A Highly Efficient Xylan-Utilization System in Aspergillus niger An76: A Functional-Proteomics Study

Genomic characterization and expression profiling of the lytic polysaccharide monooxygenases AA9 family in thermophilic fungi Thermothelomyces fergusii in response to carbon source media

Enhancing enzyme activity and stability in biomass degradation can improve substrate saccharification and, increases biorefinery efficiency. For the first time, we identified 20 lytic polysaccharide monooxygenases (LPMOs) AA9 genes in the genome of Thermothelomyces fergusii. Our results showed that TfAA9 was categorized into LPMOs1, LPMOs2, and LPMOs3 subgroups based on protein diversity. Protein- 3D structure analysis showed strong interactions between Myceliophthora thermophila AA9 proteins and 17 TfAA9 proteins. Gene ontology analysis indicated a high enrichment of cellulase activity in TfAA9 genes. KEGG pathways analysis revealed the role of TfAA9 proteins in the endohydrolysis of 1,4-beta-D-glucosidic linkages in cellulose. Numerous TfAA9s gene transcripts were up-regulated on avicel, cellobiose, and glucose, with a higher proportion on avicel. Protein concentration, endoglucanase, and cellulase activity were also boosted on avicel. However, limited fungal biomass was observed on avicel, despite the abundance of AA9 LPMOs in the T. fergusii genome. These findings expand our understanding of fungal AA9 genes and their role in lignocellulolytic degradation. The disparity between biomass and enzymatic activity suggests screening TfAA9 genes for highly active enzymes and redundant genes via heterologous expression. In short, functional characterization of these genes could contribute to improving the saccharification process of industrial raw materials.

Engineering Saccharomyces cerevisiae for targeted hydrolysis and fermentation of glucuronoxylan through CRISPR/Cas9 genome editing

BACKGROUND

The abundance of glucuronoxylan (GX) in agricultural and forestry residual side streams positions it as a promising feedstock for microbial conversion into valuable compounds. By engineering strains of the widely employed cell factory Saccharomyces cerevisiae with the ability to directly hydrolyze and ferment GX polymers, we can avoid the need for harsh chemical pretreatments and costly enzymatic hydrolysis steps prior to fermentation. However, for an economically viable bioproduction process, the engineered strains must efficiently express and secrete enzymes that act in synergy to hydrolyze the targeted polymers.

RESULTS

The aim of this study was to equip the xylose-fermenting S. cerevisiae strain CEN.PK XXX with xylanolytic enzymes targeting beechwood GX. Using a targeted enzyme approach, we matched hydrolytic enzyme activities to the chemical features of the GX substrate and determined that besides endo-1,4-β-xylanase and β-xylosidase activities, α-methyl-glucuronidase activity was of great importance for GX hydrolysis and yeast growth. We also created a library of strains expressing different combinations of enzymes, and screened for yeast strains that could express and secrete the enzymes and metabolize the GX hydrolysis products efficiently. While strains engineered with BmXyn11A xylanase and XylA β-xylosidase could grow relatively well in beechwood GX, strains further engineered with Agu115 α-methyl-glucuronidase did not display an additional growth benefit, likely due to inefficient expression and secretion of this enzyme. Co-cultures of strains expressing complementary enzymes as well as external enzyme supplementation boosted yeast growth and ethanol fermentation of GX, and ethanol titers reached a maximum of 1.33 g L- 1 after 48 h under oxygen limited condition in bioreactor fermentations.

CONCLUSION

This work underscored the importance of identifying an optimal enzyme combination for successful engineering of S. cerevisiae strains that can hydrolyze and assimilate GX. The enzymes must exhibit high and balanced activities, be compatible with the yeast's expression and secretion system, and the nature of the hydrolysis products must be such that they can be taken up and metabolized by the yeast. The engineered strains, particularly when co-cultivated, display robust growth and fermentation of GX, and represent a significant step forward towards a sustainable and cost-effective bioprocessing of GX-rich biomass. They also provide valuable insights for future strain and process development targets.

Whole-genome sequencing of biofilm-forming and chromium-resistant mangrove fungus Aspergillus niger BSC-1

Filamentous fungus Aspergillus niger has gained significant industrial and ecological value due to its great potential in enzymatic activities. The present study reports the complete genome sequence of A. niger BSC-1 which was isolated from Indian Sundarban mangrove ecosystem. The study revealed that the genome of A. niger BSC-1 was 35.1 Mbp assembled in 40 scaffolds with 49.2% GC content. A total of 10,709 genes were reported out of which 10,535 genes were predicted for encoding the proteins. BUSCO assessment showed 98.6% of genome completeness indicating high quality genome sequencing. The genome sequencing of A. niger BSC-1 revealed the presence of rodA and exgA genes for initial adhesion to surface and Ags genes for matrix formation, during biofilm growth. OrthoVenn2 analysis revealed that A.niger BSC-1 shared 9552 gene clusters with the reference strain A. niger CBS554.65. Semi-quantitative RT-PCR analysis unveiled the role of Ags1 and P-type ATPase in fungal biofilm formation and chromium (Cr) resistance, respectively. During biofilm growth the expression of Ags1 significantly (P < 0.0001; two-way ANOVA followed by Sidak's multiple comparisons test) increased with respect to planktonic culture revealing the possible involvement of Ags1 in biofilm matrix formation. Expression of P-type ATPase gene was significantly upregulated (P < 0.0001; one-way ANOVA followed by Dunnett's multiple comparisons test) with the increasing chromium concentration in the fungal culture. Besides, several other genes encoding metalloprotease, copper and zinc binding proteins, and NADH-dependent oxidoreductase were also found in the genome of A. niger BSC-1. These proteins are also involved in heavy metal tolerance and nanofabrication indicating that this filamentous fungus A. niger BSC-1 could be potentially utilized for chromium detoxification through biofilm or nanobiremediation.

Molecular modeling and dynamics simulation of alcohol dehydrogenase enzyme from high efficacy cellulosic ethanol-producing yeast mutant strain Pichia kudriavzevii BGY1-γm

One of the major constraints limiting the use of abundantly available lignocellulosic biomass as potential feedstock for alcohol industry is the lack of C₆/C₅ co-sugar fermenting yeast. The present study explores a mutant yeast Pichia kudriavzevii BGY1-γm as a potential strain for bioconversion of glucose/xylose sugars of green biomass into ethanol under batch fermentation. The mutant strain having higher alcohol dehydrogenase activity (11.31%) showed significantly higher ethanol concentration during co-fermentation of glucose/xylose sugars (14.2%) as compared to the native strain. Based on 99% sequence similarity of ADH encoding gene from the mutant with the gene sequences from other yeast strains, the ADH enzyme was identified as ADH-1 type. The study reveals first three-dimensional model of ADH-1 utilizing glucose/xylose sugars from P. kudriavzevii BGY1-γm (PkADH mutant). The refined and validated model of PkADH mutant was used for molecular docking against the substrate (acetaldehyde) and product (ethanol). Molecular docking results showed that substrate and product exhibited a binding affinity of -4.55 and -4.5 kcal/mol with PkADH mutant. Acetaldehyde and ethanol interacted at the active site of PkADH mutant via hydrogen bonds (Ser42, His69 and Asp163) and hydrophobic interactions (Cys40, Ser42, His69, Cys95, Trp123 and Asp163) to form the stable protein-ligand complex. Molecular dynamics analysis revealed that PkADH-mutant acetaldehyde and PkADH-mutant ethanol complexes were more stable than PkADH mutant. MMPBSA binding energy confirmed that binding of substrate and product results in the formation of a lower energy stable protein-ligand complex.Communicated by Ramaswamy H. Sarma.

Functional Specificity of Three α-Arabinofuranosidases from Different Glycoside Hydrolase Families in Aspergillus niger An76

α-l-Arabinofuranosidase (Abf), a debranching enzyme that can remove arabinose substituents from arabinoxylan, promotes the hydrolysis of hemicellulose in plant biomass. However, the functional specificity of Abfs from different glycoside hydrolase (GH) families on the digestion of arabinoxylan and their synergistic interaction with xylanase have not been systematically studied. In this work, we characterized three Abfs (AxhA, AbfB, and AbfC) from GH62, GH54, and GH51 families in Aspergillus niger An76. Quantitative transcriptional analysis showed that expression of the axhA gene was upregulated as a result of induction by xylose substrates, whereas expression of the abfB gene was mainly induced by arabinose. Recombinant AxhA, AbfB, and AbfC exhibited different hydrolytic performances. AxhA showed the highest catalytic activity toward wheat arabinoxylan (WAX) and tended to hydrolyze monosubstituted arabinofuranose units, whereas AbfB had higher catalytic activity on AN and debranched arabinan (DAN), having the ability to cope with mono- and disubstituted arabinofuranose units. Furthermore, AbfC had greater arabinofuranosidase activity on p-nitrophenyl-α-l-arabinofuranoside (pNP-AraF) than on other substrates. Moreover, three Abfs displayed obvious synergistic action with GH11 xylanase XynB against WAX and barley husk residues. The elucidation of the degradation mechanisms of Abfs will lay a theoretical foundation for the efficient industrialized transformation of arabinoxylans.

Profiling multi-enzyme activities of Aspergillus niger strains growing on various agro-industrial residues

Agro-industrial wastes provide potential sources of carbon for production of fungal enzymes applied for various biotechnological applications. In this study, 23 strains of Aspergillus niger were systematically investigated for their capability on production of carbohydrate-processing enzymes used in industries. The strains were grown on glucose or selected agricultural wastes comprising varied chemical compositions as the sole carbon source. As a control, glucose induced basal activities of amylase, pectinase, and xylanase in only a few strains, while the CMCase, β-glucanase, and invertase activities were detected only when the carbon source was switched to the agro-industrial biomass. According to one-way ANOVA analysis, banana peels containing lignocellulosic components with high pectin and starch contents with its easily digestible nature, were found to be the best carbon source for inducing production of most target enzymes, while the cellulose-rich sugarcane bagasse efficiently promoted maximal levels of β-glucanase and xylanase activities. The starch fiber-rich cassava pulp also effectively supported the activities of amylase and most other enzymes, but at relatively lower levels compared to those obtained with banana peel. The A. niger TL11 strain was considered the most potent strain for production of all target enzymes with the CMCase, xylanase, pectinase, β-glucanase, amylase, and invertase activities of 76.15, 601.59, 160.89, 409.20, 426.73, and 1186.94 U/mL, respectively. The results provide insights into the efficiency of various carbon sources with different chemical compositions on inducing the target enzymes as well as the dissimilarity of A. niger strains on the production of different carbohydrate-processing enzymes.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s13205-021-03086-y.

Something old, something new: challenges and developments in Aspergillus niger biotechnology

The filamentous ascomycete fungus Aspergillus niger is a prolific secretor of organic acids, proteins, enzymes and secondary metabolites. Throughout the last century, biotechnologists have developed A. niger into a multipurpose cell factory with a product portfolio worth billions of dollars each year. Recent technological advances, from genome editing to other molecular and omics tools, promise to revolutionize our understanding of A. niger biology, ultimately to increase efficiency of existing industrial applications or even to make entirely new products. However, various challenges to this biotechnological vision, many several decades old, still limit applications of this fungus. These include an inability to tightly control A. niger growth for optimal productivity, and a lack of high-throughput cultivation conditions for mutant screening. In this mini-review, we summarize the current state-of-the-art for A. niger biotechnology with special focus on organic acids (citric acid, malic acid, gluconic acid and itaconic acid), secreted proteins and secondary metabolites, and discuss how new technological developments can be applied to comprehensively address a variety of old and persistent challenges.

Synergistic mechanism of GH11 xylanases with different action modes from Aspergillus niger An76

BACKGROUND

Xylan is the most abundant hemicellulose polysaccharide in nature, which can be converted into high value-added products. However, its recalcitrance to breakdown requires the synergistic action of multiple enzymes. Aspergillus niger, possessing numerous xylan degrading isozyme-encoding genes, are highly effective xylan degraders in xylan-rich habitats. Therefore, it is necessary to explore gene transcription, the mode of action and cooperation mechanism of different xylanase isozymes to further understand the efficient xylan-degradation by A. niger.

RESULTS

Aspergillus niger An76 encoded a comprehensive set of xylan-degrading enzymes, including five endo-xylanases (one GH10 and four GH11). Quantitative transcriptional analysis showed that three xylanase genes (xynA, xynB and xynC) were up-regulated by xylan substrates, and the order and amount of enzyme secretion differed. Specifically, GH11 xylanases XynA and XynB were initially secreted successively, followed by GH10 xylanase XynC. Biochemical analyses displayed that three GH11 xylanases (XynA, XynB and XynD) showed differences in catalytic performance and product profiles, possibly because of intricate hydrogen bonding between substrates and functional residues in the active site architectures impacted their binding capacity. Among these, XynB had the best performance in the degradation of xylan and XynE had no catalytic activity. Furthermore, XynA and XynB showed synergistic effects during xylan degradation.

CONCLUSIONS

The sequential secretion and different action modes of GH11 xylanases were essential for the efficient xylan degradation by A. niger An76. The elucidation of the degradation mechanisms of these xylanase isozymes further improved our understanding of GH-encoding genes amplification in filamentous fungi and may guide the design of the optimal enzyme cocktails in industrial applications.

Insights into the cellulose degradation mechanism of the thermophilic fungus Chaetomium thermophilum based on integrated functional omics

BACKGROUND

Lignocellulose is the most abundant and renewable biomass resource on the planet. Lignocellulose can be converted into biofuels and high-value compounds; however, its recalcitrance makes its breakdown a challenge. Lytic polysaccharide monooxygenases (LPMOs) offer tremendous promise for the degradation of recalcitrant polysaccharides. Chaetomium thermophilum, having many LPMO-coding genes, is a dominant thermophilic fungus in cellulose-rich and self-heating habitats. This study explores the genome, secretomes and transcript levels of specific genes of C. thermophilum.

RESULTS

The genome of C. thermophilum encoded a comprehensive set of cellulose- and xylan-degrading enzymes, especially 18 AA9 LPMOs that belonged to different subfamilies. Extracellular secretomes showed that arabinose and microcrystalline cellulose (MCC) could specifically induce the secretion of carbohydrate-active enzymes (CAZymes), especially AA9 LPMOs, by C. thermophilum under different carbon sources. Temporal analyses of secretomes and transcripts revealed that arabinose induced the secretion of xylanases by C. thermophilum, which was obviously different from other common filamentous fungi. MCC could efficiently induce the specific secretion of LPMO2s, possibly because the insert in loop3 on the substrate-binding surface of LPMO2s strengthened its binding capacity to cellulose. LPMO2s, cellobio hydrolases (CBHs) and cellobiose dehydrogenases (CDHs) were cosecreted, forming an efficient cellulose degradation system of oxidases and hydrolases under thermophilic conditions.

CONCLUSIONS

The specific expression of LPMO2s and cosecretion of hydrolases and oxidases by the thermophilic fungus C. thermophilum play an important role in cellulose degradation. This insight increases our understanding of the cellulose degradation under thermophilic conditions and may inspire the design of the optimal enzyme cocktails for more efficient exploration of biomass resources in industrial applications.

Biofilm-Related, Time-Series Transcriptome and Genome Sequencing in Xylanase-Producing Aspergillus niger SJ1

In this study, we found that biofilm formation is a critical factor affecting the activity of Aspergillus niger SJ1 xylanase. Xylanase activity increased 8.8% from 1046.88 to 1147.74 U/mL during A. niger SJ1 immobilized fermentation with biofilm formation. Therefore, we carried out the work of genomic analysis and biofilm-related time-series transcriptome analysis of A. niger SJ1 for better understanding of the ability of A. niger SJ to produce xylanase and biofilm formation. Genome annotation results revealed a complete biofilm polysaccharide component synthesis pathway in A. niger SJ1 and five proteins regarding xylanase synthesis. In addition, results of transcriptome analysis revealed that the genes involved in the synthesis of cell wall polysaccharides and amino acid anabolism were highly expressed in the biofilm. Furthermore, the expression levels of major genes in the gluconeogenesis pathway and mitogen-activated protein kinase pathway were examined.

Mild hydrothermal pretreatment of sugarcane bagasse enhances the production of holocellulases by Aspergillus niger

Holocellulase production by Aspergillus niger using raw sugarcane bagasse (rSCB) as the enzyme-inducing substrate is hampered by the intrinsic recalcitrance of this material. Here we report that mild hydrothermal pretreatment of rSCB increases holocellulase secretion by A. niger. Quantitative proteomic analysis revealed that pretreated solids (PS) induced a pronounced up-regulation of endoglucanases and cellobiohydrolases compared to rSCB, which resulted in a 10.1-fold increase in glucose release during SCB saccharification. The combined use of PS and pretreatment liquor (PL), referred to as whole pretreated slurry (WPS), as carbon source induced a more balanced up-regulation of cellulases, hemicellulases and pectinases and resulted in the highest increase (4.8-fold) in the release of total reducing sugars from SCB. The use of PL as the sole carbon source induced the modulation of A. niger's secretome towards hemicellulose degradation. Mild pretreatment allowed the use of PL in downstream biological operations without the need for undesirable detoxification steps.

Integrated Functional-Omics Analysis of Thermomyces lanuginosus Reveals its Potential for Simultaneous Production of Xylanase and Substituted Xylooligosaccharides

Thermophiles have several beneficial properties for the conversion of biomass at high temperatures. Thermomyces lanuginosus is a thermophilic filamentous fungus that was shown to secrete 40 glycoside hydrolases and 25 proteases when grown on different carbon sources. Among the 13 identified glycoside hydrolases with high expression levels, 9 were reduced sugar glycosidases (RSGs) belonging to seven GH families, and 7 of the 10 identified proteases were exopeptidases belonging to six different protease families. High expression of RSGs and exopeptidases may allow the fungus to efficiently degrade oligosaccharides and oligopeptides in saprophytic habitats. There were no xylan side chain-degrading enzymes predicted in the genome of T. lanuginosus, and only one thermophilic GH11 xylanase (g4601.t1) and one GH43 xylosidase (g3706.t1) were detected by liquid chromatography-mass spectrometry/mass spectrometry when T. lanuginosus grown on xylan, which led to the accumulation of substituted xylooligosaccharides (SXOS) during corncob xylan degradation where SXOS output made up more than 8% of the total xylan. The SXOS are beneficial prebiotics and important inducers for enzymes secretion of microorganisms. Thus, T. lanuginosus exhibits distinct advantages in utilizing cheap raw materials producing one thermostable xylanase and the high value-added SXOS as well as microbial inoculants to compost by batch fermentation.