Cellulase is a generic term for enzymes which decompose cellulose. Cellulase produced by microorganisms is generally composed of many types of cellulase components. The cellulase components are classified by their substrate specificity into three types: cellobiohydrolase, endoglucanase, and β-glucosidase. It is considered that Aspergillus niger, a filamentous fungus which produces cellulase, produces four types of cellobiohydrolase, fifteen types of endoglucanases, and fifteen types of β-glucosidases at the maximum. Therefore, when cellulase produced by a microorganism is industrially utilized, it is used as a mixture of various cellulase components produced by the microorganism.
A filamentous fungus Acremonium cellulolyticus is characterized by producing cellulase having high saccharification ability (non-patent literature 1), and it is reported that it has high usefulness for feed use or silage use (patent literatures 1-3). The cellulase components contained (patent literatures 4-10) have been studied in detail, and it is clarified that many kinds of cellulase components are secreted similarly to other filamentous fungi.
It is considered that several types of specific enzyme components in many types of cellulase components are important for a certain limited use. Therefore, if the cellulase component composition of cellulase produced by a microorganism can be optimized according to the use, it is expected that cellulase having higher activity can be obtained. The best way to accomplish this is to overexpress a specific enzyme by the introduction of its specific enzyme gene, or to disrupt a specific enzyme gene, using gene recombination techniques.
However, only two types of cellobiohydrolases (patent literatures 4 and 5) and a type of β-glucosidase (patent literature 10) were isolated in Acremonium cellulolyticus, and thus, enhanced expression by gene introduction or suppressed expression by gene disruption could not be carried out with respect to the other cellulases.
Under these circumstances, the isolation of genes for polysaccharide-degrading enzymes such as endoglucanase and β-glucosidase has been desired to optimize the composition of cellulase components produced by Acremonium cellulolyticus, using gene recombination techniques.