Biomedical Engineering Reference
In-Depth Information
is the natural inducer of cellulases in this fungus and this disaccharide
is believed to be formed from cellobiose by the transglycosylating
activity of β-glucosidase I (Gritzali and Brown 1979, Vaheri et al
.
1979).
However, sophorose is not a universal inducer of cellulases in all fungi,
as it has been shown not to induce cellulases in
Penicillium janthinellum,
Phanerochaete chysosporium, Aspergillus nidulans
and
A
.
niger
(Bisaria and
Mishra 1989, Mernitz et al
.
1996, Gielkens et al
.
1999). Cellobiose, a β-1,4-
linked glucose disaccharide and the main reaction product of the action
of cellobiohydrolases, has also been reported to induce cellulases in many
species of fungi including
Tr. reesei
,
A. nidulans
and
P. janthinellum
(Mernitz
et al
.
1996, Ilmen et al
.
1997, Chikamatsu et al
.
1999, Ding et al
.
2001). These
reports are somewhat controversial due to variations in culture conditions.
The disaccharide lactose also promotes induction of cellulases,
although induction by this disaccharide is still not understood as it is
not a natural component of plant cell wall polysaccharides. Lactose is
a disaccharide comprised of glucose and its C4 epimer galactose and is
cleaved by extracellular β-galactosidase into glucose and galactose. It is
thought that galactose enters the Leloir pathway consisting of a sequence
of enzymatic steps catalysed by galactokinase (Gal1), galactose-1-phosphate
uridylyltransferase(Gal7) and UDP-galactose 4-epimerase (Gal10) and
signals the induction of cellulases in this manner. However, when galactose
is used as a sole carbon source it also enters the Lelior pathway but cellulase
induction is not observed. Clearly the intracellular galactose utilisation
pathway alone cannot explain the observed cellulase expression on lactose
(Margolles-Clark et al
.
1997, Seiboth et al
.
2004, Karaffa et al
.
2006). It is
also clear that lactose induction is mediated, at least partially, through a
different pathway to sophorose since the genomic deletion of the
gal1
gene,
affects only lactose-mediated induction (Seiboth et al
.
2004). More recent
studies suggest the enzyme D-xylose reductase (Xyl1) may be important
in induction of cellulases by lactose. Induction of cellulases by lactose was
impaired in Δ
xyl1
strains of
Tr. reesei
. However, the exact role played by
Xyl1 is not yet fully understood (Seiboth et al
.
2007).
As well as lactose and the proposed natural inducer sophorose, a
number of other oligosaccharides and derivatives thereof, induce expression
of cellulases when added to liquid culture. These include gentiobiose (β-1,6
glucose disaccharide), δ-cellobiono-1,5 lactone, laminaribiose, xylobiose
and the monosaccharide L-sorbose (Vaheri et al
.
1979, Durand et al. 1988,
Margolles-Clark et al
.
1997, Nogawa et al
.
2001).
Glucose has been shown to repress the majority of cellulases and this
repression has been shown to over-ride induction as the addition of glucose
to an induced culture results in complete repression of cellulase gene
expression (El-Gogary et al
.
1989, Ilmen et al
.
1997). Signifi cant levels of
