Biomedical Engineering Reference
In-Depth Information
the
expression of (hemi)cellulolytic genes. However
if the concentration
of xylose is high, CreA-mediated
repression occurs, resulting in reduced
expression of these genes. The repression effect is stronger
in the presence of
glucose, which largely prevents the expression of (hemi)cellulolytic
genes.
XlnR-mediated expression
of other (hemi)cellulolytic genes results in the
release of arabinose,
cellobiose, ferulic acid and galactose by the enzymes
encoded by these genes. These compounds induce the expression
of other
genes (de Vries and Visser 2001).
Analysis of the promoter region of
H. Jecorina
β-galactosidase identifi ed
six sites for the CCAAT-binding Hap 2/3/5 complex (Seiboth et al. 2005).
This sequence is known as an upstream activating sequence in higher
eukaryotes (Johnson and McKnight 1989) as well as in fi lamentous fungi
(Littlejohn and Hynes 1992), (Nagata et al. 1993). With respect to fungi, the
only CCAAT box binding protein characterized in detail is the
S. cerevisiae
HAP complex, which consists of at least three subunits (Pinkham and
Guarente 1985); (McNabb et al. 1997). Genes, which have been shown
to be positively regulated by HAP-like complexes in fungi include
cbh2
from
Trichoderma reesei
(or
Hypocrea jecorina
) (Zeilinger et al. 1998), taka-
amylase (Kato et al. 1997) and isopencillin N synthase (Bergh et al. 1996)
from
A. nidulans
. The promoter region of
H. Jecorina
β-galactosidase
gene also contained three single sites and one double site for the carbon
catabolite repressor Cre1 and one site each for the cellulose and xylanase
repressor Ace 1. No consensus binding sites for XlnR were found.
H.
Jecorina
β-galactosidase transcript was most abundant during growth of
H.
Jecorina
on L-arabinose and L-arabinitol and was present at lower levels on
D-galactose and D-xylose. The expression level on lactose was low relative
to that on D-galactose and D-xylose and no transcript accumulated during
growth on D-glucose and glycerol. In a Cre1-depressed
H. Jecorina
strain
basal levels of the β-galactosidase transcript accumulated during growth on
D-glucose, glycerol and D-xylose and high levels were found during growth
on D-galactose and lactose. These results indicate that Cre1 dependent
carbon catabolite repression interferes with
H. Jecorina
β-galactosidase
transcription depending on the carbon source at either the basal or induced
level of transcription (Seiboth et al. 2005).
Evidence has been obtained for the role of galacturonic acid as a general
inducer of pectinolytic enzymes
in
A. niger
. Several genes encoding pectin
main-chain cleaving
enzymes (
pelA, plyA, pgaX
and
rglA
) and a gene encoding
pectin
methylesterase (
pmeA
) are expressed in the presence of galacturonic
acid (Parenicova 2000). Genes encoding arabinofuranosidases (
abfA
and
abfB
),
endoarabinase (
abnA
), endogalactanase (
galA
) and β-galactosidase
(
lacA
), all of which act on pectin side-chains, are also expressed
on
galacturonic acid (de Vries et al. 2002). Apart from transcriptional activation
by XlnR (xylan degradation) and induction on galacturonic acid (released
