Background: During the process of bioethanol production, cellulose is hydrolyzed into its monomeric soluble units. For efficient hydrolysis, a chemical and/or mechanical pretreatment step is required. Such pretreatment is designed to increase enzymatic digestibility of the cellulose chains inter alia by de-crystallization of the cellulose chains and by removing barriers, such as lignin from the plant cell wall. Biological pretreatment, in which lignin is decomposed or modified by white-rot fungi, has also been considered. One disadvantage in biological pretreatment, however, is the consumption of the cellulose by the fungus. Thus, fungal species that attack lignin with only minimal cellulose loss are advantageous. The secretomes of white-rot fungi contain carbohydrate-active enzymes (CAZymes) including lignin-modifying enzymes. Thus, modification of secretome composition can alter the ratio of lignin/cellulose degradation.
Results: Pleurotus ostreatus PC9 was genetically modified to either overexpress or eliminate (by gene replacement) the transcriptional regulator CRE1, known to act as a repressor in the process of carbon catabolite repression. The cre1-overexpressing transformant demonstrated lower secreted cellulolytic activity and slightly increased selectivity (based on the chemical composition of pretreated wheat straw), whereas the knockout transformant demonstrated increased cellulolytic activity and significantly reduced residual cellulose, thereby displaying lower selectivity. Pretreatment of wheat straw using the wild-type PC9 resulted in 2.8-fold higher yields of soluble sugar compared to untreated wheat straw. The overexpression transformant showed similar yields (2.6-fold), but the knockout transformant exhibited lower yields (1.2-fold) of soluble sugar. Based on proteomic secretome analysis, production of numerous CAZymes was affected by modification of the expression level of cre1.
Conclusions: The gene cre1 functions as a regulator for expression of fungal CAZymes active against plant cell wall lignocelluloses, hence altering the substrate preference of the fungi tested. While the cre1 knockout resulted in a less efficient biological pretreatment, i.e., less saccharification of the treated biomass, the converse manipulation of cre1 (overexpression) failed to improve efficiency. Despite the inverse nature of the two genetic alterations, the expected "mirror image" (i.e., opposite regulatory response) was not observed, indicating that the secretion level of CAZymes, was not exclusively dependent on CRE1 activity.
Keywords: Biological pretreatment; CAZymes; Decomposition of lignocellulose; Secretome; White-rot fungi; cre1.