Biomedical Engineering Reference
In-Depth Information
Several CREI/CREA mutants have been isolated to date. In A. nidulans ,
the most severe mutation cre A30, which results from a pericentric inversion
of chromosome I, gives rise to a truncated creA gene (Arst et al . . 1990). In Tr.
reesei , the mutant strain rut C30 contains a truncated CREI that only has one
zinc fi nger. This strain is well-known for its overproduction of cellulolytic
enzymes, with most of the cellulase and many of the hemicellulase genes
being expressed to some extent on glucose. Transformation of an intact cre1
gene into the rut C30 mutant strain of Tr. reesei restored glucose repression,
thereby abolishing overproduction of cellulases and hemicellulases and
indicating that the expression of these enzymes is mediated by CREI
(Ilmen et al. 1996b). As yet, no complete loss-of-function creA mutant has
been isolated from Aspergillus sp. (Dowzer and Kelly 1991). The deletion
of the creA gene in A. nidulans appears to be lethal. It is possible that total
de-repression of all systems normally under CREA control is lethal to the
cell, thus explaining why a loss-of-function mutant has not been isolated
to date. It is also possible that CREA has roles beyond acting as a repressor
and may also have some positive function(s) (Dowzer and Kelly 1991).
Xylanase Regulator (XlnR/Xyr1)
The transcriptional regulator XlnR was fi rst isolated from A. niger (van Peij
et al . . 1998). XlnR is a zinc binuclear cluster protein and a member of the
GAL4 family of transcription factors. It is a positively acting transcription
factor and a central regulator that controls the transcription of more
than 20 genes encoding hemicellulases and cellulases in Aspergillus . In
A. niger , XlnR has been shown to positively regulate the expression
of two endoxylanases, a β-xylosidase, an α-glucuronidase, an acetyl
xylan esterase, an arabinoxylan arabinofuranohydrolase and a feruloyl
esterase (van Peij et al. 1998). XlnR also regulates the expression of two
cellobiohydrolases (Gielkens et al . , 1999), three endoglucanases (van Peij
et al. 1998, Hasper et al. 2002) and an α- and a β-galactosidase (de Vries et
al. 1999a). In addition, XlnR has been shown to regulate the expression of
an intracellular enzyme, D-xylose reductase, which is involved in xylose
catabolism; this is the fi rst example of a link between the regulation of
intracellular and extracellular catabolic enzymes in fi lamentous fungi
(Hasper et al. 2000).
The functional domains of the XlnR protein include a N-terminal zinc
binuclear cluster domain which forms the DNA-binding domain, a central
coiled-coil domain that may be involved in nuclear translocation and a
C-terminal domain (van Peij et al. 1998). The DNA-binding domain of XlnR
has been shown to bind to the sequence 5'-GGCTAA-3'. Studies have also
shown that variation within the six base pair site still allows binding as
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