Biomedical Engineering Reference
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in expression of all the main cellulase and xylanase genes under inducing
conditions (Aro et al. 2003). The latter results suggest that the ACEI protein
is in fact a repressor of cellulase and xylanase expression. The difference
in results may be explained by the fact that the ACEI protein expressed
in yeast was truncated and lacked 242 amino acids from the
N
-terminus.
It is thought that the
N
-terminus may be of functional importance as it is
conserved among the putative ACEI proteins from
T. emersonii
,
A. nidulans
and
Neurospora crassa
(Aro et al. 2003).
More recent studies have shown ACEI to function as a negative regulator
of
xyn1
expression in
Tr. reesei
(Rauscher et al. 2006). EMSA analysis, carried
out using heterologously expressed proteins, revealed binding of Xyr1 and
ACEI to the
xyn1
upstream regulatory region. Moreover, binding assays
carried out using cell-free extracts from growth of a wild-type
Tr. reesei
strain and an
ace1
deletion strain on glucose found that ACEI participates
in a repression specifi c complex with the 5'-GGCTAA-3' motif and that
ACEI competes with Xyr1 for the downstream GGCTAA part of the motif
(5'-GGCTAAATGCGACATCTTAGCC-3').
It has been speculated that ACEI may also have functions other than
as a repressor of cellulase and xylanase expression. Evidence that
ace1
may
regulate the expression of genes other than cellulase and xylanase genes
was found when deletion of the
ace1
gene in
Tr. reesei
resulted in a clear
reduction in the growth of the fungus on sorbitol (Aro et al. 2003). Indeed,
it is thought that ACEI may play a more general regulatory role since the
stzA
gene of
A. nidulans
,
which shows 58% identity to the
ace1
gene of
Tr.
reesei
, has been deposited in the database as a gene encoding a protein that
alleviates sensitivity to salt and DNA damaging agents.
Activator of Cellulase Expression (ACEII)
The
ace2
gene was fi rst isolated from
Tr. reesei
using a yeast-based cloning
system that selected for factors binding to or activating the
cbh1
promoter
(Aro et al. 2001). The
ace2
gene encodes a protein of 341 amino acids and
has a predicted molecular mass of 38 kDa. It is a zinc binuclear cluster
protein belonging to a class of proteins that is exclusive to fungi. The
ace2
gene has not been isolated from any other species and the genomes of
A. nidulans, N. crassa
and
Magnaporthe grisea
do not appear to contain an
ace2
homologue (Aro et al. 2005).
Deletion of the
ace2
gene results in reduced expression of the
cbh1,
cbh2, egl1
and
egl2
genes when the fungus was grown on cellulose, thus
indicating that ACEII may act as an activator of cellulase expression (Aro et
al. 2001). This was further confi rmed by Northern analysis, which revealed
that
cbh1, cbh2
and egl2
transcript levels were all lower in the
ace2
deletion
strain. However, deletion of
ace2
did not have any effect on induction by