Induction and Repression of Carbohydrate Degrading Enzymes in Fungi with Special Reference to their Coding Genes - Biotechnology of Fungal Genes - page 55

individual cellulase transcripts have been detected in cultures once glucose
has been completely metabolized. This 'induction' was initially thought to
be a natural starvation survival mechanism. However, cellulase transcripts
were not detected in cultures lacking a carbon source. Thus, it would appear
that the expression of these cellulase transcripts is the result of derepression
of cellulase gene expression or may be due to the effect of some inducing
compound produced by transglycosylation, either from glucose present
previously in the culture medium or other oligosaccharides released from
the fungal cell wall (Sternberg and Mandels 1979, Ilmen et al . 1997).
Expression of Xylanases
Xylan is a high molecular mass polymer, which is unable to penetrate the
fungal cell wall. For this reason it is thought that the monosaccharides
and small oligosaccharides derived from xylan play a key role in the
regulation of xylanase biosynthesis. Generally crude plant compounds,
such as sugar beet pulp and wheat bran are used for the induction and
purifi cation of xylanases due to the low cost of these substrates. However,
in order to determine the inducer of a particular xylanase it is necessary
to examine the production of xylanases on individual monosaccharides.
Xylanase induction by D-xylose has been reported in Talaromyces
emersonii, Tr. reesei and Aspergillus sp. (Zeilinger et al . 1996, van Peij et al .
1998, Reen et al . 2003). However, induction is concentration dependent in
that low D-xylose concentrations induce expression of xylanases while high
concentrations have a repressive effect (De Vries et al. 1999b, Reen et al .
2003). All of the main xylanases expressed in the presence of xylan or xylose
are repressed in the presence of glucose and this repression is thought to be
mediated by the CRE protein in A. niger and Tr. reesei (de Vries et al. 1999a).
The disaccharide, xylobiose, has been reported to induce expression
of several of the xylanase genes as well as genes encoding some of the
side-chain cleaving enzymes in Tr. reesei (Margolles-Clark et al . 1997).
Induction of xylanase expression has also been reported on cellulose and
sophorose (Zeilinger et al . 1996). Arabinose and L-arabitol have been shown
to induce expression of genes encoding enzymes involved in arabinoxylan
degradation in A. niger (van der Veen et al . 1993, Flipphi et al. 1994, Gielkens
et al. 1997). L-Arabitol is believed to be the true inducer of this system in
A. nidulans (de Vries et al. 1994). D-Galacturonic acid, one of the major
monosaccharide constituents of pectin, induces the expression of a large
number of genes encoding pectinolytic enzymes in A. niger (de Vries et al.
2002a). D-Galacturonic acid has also been reported to induce the expression
of α-glucuronidase in A. niger (de Vries et al. 2002b).