Biomedical Engineering Reference
In-Depth Information
sophorose, the proposed inducer of cellulases in
Tr. reesei
, thus suggesting
that the induction of cellulases by sophorose and cellulose may activate
partially different regulatory pathways.
ACEII binds to the consensus sequence 5'-GGSTAA-3' (Aro et al. 2001,
Wurleitner et al. 2003). ACEII binding sites have been found in many of
the major cellulase and xylanase promoters in
Tr. reesei
. For example, the
cbh1
and
cbh2
promoter regions contain six and three sites, respectively,
the
egl1
promoter contains one site and the
xyn1
promoter contains two
sites. To date, ACEII has been shown to bind to the consensus sequence
5'-GGSTAA-3' in the
Tr. reesei
cbh1
and
xyn2
promoters (Aro et al. 2001,
Wurleitner et al. 2003).
The effect of
ace2
deletion on xylanase expression has also been
investigated. Expression levels of the
xyn1
gene remained unchanged,
whereas
xyn2
expression was reduced to 30-45% of that of the wild type
(Aro et al. 2001). The consensus sequence 5'-GGGTAAATTGG-3', known as
the xylanase-activating element (XAE), has been shown to be responsible
for the binding of ACEII and the Hap2/3/5 complex to the
xyn2
promoter,
with ACEII binding to the 5'-GGTAAA-3' motif (Wurleitner et al. 2003).
In vivo
footprinting demonstrated that these trans-acting factors are bound
under repressing and inducing conditions and that both basal transcription
and induction is dependent on their binding. Mutations within the
ACEII binding site (AA
-217/-218
to TT and a G
-221
to T) led to complete loss
of ACEII binding to its target in the XAE and completely abolished both
basal and inducible
xyn2
expression. More recent studies have established
that ACEII is responsible for the basal level of
xyn2
transcription and also
that ACEII can regulate
xyn2
expression both directly by binding to the
xyn2
promoter or indirectly by having an impact on the expression of
xyr1
(Stricker et al. 2008).
Hap2/3/5 Complex
The pentanucleotide sequence, 5'-CCAAT-3', is found in the 5' regions of
approximately 30% of eukaryotic genes. The sequence may be found in
the forward or reverse direction and is usually located between -60 bp and
-100 bp from the start point of transcription (Mantovani 1998, Brakhage et
al. 1999). The frequency of occurrence of the CCAAT sequence is higher
in TATA-less promoters than in TATA-containing promoters. In TATA-
containing promoters, the CCAAT box is normally located between -80
bp and -100 bp and not closer than -50 bp to the start codon. In TATA-less
promoters, the 5'-CCAAT-3' sequence is found closer (-66 bp on average)
to the ATG. A conserved multimeric protein complex binds to CCAAT
sequences present in the promoters of eukaryotic genes ranging from fungi