A novel major facilitator transporter TrSTR1 is essential for pentose utilization and involved in xylanase induction in Trichoderma reesei

Structural and biochemical insights of xylose MFS and SWEET transporters in microbial cell factories: challenges to lignocellulosic hydrolysates fermentation

The production of bioethanol from lignocellulosic biomass requires the efficient conversion of glucose and xylose to ethanol, a process that depends on the ability of microorganisms to internalize these sugars. Although glucose transporters exist in several species, xylose transporters are less common. Several types of transporters have been identified in diverse microorganisms, including members of the Major Facilitator Superfamily (MFS) and Sugars Will Eventually be Exported Transporter (SWEET) families. Considering that Saccharomyces cerevisiae lacks an effective xylose transport system, engineered yeast strains capable of efficiently consuming this sugar are critical for obtaining high ethanol yields. This article reviews the structure-function relationship of sugar transporters from the MFS and SWEET families. It provides information on several tools and approaches used to identify and characterize them to optimize xylose consumption and, consequently, second-generation ethanol production.

Genome-wide prediction and transcriptome analysis of sugar transporters in four ascomycete fungi

The import of plant-derived small sugars by sugar transporters (STs) has received increasing interest due to its important biological role and great industrial potential. STs are important targets of genetic engineering to improve fungal plant biomass conversion. Comparatively analysis of the genome-wide prevalence and transcriptomics of STs was performed in four filamentous fungi: Aspergillus niger, Aspergillus nidulans, Penicillium subrubescens and Trichoderma reesei. Using phylogenetic analysis and literature mining, their predicted STs were divided into ten subfamilies with putative sugar specificities assigned. In addition, transcriptome analysis revealed complex expression profiles among different STs subfamilies and fungal species, indicating a sophisticated transcriptome regulation and functional diversity of fungal STs. Several STs showed strong co-expression with other genes involved in sugar utilization, encoding CAZymes and sugar catabolic enzymes. This study provides new insights into the diversity of STs at the genomic/transcriptomic level, facilitating their biochemical characterization and metabolic engineering.

Effect of the res2 transcription factor gene deletion on protein secretion and stress response in the hyperproducer strain Trichoderma reesei Rut-C30

BACKGROUND

The fungus Trichoderma reesei is one of the most used industrial cellulase producers due to its high capacity of protein secretion. Strains of T. reesei with enhanced protein secretion capacity, such as Rut-C30, have been obtained after several rounds of random mutagenesis. The strain was shown to possess an expanded endoplasmic reticulum, but the genetic factors responsible for this phenotype remain still unidentified. Recently, three new transcription factors were described in Neurospora crassa which were demonstrated to be involved in protein secretion. One of them, RES2, was involved in upregulation of secretion-related genes. The aim of our present study was therefore to analyze the role of RES2, on protein secretion in the T. reesei Rut-C30 strain.

RESULT

Deletion of the res2 gene in Rut-C30 resulted in slightly slower growth on all substrates tested, and lower germination rate as well as lower protein secretion compared to the parental strain Rut-C30. Transcriptomic analysis of the Rut-C30 and the Δres2 mutant strain in secretion stress conditions showed remarkably few differences : 971 genes were differentially expressed (DE) in both strains while 192 genes out of 1163 (~ 16.5%) were DE in Rut-C30 only and 693 out of 1664 genes (~ 41.6%) displayed differential expression solely in Δres2. Notably, induction of protein secretion by cultivating on lactose and addition of secretion stress inducer DTT induced many genes of the secretion pathway similarly in both strains. Among the differentially expressed genes, those coding for amino acid biosynthesis genes, transporters and genes involved in lipid metabolism were found to be enriched specifically in the Δres2 strain upon exposure to lactose or DTT. Besides, redox homeostasis and DNA repair genes were specifically upregulated in the Δres2 strain, indicating an altered stress response.

CONCLUSION

These results indicate that in the T. reesei Rut-C30 strain, RES2 does not act as a master regulator of the secretion pathway, but it contributes to a higher protein secretion by adjusting the expression of genes involved in different steps of protein synthesis and the secretion pathway.

Cysteine facilitates the lignocellulolytic response of Trichoderma guizhouense NJAU4742 by indirectly up-regulating membrane sugar transporters

BACKGROUND

Filamentous fungi possess a rich CAZymes system, which is widely studied and applied in the bio-conversion of plant biomass to alcohol chemicals. Carbon source acquisition is the fundamental driver for CAZymes-producing sustainability and secondary metabolism, therefore, a deeper insight into the regulatory network of sugar transport in filamentous fungi has become urgent.

RESULTS

This study reports an important linkage of sulfur assimilation to lignocellulose response of filamentous fungus. Inorganic sulfur addition facilitated biodegradation of rice straw by Trichoderma guizhouense NJAU4742. Cysteine and glutathione were revealed as major intracellular metabolites responsive to sulfur addition by metabolomics, cysteine content was increased in this process and glutathione increased correspondingly. Two membrane sugar transporter genes, Tgmst1 and Tgmst2, were identified as the critical response genes significantly up-regulated when intracellular cysteine increased. Tgmst1 and Tgmst2 were both positively regulated by the glucose regulation-related protein (GRP), up-regulation of both Tgmst1 and Tggrp can cause a significant increase in intracellular glucose. The transcriptional regulatory function of GRP mainly relied on GSH-induced glutathionylation, and the transcription activating efficiency was positively related to the glutathionylation level, furthermore, DTT-induced deglutathionylation resulted in the down-regulation of downstream genes.

CONCLUSIONS

Inorganic sulfur addition induces a rise in intracellular Cys content, and the conversion of cysteine to glutathione caused the increase of glutathionylation level of GRP, which in turn up-regulated Tgmst1 and Tgmst2. Subsequently, the sugar transport efficiency of single cells was improved, which facilitated the maintenance of vigorous CAZymes metabolism and the straw-to-biomass conversion.

Space exposure enhanced pectin-degrading enzymes expression and activity in Aspergillus costaricaensis

Aspergillus is a well-studied fungal genus that is widely used in the processing of plant biomass in industries. This study investigated the effects of space exposure on the ability of Aspergillus costaricaensis, a filamentous fungus isolated from rotten orange peel, to degrade pectin. These fungal spores were carried into space by the Long March 5B carrier rocket and exposed to cosmic radiation for 79 h. After the flight, these spores were resuscitated, and then the growing strains were screened with pectin as the sole carbon source, and the pectinase activity was evaluated. A mutant with increased biomass accumulation ability and pectin-degrading activity compared to the ground control strain was obtained. Comparative transcriptome analysis revealed that several CAZymes genes were significantly upregulated in the mutant, especially those related to pectin degradation. Among the 44 pectinases identified from the annotated genome, 42 were up-regulated. The activities of these pectinases are able to synergistically break down the structure of pectin. In addition, the expression of some genes involved in metabolism, sugar transport, and stress response was altered. These results imply that space exposure might serve as a potential mutagenesis breeding technique, offering the opportunity to acquire biomass-degrading microbial strains with potential for industrial application.

Discovery of ER-localized sugar transporters for cellulase production with lac1 being essential

BACKGROUND

In the process of cellulose hydrolysis, carbohydrate hydrolysates are transported into cells through membrane transporters, and then affect the expression of cellulase-encoding genes. Sugar transporters play a crucial role in cellulase production in lignocellulolytic fungi, of which relatively few have been functionally validated to date and are all reported to be on cell membrane.

RESULT

Through transcriptome analysis and qRT-PCR, three putative MFS sugar transporters GST, MFS, and LAC1 were found to display significantly higher mRNA levels in T. reesei grown on cellulose than on glucose. The individual deletion of these three genes compromised cellulase production and delayed sugar absorption by 24 h in T. reesei. Nevertheless, they transported pretty low level of sugars, including galactose, lactose, and mannose, and did not transport glucose, when expressed in yeast system. Meanwhile, all three transporters were unexpectedly found to be intracellular, being located in endoplasmic reticulum (ER). Particularly, the knockout of lac1 almost abolished cellulase production, and significantly inhibited biomass generation regardless of sugar types, indicating that lac1 is essential for cellulase production and biomass formation. The absence of lac1 upregulated genes involved in ribosome biogenesis, while downregulated genes in cellulase production, protein processing in ER (particularly protein glycosylation), and lipid biosynthesis. The inhibition of lac1 deletion on the transcriptional levels of genes related to cellulase biosynthesis was restored after 72 h, but the cellulase production was still inhibited, indicating lac1 might pose a post-transcription regulation on cellulase production that are independent on the known cellulase regulation mediated by CRT1 and XYR1.

CONCLUSION

For the first time, intracellular sugar transporters (mfs, gst, and lac1) facilitating cellulase production were identified, which was distributed in ER. Their sugar transporting ability was very weak, indicating that they might be related to sugar utilization inside cells rather than the cellular sugar uptake. More importantly, sugar transporter lac1 is first found to be essential for cellulase production and biomass formation by affecting protein processing in ER (particularly protein glycosylation) and lipid biosynthesis. The effect of LAC1 on cellulase production seems to be post-transcriptional at late stage of cellulase production, independent on the well-known cellulase regulation mediated by CRT1 and XYR1. These findings improve the understanding of intracellular sugar transporters in fungi and their important role in cellulase synthesis.

Transmembrane transport process and endoplasmic reticulum function facilitate the role of gene cel1b in cellulase production of Trichoderma reesei

Background

A total of 11 β-glucosidases are predicted in the genome of Trichoderma reesei, which are of great importance for regulating cellulase biosynthesis. Nevertheless, the relevant function and regulation mechanism of each β-glucosidase remained unknown.

Results

We evidenced that overexpression of cel1b dramatically decreased cellulase synthesis in T. reesei RUT-C30 both at the protein level and the mRNA level. In contrast, the deletion of cel1b did not noticeably affect cellulase production. Protein CEL1B was identified to be intracellular, being located in vacuole and cell membrane. The overexpression of cel1b reduced the intracellular pNPGase activity and intracellular/extracellular glucose concentration without inducing carbon catabolite repression. On the other hand, RNA-sequencing analysis showed the transmembrane transport process and endoplasmic reticulum function were affected noticeably by overexpressing cel1b. In particular, some important sugar transporters were notably downregulated, leading to a compromised cellular uptake of sugars including glucose and cellobiose.

Conclusions

Our data suggests that the cellulase inhibition by cel1b overexpression was not due to the β-glucosidase activity, but probably the dysfunction of the cellular transport process (particularly sugar transport) and endoplasmic reticulum (ER). These findings advance the knowledge of regulation mechanism of cellulase synthesis in filamentous fungi, which is the basis for rationally engineering T. reesei strains to improve cellulase production in industry.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12934-022-01809-1.

Factors regulating cellulolytic gene expression in filamentous fungi: an overview

The growing demand for biofuels such as bioethanol has led to the need for identifying alternative feedstock instead of conventional substrates like molasses, etc. Lignocellulosic biomass is a relatively inexpensive feedstock that is available in abundance, however, its conversion to bioethanol involves a multistep process with different unit operations such as size reduction, pretreatment, saccharification, fermentation, distillation, etc. The saccharification or enzymatic hydrolysis of cellulose to glucose involves a complex family of enzymes called cellulases that are usually fungal in origin. Cellulose hydrolysis requires the synergistic action of several classes of enzymes, and achieving the optimum secretion of these simultaneously remains a challenge. The expression of fungal cellulases is controlled by an intricate network of transcription factors and sugar transporters. Several genetic engineering efforts have been undertaken to modulate the expression of cellulolytic genes, as well as their regulators. This review, therefore, focuses on the molecular mechanism of action of these transcription factors and their effect on the expression of cellulases and hemicellulases.

Elimination of the Sugar Transporter GAT1 Increased Xylanase I Production in Trichoderma reesei

The filamentous fungus Trichoderma reesei secretes large quantities of cellulases and hemicellulases that have found wide applications in industry. Compared with extensive studies on the mechanism controlling cellulase gene expression, less is known about the regulatory mechanism behind xylanase gene expression. Herein, several putative sugar transporter encoding genes that showed significant upregulation on xylan were identified in T. reesei. Deletion of one such gene, gat1, resulted in increased xylanase production but hardly affected cellulase induction. Further analyses demonstrated that deletion of gat1 markedly increased XYNI production at the transcriptional level and only exerted a minor effect on XYNII synthesis. In contrast, overexpressing gat1 caused a continuous decrease in xyn1 expression. Deletion of gat1 also affected the expression of xyn1 and pectinase genes when T. reesei was cultivated with galacturonic acid as the sole carbon source. Transcriptome analyses of Δgat1 and its parental strain identified 255 differentially expressed genes that are enriched in categories of glycoside hydrolases, lipid metabolism, transporters, and transcriptional factors. The results thus implicate a repressive role of the sugar transporter GAT1 in xyn1 expression and reveal that distinct regulatory mechanisms may exist in controlling the expression of different xylanase genes in T. reesei.

Functional characterization of a highly specific L-arabinose transporter from Trichoderma reesei

BACKGROUND

Lignocellulose biomass has been investigated as a feedstock for second generation biofuels and other value-added products. Some of the processes for biofuel production utilize cellulases and hemicellulases to convert the lignocellulosic biomass into a range of soluble sugars before fermentation with microorganisms such as yeast Saccharomyces cerevisiae. One of these sugars is L-arabinose, which cannot be utilized naturally by yeast. The first step in L-arabinose catabolism is its transport into the cells, and yeast lacks a specific transporter, which could perform this task.

RESULTS

We identified Trire2_104072 of Trichoderma reesei as a potential L-arabinose transporter based on its expression profile. This transporter was described already in 2007 as D-xylose transporter XLT1. Electrophysiology experiments with Xenopus laevis oocytes and heterologous expression in yeast revealed that Trire2_104072 is a high-affinity L-arabinose symporter with a Km value in the range of [Formula: see text] 0.1-0.2 mM. It can also transport D-xylose but with low affinity (Km [Formula: see text] 9 mM). In yeast, L-arabinose transport was inhibited slightly by D-xylose but not by D-glucose in an assay with fivefold excess of the inhibiting sugar. Comparison with known L-arabinose transporters revealed that the expression of Trire2_104072 enabled yeast to uptake L-arabinose at the highest rate in conditions with low extracellular L-arabinose concentration. Despite the high specificity of Trire2_104072 for L-arabinose, the growth of its T. reesei deletion mutant was only affected at low L-arabinose concentrations.

CONCLUSIONS

Due to its high affinity for L-arabinose and low inhibition by D-glucose or D-xylose, Trire2_104072 could serve as a good candidate for improving the existing pentose-utilizing yeast strains. The discovery of a highly specific L-arabinose transporter also adds to our knowledge of the primary metabolism of T. reesei. The phenotype of the deletion strain suggests the involvement of other transporters in L-arabinose transport in this species.

Electrophysiological characterization of a diverse group of sugar transporters from Trichoderma reesei

Trichoderma reesei is an ascomycete fungus known for its capability to secrete high amounts of extracellular cellulose- and hemicellulose-degrading enzymes. These enzymes are utilized in the production of second-generation biofuels and T. reesei is a well-established host for their production. Although this species has gained considerable interest in the scientific literature, the sugar transportome of T. reesei remains poorly characterized. Better understanding of the proteins involved in the transport of different sugars could be utilized for engineering better enzyme production strains. In this study we aimed to shed light on this matter by characterizing multiple T. reesei transporters capable of transporting various types of sugars. We used phylogenetics to select transporters for expression in Xenopus laevis oocytes to screen for transport activities. Of the 18 tested transporters, 8 were found to be functional in oocytes. 10 transporters in total were investigated in oocytes and in yeast, and for 3 of them no transport function had been described in literature. This comprehensive analysis provides a large body of new knowledge about T. reesei sugar transporters, and further establishes X. laevis oocytes as a valuable tool for studying fungal sugar transporters.

Disruption of alpha-tubulin releases carbon catabolite repression and enhances enzyme production in Trichoderma reesei even in the presence of glucose

BACKGROUND

Trichoderma reesei is a filamentous fungus that is important as an industrial producer of cellulases and hemicellulases due to its high secretion of these enzymes and outstanding performance in industrial fermenters. However, the reduction of enzyme production caused by carbon catabolite repression (CCR) has long been a problem. Disruption of a typical transcriptional regulator, Cre1, does not sufficiently suppress this reduction in the presence of glucose.

RESULTS

We found that deletion of an α-tubulin (tubB) in T. reesei enhanced both the amount and rate of secretory protein production. Also, the tubulin-disrupted (ΔtubB) strain had high enzyme production and the same enzyme profile even if the strain was cultured in a glucose-containing medium. From transcriptome analysis, the ΔtubB strain exhibited upregulation of both cellulase and hemicellulase genes including some that were not originally induced by cellulose. Moreover, cellobiose transporter genes and the other sugar transporter genes were highly upregulated, and simultaneous uptake of glucose and cellobiose was also observed in the ΔtubB strain. These results suggested that the ΔtubB strain was released from CCR.

CONCLUSION

Trichoderma reesei α-tubulin is involved in the transcription of cellulase and hemicellulase genes, as well as in CCR. This is the first report of overcoming CCR by disrupting α-tubulin gene in T. reesei. The disruption of α-tubulin is a promising approach for creating next-generation enzyme-producing strains of T. reesei.

Characterization of two sugar transporters responsible for efficient xylose uptake in an oleaginous yeast Candida tropicalis SY005

Microbial conversion of lignocellulosic feedstock to the target bioproduct requires efficient assimilation of its constituent sugars, a large part of which comprises of glucose and xylose. This study aims to identify and characterize sugar transporters capable of xylose uptake in an oleaginous strain of the industrially relevant yeast Candida tropicalis. In silico database mining resulted in two sugar transporter proteins- CtStp1 and CtStp2, containing conserved amino acid residues and motifs that have been previously reported to be involved in xylose transport in other organisms. Several softwares predicted the likelihood of 10-12 transmembrane (TM) helices to be present in both the Stps, while molecular modelling showed 12 TM helices that were organized into a typical structure found in the major facilitator superfamily of transporters. Docking with different sugars also predicted favorable interactions. Heterologous expression in a Saccharomyces cerevisiae strain harboring functional xylose metabolic genes validated the broad substrate specificity of the two Stps. Each transporter supported prominent growth of recombinant S. cerevisiae strains on six sugars including xylose at various concentrations. Expression of CtSTP1 and CtSTP2 along with the xylose metabolic genes in yeast transformants grown in presence of xylose was confirmed by transcript detection. Growth curve and sugar consumption profiles revealed uptake of both glucose and xylose simultaneously by the recombinant yeast strains, though CtStp1 showed relatively less effect of glucose repression in mixed sugars and was a better transporter of xylose than CtStp2. Such glucose-xylose utilizing efficient transporters can be effective tools for developing co-fermenting yeasts through genetic engineering in future, with noteworthy applications in renewable biomass utilization.

In silico Proteomic Analysis Provides Insights Into Phylogenomics and Plant Biomass Deconstruction Potentials of the Tremelalles

Basidiomycetes populate a wide range of ecological niches but unlike ascomycetes, their capabilities to decay plant polymers and their potential for biotechnological approaches receive less attention. Particularly, identification and isolation of CAZymes is of biotechnological relevance and has the potential to improve the cache of currently available commercial enzyme cocktails toward enhanced plant biomass utilization. The order Tremellales comprises phylogenetically diverse fungi living as human pathogens, mycoparasites, saprophytes or associated with insects. Here, we have employed comparative genomics approaches to highlight the phylogenomic relationships among thirty-five Tremellales and to identify putative enzymes of biotechnological interest encoded on their genomes. Evaluation of the predicted proteomes of the thirty-five Tremellales revealed 6,918 putative carbohydrate-active enzymes (CAZYmes) and 7,066 peptidases. Two soil isolates, Saitozyma podzolica DSM 27192 and Cryptococcus sp. JCM 24511, show higher numbers harboring an average of 317 compared to a range of 267-121 CAZYmes for the rest of the strains. Similarly, the proteomes of the two soil isolates along with two plant associated strains contain higher number of peptidases sharing an average of 234 peptidases compared to a range of 226-167 for the rest of the strains. Despite these huge differences and the apparent enrichment of these enzymes among the soil isolates, the data revealed a diversity of the various enzyme families that does not reflect specific habitat type. Growth experiment on various carbohydrates to validate the predictions provides support for this view. Overall, the data indicates that the Tremellales could serve as a rich source of both CAZYmes and peptidases with wide range of potential biotechnological relevance.

Identification and Characterization of an Efficient d-Xylose Transporter in Saccharomyces cerevisiae

d-Xylose is the most abundant hemicellulosic monomer on earth, but wild-type Saccharomyces cerevisiae has very limited d-xylose uptake capacity. We conducted bioprospecting for new sugar transporters from the d-xylose-consuming filamentous fungus Trichoderma reesei and identified three candidates belonging to the major facilitator superfamily. When they were expressed in yeast and assayed for d-xylose uptake, one of them, Xltr1p, had d-xylose transport activity that was more efficient than that of Gal2p, an endogenous yeast transporter. Site-directed mutagenesis was used to examine the functional contributions of 13 amino acid residues for the uptake of d-xylose, and these experiments identified particular amino acids that function distinctly in d-xylose vs glucose transport (e.g., F300). Excitingly, the yeast strain expressing the N326F^Xltr1p variant was able to carry a "high efficiency" transport for d-xylose but was nearly unable to utilize glucose; in contrast, the strain with the F300A^Xltr1p variant grew on glucose but lost d-xylose transport activity.

The influence of feedstock characteristics on enzyme production in Trichoderma reesei: a review on productivity, gene regulation and secretion profiles

Biorefineries, designed for the production of lignocellulose-based chemicals and fuels, are receiving increasing attention from the public, governments, and industries. A major obstacle for biorefineries to advance to commercial scale is the high cost of the enzymes required to derive the fermentable sugars from the feedstock used. As summarized in this review, techno-economic studies suggest co-localization and integration of enzyme manufacturing with the cellulosic biorefinery as the most promising alternative to alleviate this problem. Thus, cultivation of Trichoderma reesei, the principal producer of lignocellulolytic enzymes, on the lignocellulosic biomass processed on-site can reduce the cost of enzyme manufacturing. Further, due to a complex gene regulation machinery, the fungus can adjust the gene expression of the lignocellulolytic enzymes towards the characteristics of the feedstock, increasing the hydrolytic efficiency of the produced enzyme cocktail. Despite extensive research over decades, the underlying regulatory mechanisms are not fully elucidated. One aspect that has received relatively little attention in literature is the influence the characteristics of a lignocellulosic substrate, i.e., its chemical and physical composition, has on the produced enzyme mixture. Considering that the fungus is dependent on efficient enzymatic degradation of the lignocellulose for continuous supply of carbon and energy, a relationship between feedstock characteristics and secretome composition can be expected. The aim of this review was to systematically collect, appraise, and aggregate data and integrate results from studies analyzing enzyme production by T. reesei on insoluble cellulosic model substrates and lignocellulosic biomass. The results show that there is a direct effect of the substrate's complexity (rated by structure, composition of the lignin-carbohydrate complex, and recalcitrance in enzymatic saccharification) on enzyme titers and the composition of specific activities in the secretome. It further shows that process-related factors, such as substrate loading and cultivation set-up, are direct targets for increasing enzyme yields. The literature on transcriptome and secretome composition further supports the proposed influence of substrate-related factors on the expression of lignocellulolytic enzymes. This review provides insights into the interrelation between the characteristics of the substrate and the enzyme production by T. reesei, which may help to advance integrated enzyme manufacturing of substrate-specific enzymes cocktails at scale.

Enhancement of cellulase production in Trichoderma reesei RUT-C30 by comparative genomic screening

BACKGROUND

Cellulolytic enzymes produced by the filamentous fungus Trichoderma reesei are commonly used in biomass conversion. The high cost of cellulase is still a significant challenge to commercial biofuel production. Improving cellulase production in T. reesei for application in the cellulosic biorefinery setting is an urgent priority.

RESULTS

Trichoderma reesei hyper-cellulolytic mutant SS-II derived from the T. reesei NG14 strain exhibited faster growth rate and more efficient lignocellulosic biomass degradation than those of RUT-C30, another hyper-cellulolytic strain derived from NG14. To identify any genetic changes that occurred in SS-II, we sequenced its genome using Illumina MiSeq. In total, 184 single nucleotide polymorphisms and 40 insertions and deletions were identified. SS-II sequencing revealed 107 novel mutations and a full-length wild-type carbon catabolite repressor 1 gene (cre1). To combine the mutations of RUT-C30 and SS-II, the sequence of one confirmed beneficial mutation in RUT-C30, cre196, was introduced in SS-II to replace full-length cre1, forming the mutant SS-II-cre196. The total cellulase production of SS-II-cre196 was decreased owing to the limited growth of SS-II-cre196. In contrast, 57 genes mutated only in SS-II were selected and knocked out in RUT-C30. Of these, 31 were involved in T. reesei growth or cellulase production. Cellulase activity was significantly increased in five deletion strains compared with that in two starter strains, RUT-C30 and SS-II. Cellulase production of T. reesei Δ108642 and Δ56839 was significantly increased by 83.7% and 70.1%, respectively, compared with that of RUT-C30. The amount of glucose released from pretreated corn stover hydrolyzed by the crude enzyme from Δ108642 increased by 11.9%.

CONCLUSIONS

The positive attribute confirmed in one cellulase hyper-producing strain does not always work efficiently in another cellulase hyper-producing strain, owing to the differences in genetic background. Genome re-sequencing revealed novel mutations that might affect cellulase production and other pathways indirectly related to cellulase formation. Our strategy of combining the mutations of two strains successfully identified a number of interesting phenotypes associated with cellulase production. These findings will contribute to the creation of a gene library that can be used to investigate the involvement of various genes in the regulation of cellulase production.

Gene Co-expression Network Reveals Potential New Genes Related to Sugarcane Bagasse Degradation in Trichoderma reesei RUT-30

The biomass-degrading fungus Trichoderma reesei has been considered a model for cellulose degradation, and it is the primary source of the industrial enzymatic cocktails used in second-generation (2G) ethanol production. However, although various studies and advances have been conducted to understand the cellulolytic system and the transcriptional regulation of T. reesei, the whole set of genes related to lignocellulose degradation has not been completely elucidated. In this study, we inferred a weighted gene co-expression network analysis based on the transcriptome dataset of the T. reesei RUT-C30 strain aiming to identify new target genes involved in sugarcane bagasse breakdown. In total, ~70% of all the differentially expressed genes were found in 28 highly connected gene modules. Several cellulases, sugar transporters, and hypothetical proteins coding genes upregulated in bagasse were grouped into the same modules. Among them, a single module contained the most representative core of cellulolytic enzymes (cellobiohydrolase, endoglucanase, β-glucosidase, and lytic polysaccharide monooxygenase). In addition, functional analysis using Gene Ontology (GO) revealed various classes of hydrolytic activity, cellulase activity, carbohydrate binding and cation:sugar symporter activity enriched in these modules. Several modules also showed GO enrichment for transcription factor activity, indicating the presence of transcriptional regulators along with the genes involved in cellulose breakdown and sugar transport as well as other genes encoding proteins with unknown functions. Highly connected genes (hubs) were also identified within each module, such as predicted transcription factors and genes encoding hypothetical proteins. In addition, various hubs contained at least one DNA binding site for the master activator Xyr1 according to our in silico analysis. The prediction of Xyr1 binding sites and the co-expression with genes encoding carbohydrate active enzymes and sugar transporters suggest a putative role of these hubs in bagasse cell wall deconstruction. Our results demonstrate a vast range of new promising targets that merit additional studies to improve the cellulolytic potential of T. reesei strains and to decrease the production costs of 2G ethanol.

New Genomic Approaches to Enhance Biomass Degradation by the Industrial Fungus Trichoderma reesei

The filamentous fungi Trichoderma reesei is one of the most well-studied cellulolytic microorganisms. It is the most important fungus for the industrial production of enzymes to biomass deconstruction being widely used in the biotechnology industry, mainly in the production of biofuels. Here, we performed an analytic review of the holocellulolytic system presented by T. reesei as well as the transcriptional and signaling mechanisms involved with holocellulase expression in this fungus. We also discuss new perspectives about control of secretion and cellulase expression based on RNA-seq and functional characterization data of T. reesei growth in different carbon sources, which comprise glucose, cellulose, sophorose, and sugarcane bagasse.

Characterization of a novel sugar transporter involved in sugarcane bagasse degradation in Trichoderma reesei

BACKGROUND

Trichoderma reesei is a saprophytic fungus implicated in the degradation of polysaccharides present in the cell wall of plants. T. reesei has been recognized as the most important industrial fungus that secretes and produces cellulase enzymes that are employed in the production of second generation bioethanol. A few of the molecular mechanisms involved in the process of biomass deconstruction by T. reesei; in particular, the effect of sugar transporters and induction of xylanases and cellulases expression are yet to be known.

RESULTS

In our study, we characterized a novel sugar transporter, which was previously identified by our group through in silico analysis of RNA-seq data. The novel T. reesei 69957-sugar transport system (Tr69957) is capable of transporting xylose, mannose, and cellobiose using a T. reesei 69957-sugar transport system in Saccharomyces cerevisiae. The deletion of Tr69957 in T. reesei affected the fungal growth and biomass accumulation, and the sugar uptake in the presence of mannose, cellobiose, and xylose. Molecular docking studies revealed that Tr69957 shows reduced protein-ligand binding energy for interactions towards disaccharides in comparison with monosaccharides. Furthermore, the deletion of Tr69957 affected the gene expression of cellobiohydrolases (cel7a and cel6a), β-glucosidases (cel3a and cel1a), and xylanases (xyn1 and xyn2) in the cultures of parental and mutant strains in the presence of cellobiose and sugarcane bagasse (SCB).

CONCLUSION

The transporter Tr69957 of T. reesei can transport cellobiose, xylose, and mannose, and can affect the expression of a few genes encoding enzymes, such as cellulases and xylanases, in the presence of SCB. We showed for the first time that a filamentous fungus (T. reesei) contains a potential mannose transporter that may be involved in the degradation of cellulose.

Transient Silencing of DNA Repair Genes Improves Targeted Gene Integration in the Filamentous Fungus Trichoderma reesei

Trichoderma reesei is a filamentous fungus that is used worldwide to produce industrial enzymes. Industrial strains have traditionally been created though systematic strain improvement using mutagenesis and screening approaches. It is also desirable to specifically manipulate the genes of the organism to further improve and to modify the strain. Targeted integration in filamentous fungi is typically hampered by very low frequencies of homologous recombination. To address this limitation, we have developed a simple transient method for silencing genes in T. reesei Using gene-specific small interfering RNAs (siRNAs) targeted to mus53, we could achieve up to 90% knockdown of mus53 mRNA. As a practical example, we demonstrated that transient silencing of DNA repair genes significantly improved homologous integration of DNA at a specific locus in a standard protoplast transformation. The best transient silencing of mus53 with siRNAs in protoplasts could achieve up to 59% marker gene integration.IMPORTANCE The previous solution for improving targeted integration efficiency has been deleting nonhomologous end joining (NHEJ) DNA repair genes. However, deleting these important repair genes may lead to unintended consequences for genomic stability and could lead to the accumulation of spontaneous mutations. Our method of transiently silencing NHEJ repair pathway genes allows recovery of their important repair functions. Here we report a silencing approach for improving targeted DNA integration in filamentous fungi. Furthermore, our transient silencing method is a truly flexible approach that is capable of knocking down the expression of a target gene in growing mycelial cultures, which could facilitate the broad study of gene functions in T. reesei.

Comparative transcriptome analysis reveals different strategies for degradation of steam-exploded sugarcane bagasse by Aspergillus niger and Trichoderma reesei

Background

Second generation (2G) ethanol is produced by breaking down lignocellulosic biomass into fermentable sugars. In Brazil, sugarcane bagasse has been proposed as the lignocellulosic residue for this biofuel production. The enzymatic cocktails for the degradation of biomass-derived polysaccharides are mostly produced by fungi, such as Aspergillus niger and Trichoderma reesei. However, it is not yet fully understood how these microorganisms degrade plant biomass. In order to identify transcriptomic changes during steam-exploded bagasse (SEB) breakdown, we conducted a RNA-seq comparative transcriptome profiling of both fungi growing on SEB as carbon source.

Results

Particular attention was focused on CAZymes, sugar transporters, transcription factors (TFs) and other proteins related to lignocellulose degradation. Although genes coding for the main enzymes involved in biomass deconstruction were expressed by both fungal strains since the beginning of the growth in SEB, significant differences were found in their expression profiles. The expression of these enzymes is mainly regulated at the transcription level, and A. niger and T. reesei also showed differences in TFs content and in their expression. Several sugar transporters that were induced in both fungal strains could be new players on biomass degradation besides their role in sugar uptake. Interestingly, our findings revealed that in both strains several genes that code for proteins of unknown function and pro-oxidant, antioxidant, and detoxification enzymes were induced during growth in SEB as carbon source, but their specific roles on lignocellulose degradation remain to be elucidated.

Conclusions

This is the first report of a time-course experiment monitoring the degradation of pretreated bagasse by two important fungi using the RNA-seq technology. It was possible to identify a set of genes that might be applied in several biotechnology fields. The data suggest that these two microorganisms employ different strategies for biomass breakdown. This knowledge can be exploited for the rational design of enzymatic cocktails and 2G ethanol production improvement.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-017-3857-5) contains supplementary material, which is available to authorized users.

Identification of a Novel L-rhamnose Uptake Transporter in the Filamentous Fungus Aspergillus niger

The study of plant biomass utilization by fungi is a research field of great interest due to its many implications in ecology, agriculture and biotechnology. Most of the efforts done to increase the understanding of the use of plant cell walls by fungi have been focused on the degradation of cellulose and hemicellulose, and transport and metabolism of their constituent monosaccharides. Pectin is another important constituent of plant cell walls, but has received less attention. In relation to the uptake of pectic building blocks, fungal transporters for the uptake of galacturonic acid recently have been reported in Aspergillus niger and Neurospora crassa. However, not a single L-rhamnose (6-deoxy-L-mannose) transporter has been identified yet in fungi or in other eukaryotic organisms. L-rhamnose is a deoxy-sugar present in plant cell wall pectic polysaccharides (mainly rhamnogalacturonan I and rhamnogalacturonan II), but is also found in diverse plant secondary metabolites (e.g. anthocyanins, flavonoids and triterpenoids), in the green seaweed sulfated polysaccharide ulvan, and in glycan structures from viruses and bacteria. Here, a comparative plasmalemma proteomic analysis was used to identify candidate L-rhamnose transporters in A. niger. Further analysis was focused on protein ID 1119135 (RhtA) (JGI A. niger ATCC 1015 genome database). RhtA was classified as a Family 7 Fucose: H+ Symporter (FHS) within the Major Facilitator Superfamily. Family 7 currently includes exclusively bacterial transporters able to use different sugars. Strong indications for its role in L-rhamnose transport were obtained by functional complementation of the Saccharomyces cerevisiae EBY.VW.4000 strain in growth studies with a range of potential substrates. Biochemical analysis using L-[³H(G)]-rhamnose confirmed that RhtA is a L-rhamnose transporter. The RhtA gene is located in tandem with a hypothetical alpha-L-rhamnosidase gene (rhaB). Transcriptional analysis of rhtA and rhaB confirmed that both genes have a coordinated expression, being strongly and specifically induced by L-rhamnose, and controlled by RhaR, a transcriptional regulator involved in the release and catabolism of the methyl-pentose. RhtA is the first eukaryotic L-rhamnose transporter identified and functionally validated to date.

The growth of filamentous fungi on plant biomass, which occurs through the utilization of its components (e.g. D-glucose, D-xylose, L-arabinose, L-rhamnose) as carbon sources, is a highly regulated event. L-rhamnose (6-deoxy-L-mannose) is a deoxy-sugar present in plant cell wall pectic polysaccharides (mainly rhamnogalacturonan I and rhamnogalacturonan II), but also in diverse plant secondary metabolites, ulvan from green seaweeds and glycan structures from virus and bacteria. The utilization, transformation or detoxification of this monosaccharide by fungi involves a first step of chemical hydrolysis, performed by alpha-L-rhamnosidases, and a second step of transport into the cell, prior to its metabolization. While many rhamnosidases have been identified, not a single eukaryotic plasma membrane L-rhamnose transporter is known to date. In this study we identified and characterized, for the first time, a fungal L-rhamnose transporter (RhtA), from the industrial workhorse Aspergillus niger. We also found that RhtA putative orthologs are conserved throughout different fungal orders, opening the possibility of identifying new transporters of its kind.

Identification and functional characterization of novel xylose transporters from the cell factories Aspergillus niger and Trichoderma reesei

BACKGROUND

Global climate change and fossil fuels limitations have boosted the demand for robust and efficient microbial factories for the manufacturing of bio-based products from renewable feedstocks. In this regard, efforts have been done to enhance the enzyme-secreting ability of lignocellulose-degrading fungi, aiming to improve protein yields while taking advantage of their ability to use lignocellulosic feedstocks. Access to sugars in complex polysaccharides depends not only on their release by specific hydrolytic enzymes, but also on the presence of transporters capable of effectively transporting the constituent sugars into the cell. This study aims to identify and characterize xylose transporters from Aspergillus niger and Trichoderma reesei, two fungi that have been industrially exploited for decades for the production of lignocellulose-degrading hydrolytic enzymes.

RESULTS

A hidden Markov model for the identification of xylose transporters was developed and used to analyze the A. niger and T. reesei in silico proteomes, yielding a list of candidate xylose transporters. From this list, three A. niger (XltA, XltB and XltC) and three T. reesei (Str1, Str2 and Str3) transporters were selected, functionally validated and biochemically characterized through their expression in a Saccharomyces cerevisiae hexose transport null mutant, engineered to be able to metabolize xylose but unable to transport this sugar. All six transporters were able to support growth of the engineered yeast on xylose but varied in affinities and efficiencies in the uptake of the pentose. Amino acid sequence analysis of the selected transporters showed the presence of specific residues and motifs recently associated to xylose transporters. Transcriptional analysis of A. niger and T. reesei showed that XltA and Str1 were specifically induced by xylose and dependent on the XlnR/Xyr1 regulators, signifying a biological role for these transporters in xylose utilization.

CONCLUSIONS

This study revealed the existence of a variety of xylose transporters in the cell factories A. niger and T. reesei. The particular substrate specificity and biochemical properties displayed by A. niger XltA and XltB suggested a possible biological role for these transporters in xylose uptake. New insights were also gained into the molecular mechanisms regulating the pentose utilization, at inducer uptake level, in these fungi. Analysis of the A. niger and T. reesei predicted transportome with the newly developed hidden Markov model showed to be an efficient approach for the identification of new xylose transporting proteins.