolysate was used as a carbon source, the α-amylase and α-glucosidase activities were 590.2 and 362.7 U/mL, respectively. Thus, we successfully generated an Aspergillus nidulans strain showing amylolytic enzyme production in response to non-edible xylan-containing hemicellulosic biomass by transforming it with the chimeric transcription factor AXTF. Furthermore, the use of genes encoding engineered transcription factors is advantageous because introducing such genes into an industrial Aspergillus strain has similar simultaneous effects on multiple amylase genes controlled by AmyR.