The most studied aerobic cellulolytic microorganism is the fungus, Hypocrea jecorina, orig­inally called Trichoderma reesei. It was isolated and studied by Drs Reese and Mandels at the

Army Quartermaster Lab in Natick, MA, during World War II, because it was degrading the cotton fabrics used by the army for tents, gun straps, etc. on islands in the Pacific Ocean (56). The original goal of this work was to find cellulase inhibitors, which was not achieved, as only the toxic ions, Hg and Ag, are good inhibitors. However, this group also carried out many studies on the organism and its crude cellulase, which then led to the development of high producing mutant strains by Dr Eveleigh that were used to develop the strains used for industrial cellulase production by several companies (57). The most abundant cellulase produced by T. reesei is the reducing end-specific exocellulase, Cel7A (cellobiohydrolase I), which makes up about 70% of the cellulase protein secreted by T. reesei (58). The next most abundant cellulase is Cel6A (CBH II), which makes up a further 10% of T. reesei secreted cellulase. T. reesei crude cellulase contains seven endoglucanases of which Cel7B (EGLI) is the most abundant. In addition, Cel5A (EGLII), Cel12A (EGLIII), Cel61A (EGLIV), Cel45A (EGLV), Cel5B, and Cel61B are also present in T. reesei secreted cellulase. Most of the T. reesei cellulases contain a family I CBM except Cel5B, Cel12A, and Cel61B. It is not clear why T. reesei produces so many endocellulases, but Cel12A was shown to have expansin activity as well as cellulase activity (59). Expansin is present in plants and it appears to disrupt the hydrogen bonds that bind different carbohydrate chains together in plant cell walls, so that it may make the chains more accessible to hydrolytic enzymes. The least studied endocellulase is Cel61A, which has extremely low cellulase activity. It is quite surprising that when a set of thermophilic fungal cellulases were screened for the ability to stimulate the activity of T. reesei crude cellulase, a number of them were able to increase it about threefold and the component that was most active in giving this stimulation was a family 61 enzyme (60). Little is known about the role of Cel45A in cellulose degradation (61). Another protein se­creted by T. reesei is swollenin, which is a low molecular weight protein that has no catalytic activity but appears to disrupt the structure of cellulose microfibers, possibly by breaking hydrogen bonds (62).

Most of the T. reesei cellulases are glycosylated and glycosylation appears to protect the cellulases from proteolysis (63). The linker peptide is particularly susceptible to proteolysis and T. reesei secretes proteases (64), so that protection from proteolysis maybe an important role for the O-linked glycosylation found on the linker peptide (65). The role of the N-linked glycosylation on the CD is unclear at this time. There is a great deal of heterogeneity in the glycosylation of any given cellulase and this causes heterogeneity of each enzyme during gel electrophoresis and column chromatography (66).

There have been extensive studies of the regulation of cellulase synthesis in T. reesei and it appears that regulation is complex (67). Glucose strongly represses cellulase synthesis and the (3-1,2-linked glucose disaccharide, sepharose, induces synthesis. A number of transcription factors have been identified in T. reesei, which can bind to cellulase promoters, and some of these are activators and some are repressors. The exact mechanisms that regulate cellulase synthesis are still not completely understood.