Biomedical Engineering Reference
In-Depth Information
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.
Expression of Cellulases
Many hypotheses have been proposed for the methods by which cellulases
are induced in fi lamentous fungi and results to-date would appear to
suggest that differences do indeed exist between fungi. Among the best
studied example is the fungus
Tr. reesei
, a brown-rot fungus that is widely
used in industry. El-Gogary and colleagues (1989) proposed that low
level constitutive expression of cellulases in
Tr. reesei
liberates soluble
cellooligosaccharides from cellulose that could in turn induce high-level
expression of a more complete cellulose-degrading system (Gritzali and
Brown 1979, El-Gogary et al
.
1989). However, Ilmen and co-workers,
(1997) could not confi rm these earlier results as none of the major cellulase
transcripts were detected in glucose-induced cultures of
Tr. reesei
in the
latter study.
Conidial bound cellulases have also been suggested as being essential
for initial hydrolysis of cellulose and for the release of a soluble cellulose
derivative, which would then promote high level expression of the complete
cellulase system (Messner and Kubicek 1991). These confl icting schools
of thought highlight the need for further investigation into the induction
of cellulases in other fungi in the anticipation that differences potentially
exist between fungi and that induction is clearly not as simple as fi rst
proposed.
Inducing and Repressing Substrates
Sophorose (β-1,2-linked glucose disaccharide) has been shown to be a
potent inducer of cellulases in
Tr. reesei
. It is widely accepted that sophorose
