Biomedical Engineering Reference
In-Depth Information
The Glucose Repressor Protein-CRE
Carbon catabolite repression (CCR) is a well-known mechanism by which
many fungi adjust their carbon catabolism in response to the carbon
source available. CCR is the preferential use of easily metabolized and
energetically favourable carbon sources, such as glucose, over alternative
carbon sources resulting in the repression of genes encoding enzymes
required for the utilisation of less favourable carbon sources. While glucose
is probably the most repressive and most studied carbon source, carbon
repression may be caused by various carbon sources.
Carbon catabolite repression has been demonstrated in many fungal
species, for example, in species of
Aspergillus
,
Trichoderma
,
Sclerotinia
and
T.
emersonii
(Ilmen et al. 1997, Margolles-Clark et al. 1997, Ruijter et al. 1997,
Vautard-Mey and Fevre 2000, Grassick et al. 2004). In many fungi, carbon
repression is mediated by the carbon catabolite repressor protein, CRE.
When the expression of an enzyme, normally produced during growth on
an inducing carbon source, is markedly or completely reduced by addition
of a known “repressing carbon source”, the enzyme (or encoding gene) is
subject to carbon catabolite repression (Ruijter and Visser 1997).
The
cre
gene has been isolated from a number of fi lamentous fungi (
A.
nidulans
,
A. niger
,
Botrytis cinerea
,
Humicola grisea
,
Sclerotinia sclerotiorum
,
Thermoascus aurantiacus
,
T. emersonii
,
Tr. harzianum
and
Tr. reesei
) and encodes
a transcription factor that contains two zinc fi ngers of the Cys
2
-His
2
type
(Dowzer and Kelly 1991, Strauss et al. 1995). The CREI/CREA proteins have
been shown to bind to the consensus sequence 5'-SYGGRG-3' (S = C or G,
Y = C or T and R = A or G) in the upstream region of the genes that they
regulate. CRE binding sites situated within the promoter regions of the
Tr.
reesei cbh1
and
xyn1
genes and the
A. nidulans xlnA
gene have been shown to
be functional
in vivo
(Ilmen et al. 1996a, Mach et al. 1996). This binding has
also been shown to be context dependent (Cubero and Scazzocchio 1994).
It has been speculated that direct CRE repression requires the presence of
two closely spaced CRE binding sites in order for repression to occur. This
theory has been supported by the fact that each of the functional binding
sites characterized to date has a second CRE site located close by (Ilmen et
al. 1996a, Mach et al. 1996, Orejas et al. 1999).
The activity of the CREI protein from
Sc. sclerotiorum
is controlled by its
nuclear translocation (Vautard-Mey et al. 1999). Subcellular localisation of
the
Sc. sclerotiorum
CREI protein was determined by Western blot analysis
of nuclear and cytosolic fractions; subcellular localisation of CREI was
observed to vary depending on the carbon source used. In cultures grown
on glucose, CREI was detected in the nucleus, whereas in cultures grown
on glucose plus pectin, an inducing carbon source, CREI was localized in
the cytosol. When the fungus was grown on pectin, only a weak signal for
