Purification, characterization and gene analysis of exo-cellulase II (Ex-2) from the white rot basidiomycete Irpex lacteus

A new exo-type cellulase, named exo-cellulase II (Ex-2), was purified from the crude enzyme preparation of Irpex lacteus. Ex-2 was very similar to the previously characterized exo-cellulase I(Ex-1)with respect to enzymatic fe atures such as optimal pH, temperature, heat stability, and catalytic activ ity, However, Ex-2 exhibited greater pH stability than Ex-1. The molecular mass and carbohydrate content of Ex-2 (56,000, 4.0%) were different from th ose of Ex-1 (53,000, 2.0%). A cellulase gene (named cel2) encoding both Ex- 2 and Ex-1 was isolated from an I. lacteus genomic library. The cel2 gene w as found to consist of 1569 bp with an open reading frame encoding 523 amin o acids, interrupted by two introns. The deduced amino acid sequences revea led that cel2 ORF has a modular structure consisting of a catalytic domain and a fungal-type cellulose-binding domain (CBD) separated by a serine-rich linker region. The catalytic domain was homologous to those of fungal cell obiohydrolases belonging to family 7 of the glycosyl hydrolases. Northern b lot analysis showed that expression of the cel2 gene was induced by various cellulosic substrates and repressed by glucose, fructose, and lactose.