Mechanism by which cellulose triggers cellobiohydrolase I gene expression in Trichoderma reesei.

Abstract

The expression of cellobiohydrolase I mRNA from Trichoderma reesei, measured by Northern blot hybridization, is controlled by the nature ofcarbon sources used in the culture medium. Cellulose and the soluble disaccharide sophorose, but not glycerol or glucose, act as inducers. Cellobiohydrolase I mRNA was undetectable when antibodies to the major members of the cellulolytic system were present in the culture medium prior to the addition of cellulose. These antibodies had no repressive effect if sophorose was used as an inducer. The results strongly suggest that a low constitutive cellulolytic system catalyzes the formation of a soluble inducer from cellulose and that this inducer triggers the expression of the cellobiohydrolase I gene transcript, most probably at the transcription level.In nature, the cycling of carbon is of the utmost importance to living systems. It is estimated that the photosynthetic process produces 1.5 x 1011 tons ofdry plant material annually, almost half of which is cellulose (1). This plant polysaccharide is used as an energy carbon source by numerous and diverse microorganisms, including fungi and bacteria occupying a variety of habitats (2). Solubilization of this insoluble polymer is via extracellular cellulase systems that catalyze the hydrolysis of cellulose to glucose. Among the best characterized of these systems are the inducible cellulases of the filamentous fungus Trichoderma reesei (3). This cellulase system consists of three The utilization of cellulose by T. reesei is enigmatic, as the product of cellulolysis (glucose) represses the expression of the cellulase system (5). At present, it is not understood how an insoluble polymer such as cellulose, which is unable to enter the fungal cell, can regulate expression of the cellulolytic enzyme system. It has been suggested (6, 7) that T. reesei expresses low, constitutive, levels of the cellulase system and that the activity of these enzymes on cellulose produces a soluble inducer, which can enter the cell and effect induction. Sophorose (2-0-,3-glucopyranosyl-Dglucose) is the most potent soluble inducer of the cellulase system in T. reesei so far identified (7,8). Sophorose as well as other glucose disaccharides were detected during growth of T. reesei on cellobiose (6) and after treatment of cellulose with the T. reesei cellulase system (9), most probably produced by the transglycosylation activity of,-glucosidase (10, 11).In preparation for investigating the molecular mechanisms responsible for regulating cellulase gene expression, we have prepared antibodies to the major members of the cellulolytic system and have also isolated a clone carrying the gene encoding cellobiohydrolase I. Using these antibodies to block the activity of the cellulase system and the CBH-I clone as a DNA probe, we present evidence that low constitutive levels of the cellulase system are responsible for triggering the expression of the CBH-I gene at the pretranslational level. Methods. Preparation of enzymes and antibodies. Enzymes were purifi...

Origin of oxidized cellulose degradation products and mechanism of their promotion of cellobiohydrolase I biosynthesis in Trichoderma reesei

“…The mechanism by which cellulase formation is triggered by the presence of cellulose in the fungus Trichoderma reesei has recently become a major focus of research: the presence of extracellular cellulose is 'recognized' by conidial-surface-bound constitutive cellulases (Kubicek, 1987;Kubicek et al, 1988;El Gogary et al, 1989), particularly cellobiohydrolase I1 (CBH 11) (Messner et al, 1991), which carry out the initial attack on the cellulose molecule. The small amounts of degradation products arising thereby may then be taken up by the fungus and act as inducers of further cellulase formation .…”

Crel, the carbon catabolite repressor protein from Trichoderma reesei

“…The formation of some of these enzymes (e.g. cellobiohydrolase I; endo-fl-l,4-xylanase I) is repressed by glucose [15,16]. It has been reported that the 5'-upstream nt-sequence of the T. reesei gene encoding cellobiohydrolase I (cbhl) shows consensus sequences for binding of a potential CreA-homologue [17].…”

Cloning and Transcript Analysis of Multiple Genes Encoding the Glycoside Hydrolase Family 6 Enzyme fromCoprinopsis cinerea

“…In addition, it has been reported that basal expression of CBHII is necessary for the formation of the inducer of the cellulase gene at the growth stage of mycelia on cellulose culture. [40][41][42] These facts indicate that CBHII has a role not only in cellulose hydrolysis but also in the formation of the inducer of cellulases. Recently, Suzuki et al 43) reported a significant transcript level of the CBHII gene from P. chrysosporium, which has a singlecopy gene of the GH6 enzyme on its genome, in a culture containing no carbon source.…”