ABSTRACT
The production of complete plant cell wall degrading enzyme systems by fungi requires an enormous amount of energy and thus places a signifi cant metabolic pressure on a fungus. Consequently, fungi have developed complex multi-component regulatory systems that repress expression of cell wall hydrolases in the presence of simple carbon sources, such as glucose and induce expression of a more complete enzyme system in the presence of more complex carbon sources, such as cellulose. The
method by which an enormous molecule such as cellulose mediates the expression of enzymes required for its complete hydrolysis is a conundrum that has proven somewhat elusive to solve. There are many confl icting hypotheses regarding cellulase induction, but it is generally thought that cellooligosaccharides or glucose are modifi ed by a transglycosylation reaction and act as soluble inducers that signal the expression of cellulases in fungi. For example, it has been suggested from experimental fi ndings that the β-1,2-linked glucose disaccharide, sophorose, acts as the inducer of cellulases in the mesophilic fungus, Trichoderma reesei (Ilmen et al. 1997). However, the mechanism by which the cellooligosaccharides, glucose and/ or transglycosylation products are generated from cellulose and induce cellulase expression has, as yet, not been satisfactorily established. It is also quite likely that a single unifying mechanism of cellulase induction does not exist between all fungi.
