Biomedical Engineering Reference
In-Depth Information
XlnR recognized the sequence 5'GGCTAG-3' within the upstream region of
aguA
in
A. niger
(de Vries et al. 2002b), while in
A. oryzae
, XlnR recognized
the sequence 5'-GGCTRA-3' in the promoter region of the xylanase gene,
xynF1
(Marui et al. 2002).
An ortholog of XlnR, known as Xyr1, has been isolated from
Tr. reesei
(Rauscher et al. 2006). Xyr1 also functions as a general activator of hydrolases
and has been shown to be involved in the activation of a number of cellulase
and hemicellulase genes including
cbh1
,
cbh2
,
egl1
,
bg1
,
xyn1
,
xyn2
and
bxl1
(Rauscher et al. 2006, Stricker et al. 2006). Functional binding sequences
for the Xyr1 transcription factor were fi rst identifi ed in the
xyn1
promoter
where Xyr1 was found to bind to an inverted repeat of GGCTAA separated
by a 10 bp spacer (Rauscher et al. 2006). Xyr1 has since been found to bind
to the
xyn2
promoter (Stricker et al. 2008). More recent studies have shown
that Xyr1 can interact not only with the 5'-GGCTAA-3' motif but also with
several 5'-GGC(A/T)(3)-3' motifs (Furukawa et al. 2009).
Studies aimed at deciphering the transcriptional regulation of the
xyr1
gene itself have shown that
xyr1
transcription is not activated via a specifi c
inducer molecule (Mach-Aigner et al
.
2008). Instead,
xyr1
expression is
subject to carbon catabolite repression mediated by CREI. In addition, to
this CREI-dependent regulation,
xyr1
transcription has also been shown
to be repressed by ACEI. This second negative regulatory effect has been
described as a “double-double” lock system. Indeed, both ACEI and ACEII
have been proposed as modulators of
xyr1
expression as studies have shown
that deletion of the
aceI
or
aceII
gene leads to increased
xyr1
expression
(Mach-Aigner et al. 2008, Stricker et al. 2008).
Activator of Cellulase Expression I (ACEI)
The
aceI
gene was fi rst isolated from
Tr. reesei
using a genetic selection
method, which was designed to identify transcription factors binding
to, or activating, the
Tr. reesei cbh1
promoter in yeast (Saloheimo et al.
2000). ACEI is a zinc fi nger transcription factor which contains three
Cys
2
-His
2
-type zinc fi ngers. The zinc fi nger region of the
ace1
gene was
shown to bind to the
Tr. reesei cbh1
promoter when expressed as a GST
fusion protein in
E. coli
. The recombinant ACEI protein bound to eight
sites within the
cbh1
upstream region. Binding was observed at all sites
containing the sequence 5'-AGGCAAA-3' and at some sites containing
only the core sequence 5'-AGGCA-3', which were preceded by an A-T
rich region (Saloheimo et al. 2000).
Initially, ACEI was believed to be an activator of cellulase expression as
deletion of the
ace1
gene led to activation of expression of the
Tr. reesei cbh1
promoter in yeast (Saloheimo et al. 2000). Subsequent investigation revealed
that deletion of the
ace1
gene in
Tr. reesei
resulted in a 20-30 fold increase
