Biomedical Engineering Reference
In-Depth Information
Transcription Factors Regulating Plant Cell Wall
Degrading Enzymes
As already mentioned, the production of large quantities of extracellular
enzymes for the hydrolysis of plant polysaccharides is an energetically-
demanding process and therefore, these enzymes are only produced when
it is necessary for the fungus to use plant polysaccharides as an energy
source. Consequently, the expression of these enzymes is tightly regulated
in a carbon source dependent manner. To date, several transcription factors
affecting the expression of these genes have been identifi ed including
CREA/CREI (Catabolite repressor element), XlnR/Xyr1 (Xylanase
regulator), ACEI (Activator of cellulases I), ACEII (Activator of cellulases
II) and the Hap2/3/5 complex. Of these regulators, the best studied is
CREA, which has been shown to mediate glucose repression in many
ascomycetous fungi. The consensus binding sequences for these regulators
have been described and functional binding sites have been characterized
in a small number of promoters (Ilmen et al. 1996a, Takashima et al. 1996,
van Peij et al. 1998, Aro et al. 2001, Marui et al. 2002, Zeilinger et al. 2003,
Rauscher et al. 2006). Most of the data available to date describing the
regulatory pathways affecting expression of plant cell wall degrading
enzymes in fi lamentous fungi has come from studies conducted using
Tr. reesei
or
Aspergillus sp
. Consequently, much remains to be elucidated
concerning the regulation of polysaccharide degrading enzymes in other
fi lamentous fungi.
The factors identifi ed to date regulating plant cell wall degrading
enzymes in fi lamentous fungi include GAL4 family proteins (XlnR and
ACEII), Cys
2
-His
2
type zinc fi nger proteins (CREA, ACEI and PacC) and
CCAAT-box binding proteins (Hap2/3/5 complex). The Zn(II)2Cys6
binuclear cluster DNA-binding domain was first identified in the
Saccharomyces cerevisiae
GAL4 protein. These DNA-binding proteins are
exclusive to fungi and are typically, but not exclusively, transcriptional
activators (Todd and Andrainopoulos 1997). The Cys
2
-His
2
zinc fi nger
transcription factors belong to a well known class of DNA-binding proteins
known as zinc fi nger proteins. The name is derived from the fact that two
histidine and two cysteine residues co-ordinate with a zinc ion to form a
fi nger structure that is capable of binding DNA (Gommans et al. 2005).
CCAAT elements have been identifi ed in 30% of eukaryotic promoters
ranging from fungi to humans. CCAAT elements are bound by a conserved
multimeric protein complex, with each complex being named according
to the organism from which it was isolated, i.e., Hap2/3/4/5 complex of
S. cerevisiae
, Hap2/3/5 of
Tr. reesei
and AnCF of
A. nidulans
(Mantovani
1998, Kato 2005).
