Biomedical Engineering Reference
In-Depth Information
and can then act directly as inducers of the genes that encode a variety of
enzymes required for plant polymer degradation.
In eukaryotic organisms, transcriptional regulation requires the
combinatorial action of different repressors and activators bound to specifi c
sites in the upstream regulatory sequence of individual genes. Transcriptional
response to these factors may be graded or binary, modulating the level of
expression or completely turning it off. β-Galactosidase (
lac
A) from
Aspergilli
exhibits highest expression when xylose, arabinose, xylan and pectin are
used as carbon sources (de Vries et al. 1999a). However, xylanolytic and
cellulolytic enzymes are also produced when xylan is the sole carbon source.
This suggests a general system of regulation of the genes encoding these
enzymes. Expression levels of a number of xylanolytic genes have been
shown to be a result of a balance between catabolite repressor element (Cre)
mediated repression and XlnR mediated induction in
Aspergillus
species
(de Vries et al. 1999).
The carbon catabolite repressor element
(Cre) is a zinc fi nger protein
which binds to specifi c
sites (5′-SYGGRG-3′) in the promoters of a wide
range of target genes in
Aspergillus
and
Trichoderma
species
(Cubero and
Scazzocchio 1994, Strauss et al. 1995). In the presence of easily metabolizable
substrates, such
as glucose or fructose, Cre inhibits or downregulates the
expression
of the target genes. XlnR, a gene encoding a transcriptional
activator has been isolated from
A. niger
(van Peij et al. 1998a). Sequence
analysis demonstrated
that it is a member of the GAL4-like family of
transcriptional
activators. Characterization of XlnR showed that it was
responsible
for the expression of genes encoding endoxylanase and
β-xylosidase.
Analysis of the promoter region of these genes identifi ed
consensus sequence 5′-GGCTAAA-3′, within which the second G was
determined
to be essential for XlnR binding, by mobility shift assays
and
in vivo
studies. XlnR
has also been shown to be involved in the regulation
of certain cellulases as well (van Peij et al. 1998b). A subsequent
A. niger
study has demonstrated
that XlnR is also involved in the regulation of
α- and β-galactosidase
genes (
aglB
and
lacA
respectively). The absence of
β-galactosidase expression
in a XlnR-defi cient
A. niger
mutant indicates that
the expression of this gene on xylose and xylan is regulated by XlnR (de
Vries et al. 1999a) Analysis of the promoter regions of the genes
regulated
by XlnR demonstrated that the third A in the consensus sequence for the
binding site is variable; the essential consensus sequence has
therefore
been shortened to GGCTAA (van Peij et al. 1998b). Recently a model
was suggested for the role of XlnR in the regulation of (hemi)cellulose
degradation by
A. niger
(de Vries et al. 2000). XlnR is activated, during
growth of
A. niger
in the presence
of arabinoxylan, by monomeric xylose
which is already present in
the substrate or released by endoxylanase B and
β-xylosidase that
are produced in low, constitutive levels. XlnR then activates
