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
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