β-glucosidase induction and repression in the cellulolytic fungus,Trichoderma reesei

Molecular and bioinformatics analyses reveal two differentially expressed intracellular GH1 β-glucosidases from the rare alkalophilic fungus Stachybotrys microspora

The present study reports the isolation and analysis of two novel GH1 β-glucosidases from the alkalophilic fungus Stachybotrys microspora, using PCR and Nested-PCR. Three major gene fragments were obtained by PCR: the first two are very similar and constitute a novel gene, which was named Smbgl1A, and the third PCR fragment is part of a different gene, named Smbgl1B. The truncated gene sequences were completely filled using the recent partial whole genome sequencing data of S. microspora (data not yet published).

Moreover, we investigated the relative effects of glucose in comparison to cellulose rather than evaluate their absolute effects.

In fact, RT-PCR analysis showed that while Smbgl1A was expressed when the fungus was grown in the presence of cellulose but not when grown with glucose, Smbgl1B was equally expressed under both conditions.

The putative catalytic residues and the conserved glycone binding sites were identified. Zymogram analysis showed the intracellular production of β-glucosidases in S. microspora. The predicted secondary structure exhibited a classical (β/α)8 barrel fold, showing that both SmBGL1A and SmBGL1B belong to the GH1 family.

Phylogenetic studies showed that SmBGL1A and SmBGL1B belong to the same branch as β-glucosidases from Stachybotrys chlorohalonata and Stachybotrys chartarum. However, SmBGL1A and SmBGL1B form two distinct clades.

•

Isolation of two novel GH 1 β-glucosidases from Stachybotrys microspora

•

Investigation of the relative effects of glucose in comparison to cellulose

•

Zymogram analysis has shown the intracellular production of GH1 β-glucosidases.

•

Prediction of the secondary structure with the presence of a classical (β/α) 8 barrel

Trichoderma reesei histone acetyltransferase Gcn5 regulates fungal growth, conidiation, and cellulase gene expression

Gcn5 is a well-established histone acetyltransferase involved in chromatin modification by catalyzing the acetylation of specific lysine residues within the N-terminal tails of the core histones. To assess the role of chromatin remodeling in the transcriptional response of cellulolytic Trichoderma reesei to the changes of environmental conditions, we identified the T. reesei ortholog of Saccharomyces cerevisiae Gcn5 by sequence alignment and functional analysis. Heterologous expression of TrGcn5 in S. cerevisiae gcn5Δ strain restored the growth defect under nutrient limitation as well as stresses. In contrast, mutant TrGcn5 with site-directed changes of residues critical for Gcn5 histone acetyltransferase activity could not complement the growth defect. The T. reesei gcn5Δ mutant strain displayed a strongly decreased growth rate and dramatic morphological changes including misshapen hyphal cells and abolished conidiation. Moreover, the induced expression of cellulase genes was severely impaired in the gcn5Δ T. reesei with acetylation of K9 and K14 of histone H3 in the cellulase gene promoter dramatically affected in the absence of TrGcn5. The results indicate that TrGcn5 plays a critical role in filamentous growth, morphogenesis, and transcriptional activation of specific genes including cellulase encoding genes.

Differential involvement of β-glucosidases from Hypocrea jecorina in rapid induction of cellulase genes by cellulose and cellobiose

Appropriate perception of cellulose outside the cell by transforming it into an intracellular signal ensures the rapid production of cellulases by cellulolytic Hypocrea jecorina. The major extracellular β-glucosidase BglI (CEL3a) has been shown to contribute to the efficient induction of cellulase genes. Multiple β-glucosidases belonging to glycosyl hydrolase (GH) family 3 and 1, however, exist in H. jecorina. Here we demonstrated that CEL1b, like CEL1a, was an intracellular β-glucosidase displaying in vitro transglycosylation activity. We then found evidence that these two major intracellular β-glucosidases were involved in the rapid induction of cellulase genes by insoluble cellulose. Deletion of cel1a and cel1b significantly compromised the efficient gene expression of the major cellulase gene, cbh1. Simultaneous absence of BglI, CEL1a, and CEL1b caused the induction of the cellulase gene by cellulose to further deteriorate. The induction defect, however, was not observed with cellobiose. The absence of the three β-glucosidases, rather, facilitated the induced synthesis of cellulase on cellobiose. Furthermore, addition of cellobiose restored the productive induction on cellulose in the deletion strains. The results indicate that the three β-glucosidases may not participate in transforming cellobiose beyond hydrolysis to provoke cellulase formation in H. jecorina. They may otherwise contribute to the accumulation of cellobiose from cellulose as inducing signals.

The bgl1 gene of Trichoderma reesei QM 9414 encodes an extracellular, cellulose-inducible beta-glucosidase involved in cellulase induction by sophorose

We have investigated the effect of disruption of the bgl1-(beta-glucosidase l-encoding) gene of Trichoderma reesei on the formation of other beta-glucosidase activities and on the induction of cellulases. To this end the bgl1 locus was disrupted by insertion of the Aspergillus nidulans amdS (acetamidase-encoding) gene. The bgl1-disrupted strain did not produce the 75 kDa extracellular beta-glucosidase on cellulose or lactose, but still formed beta-glucosidase activity on glucose, cellobiose, xylan or beta-1,3-glucan, suggesting that the enzyme(s) exhibiting this beta-glucosidase activity is (are) not encoded by bgl1. The cellulase-inducer sophorose induced the bgl1-encoded beta-glucosidase, whereas the remaining beta-glucosidase activity was induced by methyl-beta-D-glucoside. The bgl1-gene product was mainly secreted into the medium, whereas the other beta-glucosidase activity was mainly associated with the cells. A bgl1-multicopy strain formed higher amounts of cellulases than the parent strain. Nonsaturating concentrations of sophorose efficiently induced cellobiohydrolase l formation in the bgl1-multicopy strain, but less efficiently in the bgl1-disrupted strain. The multicopy strain and the parent strain were comparably efficient at saturating sophorose concentrations. The beta-glucosidase inhibitor nojirimycin strongly inhibited induction in all strains. These data suggest that the bgl1-encoded beta-glucosidase is not identical to the plasma-membrane-bound, constitutive, methyl-beta-glucoside inducible beta-glucosidase, but represents an extracellular cellulose-induced enzyme. Both enzymes contribute to rapid induction of cellulases by modifying the inducer sophorose.

Regulatory aspects of cellulase biosynthesis and secretion

The cellulase enzyme system consists of cellobiohydrolase, endoglucanase, and beta-glucosidase and has been extensively studied with respect to its biosynthesis, properties, mode of action, application, and, most recently, secretion mechanisms. A knowledge of the factors governing the biosynthesis and secretion of these enzymes at the molecular level will be useful in maximizing enzyme productivity in extracellular fluid. Among other topics, the regulatory effects of sorbose (a noninducing sugar which is not a product of cellulose hydrolysis) on cellulase synthesis and release are described. Cellulase genes have recently been cloned into a number of microorganisms with a view to understanding the gene structure and expression and to obtaining the enzyme components in pure form. The factors governing biosynthesis and secretion of cellulases in recombinant cells are also discussed. Cellulases are known to be glycoproteins, therefore, the role of O- and N-linked glycosylation on enzyme stability and secretion is also detailed.

Sophorose induction of an intracellular b-glucosidase in Trichoderma

The disaccharide sophorose induces Trichoderma to increase a soluble intracellular b-glucosidase that hydrolyses cellobiose, sophorose, and p-nitrophenyl-b-D-glucopyranoside. Simultaneously, it depresses the activity of a similar insoluble enzyme that is associated with the mycelium. Gel electrophoresis indicates that a single enzyme is responsible for all the soluble intracellular b-glucosidase activity. Cycloheximide severely inhibits sophorose induction of this enzyme indicating that the increase in activity normally obtained with sophorose is due to the de novo formation of the enzyme. The same sugars that promote the formation and release of cellulase by Trichoderma induce an increase in the soluble intracellular b-glucosidase. A function of the soluble intracellular enzyme appears to be the hydrolysis of cellobiose, which would otherwise accumulate during cellulose degradation, and thus to prevent cellobiose inhibition of cellulase.