The galactokinase of Hypocrea jecorina is essential for cellulase induction by lactose but dispensable for growth on d-galactose

Intracellular Sugar Transporters Facilitate Cellulase Synthesis in Trichoderma reesei Using Lactose

Sugar transporters play an important role in the cellulase production of lignocellulose-degrading fungi. Nevertheless, the role and function of these transporters are still unclear. Here we first report intracellular sugar transporters assisting cellulase production in Trichoderma reesei (T. reesei) using lactose. The mRNA levels of sugar transporter genes mfs, gst, and lac1 were substantially upregulated in T. reesei cultivated on lactose, with the most abundant mRNA levels at 24 h as compared to glucose. Moreover, the individual deletion of these sugar transporters significantly inhibited cellulase production, solid cell growth, and sporulation of T. reesei, suggesting they play a supporting role in cellulase production when grown in lactose. Surprisingly, MFS, GST, and LAC1 were mainly localized in the cytoplasm, with MFS and LAC1 in the endoplasmic reticulum (ER), representing the first discovery of intracellular sugar transporters involved in cellulase biosynthesis in lactose culture. The absence of the gene lac1 noticeably inhibited most of the crucial genes related to cellulase production, including cellulase-encoding genes, transcription factors, and sugar transporters, at 24 h, which was fully relieved at 48 h or 72 h, indicating that lac1 affects cellulase production more at the early step. This research advances the understanding of the function of intracellular sugar transporters in fungi, particularly for fungal cellulase production.

Detailed analysis of the D-galactose catabolic pathways in Aspergillus niger reveals complexity at both metabolic and regulatory level

The current impetus towards a sustainable bio-based economy has accelerated research to better understand the mechanisms through which filamentous fungi convert plant biomass, a valuable feedstock for biotechnological applications. Several transcription factors have been reported to control the polysaccharide degradation and metabolism of the resulting sugars in fungi. However, little is known about their individual contributions, interactions and crosstalk. D-galactose is a hexose sugar present mainly in hemicellulose and pectin in plant biomass. Here, we study D-galactose conversion by Aspergillus niger and describe the involvement of the arabinanolytic and xylanolytic activators AraR and XlnR, in addition to the D-galactose-responsive regulator GalX. Our results deepen the understanding of the complexity of the filamentous fungal regulatory network for plant biomass degradation and sugar catabolism, and facilitate the generation of more efficient plant biomass-degrading strains for biotechnological applications.

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.

A Novel Cellobiohydrolase I (CBHI) from Penicillium digitatum: Production, Purification, and Characterization

A new cellulase producer strain of Penicillium digitatum (RV 06) was previously obtained from rotten maize grains. This work aim was to optimize the production and characterize this microorganism produced cellulase. A CMCase maximum production (1.6 U/mL) was obtained in stationary liquid culture, with an initial pH of 5.0, at 25 °C, with 1% lactose as carbon source, and cultured for 5 days. The produced enzyme was purified by ammonium sulfate precipitation and exclusion chromatography. The purified enzyme optimal temperature and pH were 60 °C and 5.2, respectively. The experimental T_m of thermal inactivation was 63.68 °C, and full activity was recovered after incubation of 7 h at 50 °C. The purified 74 kDa CMCase presented K_M for CMC of 11.2 mg/mL, V_max of 0.13 μmol/min, k_cat of 52 s^-1, and k_cat/K_M of 4.7 (mg/mL)^-1 s^-1. The purified enzyme had a high specificity for CMC and p-nitrophenyl cellobioside and released glucose and cellobiose as final products of the CMC hydrolysis. The enzyme trypsin digestion produced peptides whose masses were obtained by MALDI-TOF/TOF mass spectrometry, which was also used to obtain two peptide sequences. These peptide sequences and the mass peak profile retrieved a CBHI within the annotated genome of P. digitatum PD1. Sequence alignments and phylogenetic analysis confirmed this enzyme as a CBHI of the glycoside hydrolase family 7. The P. digitatum PD1 protein in silico structural model revealed a coil and β-conformation predominance, which was confirmed by circular dichroism of the P. digitatum RV 06 purified enzyme.

The putative β-glucosidase BGL3I regulates cellulase induction in Trichoderma reesei

BACKGROUND

The filamentous fungus Trichoderma reesei (anamorph of Hypocrea jecorina) displays increased cellulase expression while growing on inducers such as lactose or cellulose. However, the mechanism of cellulase induction in T. reesei is not yet completely characterized. Here, a protein annotated as β-glucosidase (BGL3I) was found to be involved in cellulase induction in T. reesei. The effects of BGL3I on cellulase production have not yet been fully understood.

RESULTS

Deletion of the bgl3i gene had no influence on the growth of T. reesei, but significantly increased its cellulase activities. Deletion of bgl3i also resulted in decreased extracellular galactosidase activity, but significantly increased transcription of lactose permeases, which might be involved in lactose transport. Furthermore, deletion of bgl3i enhanced the transcription levels of intracellular β-glucosidases cel1a, cel1b and the regulator xyr1, which are all essential for lactose induction in T. reesei. BGL3I was found to have a relatively high ability to hydrolyze sophorose, which is proposed to be the strongest natural inducer of cellulase synthesis in T. reesei.

CONCLUSIONS

BGL3I may take part in the complex regulating system of cellulase induction. The deletion of bgl3i offers a new strategy to improve T. reesei strain performance.

Microbial cellulases - Diversity & biotechnology with reference to mangrove environment: A review

Cellulose is an abundant natural biopolymer on earth, found as a major constituent of plant cell wall in lignocellulosic form. Unlike other compounds cellulose is not easily soluble in water hence enzymatic conversion of cellulose has become a key technology for biodegradation of lignocellulosic materials. Microorganisms such as aerobic bacteria, fungi, yeast and actinomycetes produce cellulase that degrade cellulose by hydrolysing the β-1, 4-glycosidic linkages of cellulose. In contrast to aerobic bacteria, anaerobic bacteria lack the ability to effectively penetrate into the cellulosic material which leads to the development of complexed cellulase systems called cellulosome. Among the different environments, the sediments of mangrove forests are suitable for exploring cellulose degrading microorganisms because of continuous input of cellulosic carbon in the form of litter which then acts as a substrate for decomposition by microbe. Understanding the importance of cellulase, the present article overviews the diversity of cellulolytic microbes from different mangrove environments around the world. The molecular mechanism related to cellulase gene regulation, expression and various biotechnological application of cellulase is also discussed.

Trichoderma reesei xylanase 5 is defective in the reference strain QM6a but functional alleles are present in other wild-type strains

Trichoderma reesei is a paradigm for the regulation and industrial production of plant cell wall-degrading enzymes. Among these, five xylanases, including the glycoside hydrolase (GH) family 11 XYN1 and XYN2, the GH10 XYN3, and the GH30 XYN4 and XYN6, were described. By genome mining and transcriptome analysis, a further putative xylanase, encoded by xyn5, was identified. Analysis of xyn5 from the genome-sequenced reference strain T. reesei QM6a shows that it encodes a non-functional, truncated form of XYN5. However, non-truncated orthologues are present in other genome sequenced Trichoderma spp., and sequencing of xyn5 in other T. reesei wild-type isolates shows that they harbor a putative functional xyn5 allele. In silico analysis and 3D modeling revealed that the encoded XYN5 has significant structural similarities to xylanases of the GH11 family, including a GH-typical substrate binding groove and a carboxylate pair in the active site. The xyn5 of wild-type strain TUCIM1282 was recombinantly expressed in a T. reesei strain with a (hemi)cellulase-free background and the corresponding protein purified to apparent homogeneity. The pH and temperature optima and the kinetic parameters of the purified XYN5 were pH 4, 50 °C, and V_max = 2646 nkat/mg with a K_m of 9.68 mg/ml. This functional xyn5 allele was used to replace the mutated version which led to an overall increase of the xylanolytic activity. These findings are of particular importance as GH11 xylanases are of high biotechnological relevance, and T. reesei is one of the main industrial producers of such lignocellulose-degrading enzymes.

The Different Roles of Penicillium oxalicum LaeA in the Production of Extracellular Cellulase and β-xylosidase

Cellulolytic enzyme hydrolysis of lignocellulose biomass to release fermentable sugars is one of the key steps in biofuel refining. Gene expression of fungal cellulolytic enzymes is tightly controlled at the transcriptional level. Key transcription factors such as activator ClrB/CLR2 and XlnR/XYR1, as well as repressor CreA/CRE1 play crucial roles in this process. The putative protein methyltransferase LaeA/LAE1 has also been reported to regulate the gene expression of the cellulolytic enzyme. The formation and gene expression of the cellulolytic enzyme was compared among Penicillium oxalicum wild type (WT) and seven mutants, including ΔlaeA (deletion of laeA), OEclrB (clrB overexpression), OEclrBΔlaeA (clrB overexpression with deletion of laeA), OExlnR (xlnR overexpression), OExlnRΔlaeA (xlnR overexpression with deletion of laeA), ΔcreA (deletion of creA), and ΔcreAΔlaeA (double deletion of creA and laeA). Results revealed that LaeA extensively affected the expression of glycoside hydrolase genes. The expression of genes that encoded the top 10 glycoside hydrolases assayed in secretome was remarkably downregulated especially in later phases of prolonged batch cultures by the deletion of laeA. Cellulase synthesis of four mutants ΔlaeA, OEclrBΔlaeA, OExlnRΔlaeA, and ΔcreAΔlaeA was repressed remarkably compared with their parent strains WT, OEclrB, OExlnR, and ΔcreA, respectively. The overexpression of clrB or xlnR could not rescue the impairment of cellulolytic enzyme gene expression and cellulase synthesis when LaeA was absent, suggesting that LaeA was necessary for the expression of cellulolytic enzyme gene activated by ClrB or XlnR. In contrast to LaeA positive roles in regulating prominent cellulase and hemicellulase, the extracellular β-xylosidase formation was negatively regulated by LaeA. The extracellular β-xylosidase activities improved over 5-fold in the OExlnRΔlaeA mutant compared with that of WT, and the expression of prominent β-xylosidase gene xyl3A was activated remarkably. The cumulative effect of LaeA and transcription factor XlnR has potential applications in the production of more β-xylosidase.

Sugar catabolism in Aspergillus and other fungi related to the utilization of plant biomass

Fungi are found in all natural and artificial biotopes and can use highly diverse carbon sources. They play a major role in the global carbon cycle by decomposing plant biomass and this biomass is the main carbon source for many fungi. Plant biomass is composed of cell wall polysaccharides (cellulose, hemicellulose, pectin) and lignin. To degrade cell wall polysaccharides to different monosaccharides, fungi produce a broad range of enzymes with a large variety in activities. Through a series of enzymatic reactions, sugar-specific and central metabolic pathways convert these monosaccharides into energy or metabolic precursors needed for the biosynthesis of biomolecules. This chapter describes the carbon catabolic pathways that are required to efficiently use plant biomass as a carbon source. It will give an overview of the known metabolic pathways in fungi, their interconnections, and the differences between fungal species.

Awakening the endogenous Leloir pathway for efficient galactose utilization by Yarrowia lipolytica

BACKGROUND

Production of valuable metabolites by Yarrowia lipolytica using renewable raw materials is of major interest for sustainable food and energy. Galactose is a monosaccharide found in galactomannans, hemicelluloses, gums, and pectins.

RESULTS

Yarrowia lipolytica was found to express all the Leloir pathway genes for galactose utilization, which encode fully functional proteins. Gene organization and regulation in Y. lipolytica resembles filamentous fungi rather than Saccharomyces cerevisiae. After Y. lipolytica was grown on mixture of glucose and galactose, it was then able to metabolize galactose, including when glucose concentrations were higher than 4 g/L. However, glucose was still the preferred carbon source. Nonetheless, a strain overexpressing the four ylGAL genes of the Leloir pathway was able to efficiently use galactose as its sole carbon source. This mutant was used to produce citric acid and lipids from galactose; the yields were comparable to or greater than that obtained for the parental strain (W29) on glucose.

CONCLUSIONS

The construction of a Y. lipolytica strain able to produce citric acid and lipids from galactose is a very important step in bypassing issues related to the use of food-based substrates in industrial applications.

Intracellular β-glucosidases CEL1a and CEL1b are essential for cellulase induction on lactose in Trichoderma reesei

Lactose (1,4-O-β-d-galacto-pyranosyl-d-glucose) induces cellulolytic enzymes in Trichoderma reesei and is in fact one of the most important soluble carbon sources used to produce cellulases on an industrial level. The mechanism underlying the induction is, however, not fully understood. In this study, we investigated the cellular functions of the intracellular β-glucosidases CEL1a and CEL1b in the induction of cellulase genes by lactose in T. reesei. We demonstrated that while CEL1a and CEL1b were functionally equivalent in mediating the induction, the simultaneous absence of these intracellular β-glucosidases abolished cbh1 gene expression on lactose. d-Galactose restored the efficient cellulase gene induction in the Δcel1a strain independently of its reductive metabolism, but not in the Δcel1a Δcel1b strain. A further comparison of the transcriptional responses of the Δcel1a Δcel1b strain complemented with wild-type CEL1a or a catalytically inactive CEL1a version and the Δcel1a strain constitutively expressing CEL1a or the Kluyveromyces lactis β-galactosidase LAC4 showed that both the CEL1a protein and its glycoside hydrolytic activity were indispensable for cellulase induction by lactose. We also present evidence that intracellular β-glucosidase-mediated lactose induction is further conveyed to XYR1 to ensure the efficiently induced expression of cellulase genes.

Extra- and intracellular lactose catabolism in Penicillium chrysogenum: phylogenetic and expression analysis of the putative permease and hydrolase genes

Penicillium chrysogenum is used as an industrial producer of penicillin. We investigated its catabolism of lactose, an abundant component of whey used in penicillin fermentation, comparing the type strain NRRL 1951 with the high producing strain AS-P-78. Both strains grew similarly on lactose as the sole carbon source under batch conditions, exhibiting almost identical time profiles of sugar depletion. In silico analysis of the genome sequences revealed that P. chrysogenum features at least five putative β-galactosidase (bGal)-encoding genes at the annotated loci Pc22g14540, Pc12g11750, Pc16g12750, Pc14g01510 and Pc06g00600. The first two proteins appear to be orthologs of two Aspergillus nidulans family 2 intracellular glycosyl hydrolases expressed on lactose. The latter three P. chrysogenum proteins appear to be distinct paralogs of the extracellular bGal from A. niger, LacA, a family 35 glycosyl hydrolase. The P. chrysogenum genome also specifies two putative lactose transporter genes at the annotated loci Pc16g06850 and Pc13g08630. These are orthologs of paralogs of the gene encoding the high-affinity lactose permease (lacpA) in A. nidulans for which P. chrysogenum appears to lack the ortholog. Transcript analysis of Pc22g14540 showed that it was expressed exclusively on lactose, whereas Pc12g11750 was weakly expressed on all carbon sources tested, including D-glucose. Pc16g12750 was co-expressed with the two putative intracellular bGal genes on lactose and also responded on L-arabinose. The Pc13g08630 transcript was formed exclusively on lactose. The data strongly suggest that P. chrysogenum exhibits a dual assimilation strategy for lactose, simultaneously employing extracellular and intracellular hydrolysis, without any correlation to the penicillin-producing potential of the studied strains.

The influence of Aspergillus niger transcription factors AraR and XlnR in the gene expression during growth in D-xylose, L-arabinose and steam-exploded sugarcane bagasse

The interest in the conversion of plant biomass to renewable fuels such as bioethanol has led to an increased investigation into the processes regulating biomass saccharification. The filamentous fungus Aspergillus niger is an important microorganism capable of producing a wide variety of plant biomass degrading enzymes. In A. niger the transcriptional activator XlnR and its close homolog, AraR, controls the main (hemi-)cellulolytic system responsible for plant polysaccharide degradation. Sugarcane is used worldwide as a feedstock for sugar and ethanol production, while the lignocellulosic residual bagasse can be used in different industrial applications, including ethanol production. The use of pentose sugars from hemicelluloses represents an opportunity to further increase production efficiencies. In the present study, we describe a global gene expression analysis of A. niger XlnR- and AraR-deficient mutant strains, grown on a D-xylose/L-arabinose monosaccharide mixture and steam-exploded sugarcane bagasse. Different gene sets of CAZy enzymes and sugar transporters were shown to be individually or dually regulated by XlnR and AraR, with XlnR appearing to be the major regulator on complex polysaccharides. Our study contributes to understanding of the complex regulatory mechanisms responsible for plant polysaccharide-degrading gene expression, and opens new possibilities for the engineering of fungi able to produce more efficient enzymatic cocktails to be used in biofuel production.

Insights into enzyme secretion by filamentous fungi: comparative proteome analysis of Trichoderma reesei grown on different carbon sources

Trichoderma reesei is the main industrial producer of lignocellulolytic enzymes, and the secretory behavior of this fungus strongly depends on the carbon sources. To gain insights into how the T. reesei adapts to various carbons and regulates enzyme production, the extra- and intracellular proteomes of T. reesei grown in defined medium with lactose or xylose as the carbon source were investigated. Results indicated that the composition of extracellular proteome differed considerably depending on the carbons. The main cellobiohydrolases, i.e. Cel7a/Cel6 were the most abundant cellulolytic enzymes identified in both media, and found to be more abundant in lactose-grown culture. As compared to lactose, xylose can serve as a potent inducer of xylanolytic enzymes. Interestingly, most identified intracellular proteins are involved in carbon metabolism. Enzymes involved in utilization of xylose, such as d-xylose reductase (Xyl1p) and d-xylose dehydrogenase (Xyl2p), were present at elevated levels in the culture growing on xylose but only in minor amounts in the lactose culture. However, lactose induction significantly activated the expression of key enzymes involved in glycolysis pathway and citrate cycle. Importantly, the protein Xyl1p which participates both in the lactose and the xylose catabolism was verified as a potential regulator for cellulase formation in T. reesei.

BIOLOGICAL SIGNIFICANCE

This study not only gives an overview of the ubiquitous cellular changes induced by the two conventional carbon substrates, but offers the framework for understanding the mechanisms behind the carbon-dependent induction of extracellular enzymes in T. reesei. Moreover, this study provides a potential target (Xyl1p) that could be tentatively used for metabolic engineering of T. reesei for cost-effective cellulase production.

Enhancing cellulase production in Trichoderma reesei RUT C30 through combined manipulation of activating and repressing genes

To investigate whether enzyme production can be enhanced in the Trichoderma reesei industrial hyperproducer strain RUT C30 by manipulation of cellulase regulation, the positive regulator Xyr1 was constitutively expressed under the control of the strong T. reesei pdc promoter, resulting in significantly enhanced cellulase activity in the transformant during growth on cellulose. In addition, constitutive expression of xyr1 combined with downregulation of the negative regulator encoding gene ace1 further increased cellulase and xylanase activities. Compared with RUT C30, the resulting transformant exhibited 103, 114, and 134 % greater total secreted protein levels, filter paper activity, and CMCase activity, respectively. Surprisingly, strong increases in xyr1 basal expression levels resulted in very high levels of CMCase activity during growth on glucose. These findings demonstrate the feasibility of improving cellulase production by modifying regulator expression, and suggest an attractive new single-step approach for increasing total cellulase productivity in T. reesei.

RNA interference with carbon catabolite repression in Trichoderma koningii for enhancing cellulase production

The cellulase and xylanase genes of filamentous Trichoderma fungi exist under carbon catabolite repression mediated by the regulator carbon catabolite repressor (CREI). Our objective was to find the role of CREI in a cellulase-hyperproducing mutant of Trichoderma koningii, and address whether enzyme production can be further improved by silencing the cre1 gene. cre1 partially silenced strains were constructed to improve enzyme production in T. koningii YC01, a cellulase-hyperproducing mutant. Silencing of cre1 resulted in derepression of cellulase gene expression in glucose-based cultivation. The cre1 interference strain C313 produced 2.1-, 1.4-, 0.8-, and 0.8-fold higher amounts of filter paper activity, β-1,4-exoglucanase activity (ρ-nitrophenyl-β-D-cellobioside as substrate), β-1,4-endoglucanase activity (sodium carboxymethyl cellulose as substrate), and xylanase activity, respectively, than the control strain, suggesting that silencing of cre1 resulted in enhanced enzyme production capability. In addition, downregulation of cre1 resulted in elevated expression of another regulator of xylanase and cellulase expression, xyr1, indicating that CREI also acted as a repressor of xyr1 transcription in T. koningii under inducing conditions. These results show that RNAi is a feasible method for analyzing the regulatory mechanisms of gene expression and improving xylanase and cellulase productivity in T. koningii.

Establishment of an efficient RNA silencing system in Trichoderma koningii using DsRed as a reporter

We aimed to establish an efficient RNA interference (RNAi) system in the industrially important filamentous fungus Trichoderma koningii using the DsRed protein as a reporter of the silencing process. To accomplish this, a DsRed expression cassette was transformed into T. koningii, and a recombinant strain that stably expressed DsRed was obtained. Next, a vector-directing expression of a DsRed hairpin RNA was constructed and transformed into the T. koningii recipient strain. Approximately 79 % of transformants displayed a decrease in DsRed fluorescence, and expression of DsRed in some transformants appeared to be fully suppressed. Characterization of randomly selected transformants by genomic DNA PCR analysis, real-time PCR quantification, and western blot confirmed downregulation of gene expression at different levels. The RNA silencing approach described here for T. koningii is effective, and the DsRed reporter gene provides a convenient tool for identification of silenced fungal transformants by their DsRed fluorescence compared to the control strain. The results of this study demonstrate the power of RNAi in T. koningii, which supports the use of this technology for strain development programs and functional genomics studies in industrial fungal strains.

The intracellular galactoglycome in Trichoderma reesei during growth on lactose

Lactose (1,4-0-β-D-galactopyranosyl-D-glucose) is used as a soluble carbon source for the production of cellulases and hemicellulases for-among other purposes-use in biofuel and biorefinery industries. The mechanism how lactose induces cellulase formation in T. reesei is enigmatic, however. Previous results from our laboratory raised the hypothesis that intermediates from the two galactose catabolic pathway may give rise to the accumulation of intracellular oligogalactosides that could act as inducer. Here we have therefore used high-performance anion-exchange chromatography-mass spectrometry to study the intracellular galactoglycome of T. reesei during growth on lactose, in T. reesei mutants impaired in galactose catabolism, and in strains with different cellulase productivities. Lactose, allo-lactose, and lactulose were detected in the highest amounts in all strains, and two trisaccharides (Gal-β-1,6-Gal-β-1,4-Glc/Fru and Gal-β-1,4-Gal-β-1,4-Glc/Fru) also accumulated to significant levels. Glucose and galactose, as well as four further oligosaccharides (Gal-β-1,3/1,4/1,6-Gal; Gal-β-1,2-Glc) were only detected in minor amounts. In addition, one unknown disaccharide (Hex-β-1,1-Hex) and four trisaccharides were also detected. The accumulation of the unknown hexose disaccharide was shown to correlate with cellulase formation in the improved mutant strains as well as the galactose pathway mutants, and Gal-β-1,4-Gal-β-1,4-Glc/Fru and two other unknown hexose trisaccharides correlated with cellulase production only in the pathway mutants, suggesting that these compounds could be involved in cellulase induction by lactose. The nature of these oligosaccharides, however, suggests their formation by transglycosylation rather than by glycosyltransferases. Based on our results, the obligate nature of both galactose catabolic pathways for this induction must have another biochemical basis than providing substrates for inducer formation.

GalX regulates the D-galactose oxido-reductive pathway in Aspergillus niger

Galactose catabolism in Aspergillus nidulans is regulated by at least two regulators, GalR and GalX. In Aspergillus niger only GalX is present, and its role in d-galactose catabolism in this fungus was investigated. Phenotypic and gene expression analysis of a wild type and a galX disruptant revealed that GalX regulates the d-galactose oxido-reductive pathway, but not the Leloir pathway in A. niger.

Bioprospecting thermophiles for cellulase production: a review

Most of the potential bioprospecting is currently related to the study of the extremophiles and their potential use in industrial processes. Recently microbial cellulases find applications in various industries and constitute a major group of industrial enzymes. Considerable amount of work has been done on microbial cellulases, especially with resurgence of interest in biomass ethanol production employing cellulases and use of cellulases in textile and paper industry. Most efficient method of lignocellulosic biomass hydrolysis is through enzymatic saccharification using cellulases. Significant information has also been gained about the physiology of thermophilic cellulases producers and process development for enzyme production and biomass saccharification. The review discusses the current knowledge on cellulase producing thermophilic microorganisms, their physiological adaptations and control of cellulase gene expression. It discusses the industrial applications of thermophilic cellulases, their cost of production and challenges in cellulase research especially in the area of improving process economics of enzyme production.

L-xylo-3-hexulose reductase is the missing link in the oxidoreductive pathway for D-galactose catabolism in filamentous fungi

In addition to the well established Leloir pathway for the catabolism of d-galactose in fungi, the oxidoreductive pathway has been recently identified. In this oxidoreductive pathway, D-galactose is converted via a series of NADPH-dependent reductions and NAD(+)-dependent oxidations into D-fructose. The pathway intermediates include galactitol, L-xylo-3-hexulose, and d-sorbitol. This study identified the missing link in the pathway, the L-xylo-3-hexulose reductase that catalyzes the conversion of L-xylo-3-hexulose to D-sorbitol. In Trichoderma reesei (Hypocrea jecorina) and Aspergillus niger, we identified the genes lxr4 and xhrA, respectively, that encode the l-xylo-3-hexulose reductases. The deletion of these genes resulted in no growth on galactitol and in reduced growth on D-galactose. The LXR4 was heterologously expressed, and the purified protein showed high specificity for L-xylo-3-hexulose with a K(m) = 2.0 ± 0.5 mm and a V(max) = 5.5 ± 1.0 units/mg. We also confirmed that the product of the LXR4 reaction is D-sorbitol.

D-Galactose uptake is nonfunctional in the conidiospores of Aspergillus niger

The majority of black Aspergilli (Aspergillus section Nigri), including Aspergillus niger, as well as many other Ascomycetes fail to germinate on d-galactose as a sole carbon source. Here, we provide evidence that the ability of A. niger to transport D-galactose is growth stage dependent, being absent in the conidiospores but present in the mycelia. Despite earlier claims, we could identify galactokinase activity in growing cells and all genes of the Leloir pathway (responsible for channelling D-galactose into the EMP pathway) are well induced on D-galactose (and also on lactose, D-xylose and L-arabinose) in the mycelial stage. Expression of all Leloir pathway genes was also detectable in conidiospores, although galE (encoding a galactokinase) and galD (encoding a galactose-1-phosphate uridylyl transferase) were expressed poorly. These results suggest that the D-galactose-negative phenotype of A. niger conidiospores may be due to the lack of inducer uptake.

Sorbitol dehydrogenase of Aspergillus niger, SdhA, is part of the oxido-reductive D-galactose pathway and essential for D-sorbitol catabolism

In filamentous fungi D-galactose can be catabolised through the oxido-reductive and/or the Leloir pathway. In the oxido-reductive pathway D-galactose is converted to d-fructose in a series of steps where the last step is the oxidation of d-sorbitol by an NAD-dependent dehydrogenase. We identified a sorbitol dehydrogenase gene, sdhA (JGI53356), in Aspergillus niger encoding a medium chain dehydrogenase which is involved in D-galactose and D-sorbitol catabolism. The gene is upregulated in the presence of D-galactose, galactitol and D-sorbitol. An sdhA deletion strain showed reduced growth on galactitol and growth on D-sorbitol was completely abolished. The purified enzyme converted D-sorbitol to D-fructose with K(m) of 50±5 mM and v(max) of 80±10 U/mg.

Metabolic engineering of inducer formation for cellulase and hemicellulase gene expression in Trichoderma reesei

The filamentous fungus T. reeseiis today a paradigm for the commercial scale production of different plant cell wall degrading enzymes mainly cellulases and hemicellulases. Its enzymes have a long history of safe use in industry and well established applications are found within the pulp, paper, food, feed or textile processing industries. However, when these enzymes are to be used for the saccharification of cellulosic plant biomass to simple sugars which can be further converted to biofuels or other biorefinery products, and thus compete with chemicals produced from fossil sources, additional efforts are needed to reduce costs and maximize yield and efficiency of the produced enzyme mixtures. One approach to this end is the use of genetic engineering to manipulate the biochemical and regulatory pathways that operate during enzyme production and control enzyme yield. This review aims at a description of the state of art in this area.

Unique regulatory mechanism for D-galactose utilization in Aspergillus nidulans

This study describes two novel regulators, GalX and GalR, that control d-galactose utilization in Aspergillus nidulans. This system is unique for A. nidulans since no GalR homologs were found in other ascomycetes. GalR shares significant sequence identity with the arabinanolytic and xylanolytic regulators AraR and XlnR, but GalX is more distantly related.

Fungal arabinan and L-arabinose metabolism

l-Arabinose is the second most abundant pentose beside d-xylose and is found in the plant polysaccharides, hemicellulose and pectin. The need to find renewable carbon and energy sources has accelerated research to investigate the potential of l-arabinose for the development and production of biofuels and other bioproducts. Fungi produce a number of extracellular arabinanases, including α-l-arabinofuranosidases and endo-arabinanases, to specifically release l-arabinose from the plant polymers. Following uptake of l-arabinose, its intracellular catabolism follows a four-step alternating reduction and oxidation path, which is concluded by a phosphorylation, resulting in d-xylulose 5-phosphate, an intermediate of the pentose phosphate pathway. The genes and encoding enzymes l-arabinose reductase, l-arabinitol dehydrogenase, l-xylulose reductase, xylitol dehydrogenase, and xylulokinase of this pathway were mainly characterized in the two biotechnological important fungi Aspergillus niger and Trichoderma reesei. Analysis of the components of the l-arabinose pathway revealed a number of specific adaptations in the enzymatic and regulatory machinery towards the utilization of l-arabinose. Further genetic and biochemical analysis provided evidence that l-arabinose and the interconnected d-xylose pathway are also involved in the oxidoreductive degradation of the hexose d-galactose.

Differential regulation of the cellulase transcription factors XYR1, ACE2, and ACE1 in Trichoderma reesei strains producing high and low levels of cellulase

Due to its capacity to produce large amounts of cellulases, Trichoderma reesei is increasingly being investigated for second-generation biofuel production from lignocellulosic biomass. The induction mechanisms of T. reesei cellulases have been described recently, but the regulation of the genes involved in their transcription has not been studied thoroughly. Here we report the regulation of expression of the two activator genes xyr1 and ace2, and the corepressor gene ace1, during the induction of cellulase biosynthesis by the inducer lactose in T. reesei QM 9414, a strain producing low levels of cellulase (low producer). We show that all three genes are induced by lactose. xyr1 was also induced by d-galactose, but this induction was independent of d-galactose metabolism. Moreover, ace1 was carbon catabolite repressed, whereas full induction of xyr1 and ace2 in fact required CRE1. Significant differences in these regulatory patterns were observed in the high-producer strain RUT C30 and the hyperproducer strain T. reesei CL847. These observations suggest that a strongly elevated basal transcription level of xyr1 and reduced upregulation of ace1 by lactose may have been important for generating the hyperproducer strain and that thus, these genes are major control elements of cellulase production.

Molecular regulation of arabinan and L-arabinose metabolism in Hypocrea jecorina (Trichoderma reesei)

Hypocrea jecorina (anamorph: Trichoderma reesei) can grow on plant arabinans by the aid of secreted arabinan-degrading enzymes. This growth on arabinan and its degradation product L-arabinose requires the operation of the aldose reductase XYL1 and the L-arabinitol dehydrogenase LAD1. Growth on arabinan and L-arabinose is also severely affected in a strain deficient in the general cellulase and hemicellulase regulator XYR1, but this impairment can be overcome by constitutive expression of the xyl1 encoding the aldose reductase. An inspection of the genome of H. jecorina reveals four genes capable of degrading arabinan, i.e., the alpha-L-arabinofuranosidase encoding genes abf1, abf2, and abf3 and also bxl1, which encodes a beta-xylosidase with a separate alpha-L-arabinofuranosidase domain and activity but no endo-arabinanase. Transcriptional analysis reveals that in the parent strain QM9414 the expression of all of these genes is induced by L-arabinose and to a lesser extent by L-arabinitol and absent on D-glucose. Induction by L-arabinitol, however, is strongly enhanced in a Deltalad1 strain lacking L-arabinitol dehydrogenase activity and severely impaired in an aldose reductase (Deltaxyl1) strain, suggesting a cross talk between L-arabinitol and the aldose reductase XYL1 in an alpha-L-arabinofuranosidase gene expression. Strains bearing a knockout in the cellulase regulator xyr1 do not show any induction of abf2 and bxl1, and this phenotype cannot be reverted by constitutive expression of xyl1. The loss of function of xyr1 has also a slight effect on the expression of abf1 and abf3. We conclude that the expression of the four alpha-L-arabinofuranosidases of H. jecorina for growth on arabinan requires an early pathway intermediate (L-arabinitol or L-arabinose), the first enzyme of the pathway XYL1, and in the case of abf2 and bxl1 also the function of the cellulase regulator XYR1.

A novel carbon source-dependent genetic transformation system for the versatile cell factory Hypocrea jecorina (anamorph Trichoderma reesei)

Genetic transformation is an indispensable tool for basic fungal research, as well as a useful technique for directed improvement of industrial strains. Here we describe a simple and reproducible transformation system for the filamentous fungus Hypocrea jecorina. The system is based on hxk1 (encoding hexokinase) as selectable marker, a hexokinase-negative strain and D-mannitol, which is used as selective carbon source and osmotic stabilizer. Following transformation with the hxk1 gene, the obtained transformants were able to grow on D-mannitol as sole carbon source. Transformation efficiency achieved using D-mannitol as carbon source and osmotic stabilizer was roughly five times higher than that using D-sorbitol. The utility of this system was further demonstrated by transformation of H. jecorina with the egfp (encoding the enhanced green fluorescent protein) gene. Fluorescence microscopy revealed EGFP fluorescence in positive transformants. Our results demonstrated the feasibility of exploiting a carbon source metabolic pathway for the development of promising fungal transformation systems, which provides a new molecular toolbox for genetic modifications of the cell factory H. jecorina.

Metabolic engineering strategies for the improvement of cellulase production by Hypocrea jecorina

Hypocrea jecorina (= Trichoderma reesei) is the main industrial source of cellulases and hemicellulases used to depolymerise plant biomass to simple sugars that are converted to chemical intermediates and biofuels, such as ethanol. Cellulases are formed adaptively, and several positive (XYR1, ACE2, HAP2/3/5) and negative (ACE1, CRE1) components involved in this regulation are now known. In addition, its complete genome sequence has been recently published, thus making the organism susceptible to targeted improvement by metabolic engineering. In this review, we summarise current knowledge about how cellulase biosynthesis is regulated, and outline recent approaches and suitable strategies for facilitating the targeted improvement of cellulase production by genetic engineering.

The Hypocrea jecorina (Trichoderma reesei) hypercellulolytic mutant RUT C30 lacks a 85 kb (29 gene-encoding) region of the wild-type genome

Background

The hypercellulolytic mutant Hypocrea jecorina (anamorph Trichoderma reesei) RUT C30 is the H. jecorina strain most frequently used for cellulase fermentations and has also often been employed for basic research on cellulase regulation. This strain has been reported to contain a truncated carbon catabolite repressor gene cre1 and is consequently carbon catabolite derepressed. To date this and an additional frame-shift mutation in the glycoprotein-processing β-glucosidase II encoding gene are the only known genetic differences in strain RUT C30.

Results

In the present paper we show that H. jecorina RUT C30 lacks an 85 kb genomic fragment, and consequently misses additional 29 genes comprising transcription factors, enzymes of the primary metabolism and transport proteins. This loss is already present in the ancestor of RUT C30 – NG 14 – and seems to have occurred in a palindromic AT-rich repeat (PATRR) typically inducing chromosomal translocations, and is not linked to the cre1 locus. The mutation of the cre1 locus has specifically occurred in RUT C30. Some of the genes that are lacking in RUT C30 could be correlated with pronounced alterations in its phenotype, such as poor growth on α-linked oligo- and polyglucosides (loss of maltose permease), or disturbance of osmotic homeostasis.

Conclusion

Our data place a general caveat on the use of H. jecorina RUT C30 for further basic research.

Lack of aldose 1-epimerase in Hypocrea jecorina (anamorph Trichoderma reesei): a key to cellulase gene expression on lactose

The heterodisaccharide lactose (1,4-O-beta-D-galactopyranosyl-D-glucose) induces cellulase formation in the ascomycete Hypocrea jecorina (= Trichoderma reesei). Lactose assimilation is slow, and the assimilation of its beta-D-galactose moiety depends mainly on the operation of a recently described reductive pathway and depends less on the Leloir pathway, which accepts only alpha-D-galactose. We therefore reasoned whether galactomutarotase [aldose 1-epimerase (AEP)] activity might limit lactose assimilation and thus influence cellulase formation. We identified three putative AEP-encoding genes (aep1, aep2, aep3) in H. jecorina, of which two encoded intracellular protein (AEP1 and AEP2) and one encoded an extracellular protein (AEP3). Although all three were transcribed, only the aep3 transcript was detected on lactose. However, no mutarotase activity was detected in the mycelia, their cell walls, or the extracellular medium during growth on lactose. Therefore, the effect of galactomutarotase activity on lactose assimilation was studied with H. jecorina strains expressing the C-terminal galactose mutarotase part of the Saccharomyces cerevisiae Gal10. These strains showed increased growth on lactose in a gene copy number-dependent manner, although their formation of extracellular beta-galactosidase activity and transcription of the genes encoding the first steps in the Leloir and the reductive pathway was similar to the parental strain QM9414. Cellulase gene transcription on lactose dramatically decreased in these strains, but remained unaffected during growth on cellulose. Our data show that cellulase induction in H. jecorina by lactose requires the beta-anomer of D-galactose and reveal the lack of mutarotase activity during growth on lactose as an important key for cellulase formation on this sugar.

Galactose metabolism in yeast-structure and regulation of the leloir pathway enzymes and the genes encoding them

The enzymes of the Leloir pathway catalyze the conversion of galactose to a more metabolically useful version, glucose-6-phosphate. This pathway is required as galactose itself cannot be used for glycolysis directly. In most organisms, including the yeast Saccharomyces cerevisiae, five enzymes are required to catalyze this conversion: a galactose mutarotase, a galactokinase, a galactose-1-phosphate uridyltransferase, a UDP-galactose-4-epimerase, and a phosphoglucomutase. In yeast, the genes encoding these enzymes are tightly controlled at the level of transcription and are only transcribed under specific sets of conditions. In the presence of glucose, the genes encoding the Leloir pathway enzymes (often called the GAL genes) are repressed through the action of a transcriptional repressor Mig1p. In the presence of galactose, but in the absence of glucose, the concerted actions of three other proteins Gal4p, Gal80p, and Gal3p, and two small molecules (galactose and ATP) enable the rapid and high-level activation of the GAL genes. The precise molecular mechanism of the GAL genetic switch is controversial. Recent work on solving the three-dimensional structures of the various GAL enzymes proteins and the GAL transcriptional switch proteins affords a unique opportunity to delve into the precise, and potentially unambiguous, molecular mechanism of a highly exploited transcriptional circuit. Understanding the details of the transcriptional and metabolic events that occur in this pathway can be used as a paradigm for understanding the integration of metabolism and transcriptional control more generally, and will assist our understanding of fundamental biochemical processes and how these might be exploited.

The Botrytis cinerea hexokinase, Hxk1, but not the glucokinase, Glk1, is required for normal growth and sugar metabolism, and for pathogenicity on fruits

Hexose kinases play a central role in the initiation of sugar metabolism of living organisms and have also been implicated in carbon catabolite repression in yeasts and plants. In this study, the genes encoding glucokinase (Glk1) and hexokinase (Hxk1) from the plant-pathogenic ascomycete Botrytis cinerea were isolated and functionally characterized. Glk1-deficient mutants were indistinguishable from the wild-type in all growth parameters tested. In contrast, Deltahxk1 mutants lacking Hxk1 showed a pleiotropic growth defect. On artificial media, vegetative growth was retarded, and conidia formation strongly reduced. No or only marginal growth of Deltahxk1 mutants was observed when fructose, galactose, sucrose or sorbitol were used as carbon sources, and fructose inhibited growth of the mutant in the presence of other carbon sources. B. cinerea mutants containing hxk1 alleles with point mutations leading to enzymically inactive enzymes showed phenotypes similar to the Deltahxk1 disruption mutant, indicating that loss of hexose phosphorylation activity of Hxk1 is solely responsible for the pleiotropic growth defect. Virulence of the Deltahxk1 mutants was dependent on the plant tissue: on leaves, lesion formation was only slightly retarded compared to the wild-type, whereas only small lesions were formed on apples, strawberries and tomatoes. The low virulence of Deltahxk1 mutants on fruits was correlated with their high contents of sugars, in particular fructose. Heterologous expression of Hxk1 and Glk1 in yeast allowed their enzymic characterization, revealing kinetic properties similar to other fungal hexokinases and glucokinases. Both Deltaglk1 and Deltahxk1 mutants showed normal glucose repression of secreted lipase 1 activity, indicating that, in contrast to yeast, B. cinerea hexose kinases are not involved in carbon catabolite repression.

Characterization of the bga1-encoded glycoside hydrolase family 35 beta-galactosidase of Hypocrea jecorina with galacto-beta-D-galactanase activity

The extracellular bga1-encoded beta-galactosidase of Hypocrea jecorina (Trichoderma reesei) was overexpressed under the pyruvat kinase (pki1) promoter region and purified to apparent homogeneity. The monomeric enzyme is a glycoprotein with a molecular mass of 118.8 +/- 0.5 kDa (MALDI-MS) and an isoelectric point of 6.6. Bga1 is active with several disaccharides, e.g. lactose, lactulose and galactobiose, as well as with aryl- and alkyl-beta-D-galactosides. Based on the catalytic efficiencies, lactitol and lactobionic acid are the poorest substrates and o-nitrophenyl-beta-D-galactoside and lactulose are the best. The pH optimum for the hydrolysis of galactosides is approximately 5.0, and the optimum temperature was found to be 60 degrees C. Bga1 is also capable of releasing D-galactose from beta-galactans and is thus actually a galacto-beta-D-galactanase. beta-Galactosidase is inhibited by its reaction product D-galactose and the enzyme also shows a significant transferase activity which results in the formation of galacto-oligosaccharides.

Xyr1 receives the lactose induction signal and regulates lactose metabolism in Hypocrea jecorina

This study reports the vital regulatory influence of Xyr1 (xylanase regulator 1) on the transcription of hydrolytic enzyme-encoding genes and hydrolase formation on lactose in Hypocrea jecorina. While the transcription of the xyr1 gene itself is achieved by release of carbon catabolite repression, the transcript formation of xyn1 (xylanase 1) is regulated by an additional induction mechanism mediated by lactose. Xyr1 has an important impact on lactose metabolism by directly activating xyl1 (xylose reductase 1) transcription and indirectly influencing transcription of bga1 (beta-galactosidase 1). The latter is achieved by regulating the conversion of D-galactose to the inducing carbon source galactitol.

Induction of the gal pathway and cellulase genes involves no transcriptional inducer function of the galactokinase in Hypocrea jecorina

The Saccharomyces cerevisiae galactokinase ScGal1, a key enzyme for D-galactose metabolism, catalyzes the conversion of D-galactose to D-galactose 1-phosphate, whereas its catalytically inactive paralogue, ScGal3, activates the transcription of the GAL pathway genes. In Kluyveromyces lactis the transcriptional inducer function and the galactokinase activity are encoded by a single bifunctional KlGal1. Here, we investigated the cellular function of the single galactokinase GAL1 in the multicellular ascomycete Hypocrea jecorina (=Trichoderma reesei) in the induction of the gal genes and of the galactokinase-dependent induction of the cellulase genes by lactose (1,4-O-beta-D-galactopyranosyl-D-glucose). A comparison of the transcriptional response of a strain deleted in the gal1 gene (no putative transcriptional inducer and no galactokinase activity), a strain expressing a catalytically inactive GAL1 version (no galactokinase activity but a putative inducer function), and a strain expressing the Escherichia coli galK (no putative transcriptional inducer but galactokinase activity) showed that, in contrast to the two yeasts, both the GAL1 protein and the galactokinase activity are fully dispensable for induction of the Leloir pathway gene gal7 by D-galactose and that only the galactokinase activity is required for cellulase induction by lactose. The data document a fundamental difference in the mechanisms by which yeasts and multicellular fungi respond to the presence of D-galactose, showing that the Gal1/Gal3-Gal4-Gal80-dependent regulatory circuit does not operate in multicellular fungi.

Induction of extracellular β-galactosidase (Bga1) formation by d-galactose in Hypocrea jecorina is mediated by galactitol

The ability of Hypocrea jecorina (Trichoderma reesei) to grow on lactose strongly depends on the formation of an extracellular glycoside hydrolase (GH) family 35 beta-galactosidase, encoded by the bga1 gene. Previous studies, using batch or transfer cultures of pregrown cells, had shown that bga1 is induced by lactose and d-galactose, but to a lesser extent by galactitol. To test whether the induction level is influenced by the different growth rates attainable on these carbon sources, bga1 expression was compared in carbon-limited chemostat cultivations at defined dilution (=specific growth) rates. The data showed that bga1 expression by lactose, d-galactose and galactitol positively correlated with the dilution rate, and that galactitol and d-galactose induced the highest activities of beta-galactosidase at comparable growth rates. To know more about the actual inducer for beta-galactosidase formation, its expression in H. jecorina strains impaired in the first steps of the two d-galactose-degrading pathways was compared. Induction by d-galactose and galactitol was still found in strains deleted in the galactokinase-encoding gene gal1, which is responsible for the first step of the Leloir pathway of d-galactose catabolism. However, in a strain deleted in the aldose/d-xylose reductase gene xyl1, which performs the reduction of d-galactose to galactitol in a recently identified second pathway, induction by d-galactose, but not by galactitol, was impaired. On the other hand, induction by d-galactose and galactitol was not affected in an l-arabinitol 4-dehydrogenase (lad1)-deleted strain which is impaired in the subsequent step of galactitol degradation. These results indicate that galactitol is the actual inducer of Bga1 formation during growth on d-galactose in H. jecorina.