Biomedical Engineering Reference
In-Depth Information
In 1979, a fast-growing cellulolytic fungus, P. decumbens 114-2, was isolated
from soil by scientists at Shandong University [
64
], which they then studied in
detail for almost 30 years [
6
-
8
,
44
,
64
,
75
,
77
,
91
-
97
]. Some cellulases (cel7B and
cel5A) from P. decumbens have been purified and characterized, establishing the
strain improvement strategy of this fungus [
93
,
95
]. The industrial production of
cellulolytic enzymes from this species has been conducted since 1996.
3.2 Mechanisms of Cellulase Synthesis
Apart from the development of economically feasible systems for cellulose deg-
radation, there has also been continuing interest in understanding the mechanisms
of cellulase synthesis and production to identify feasible approaches for increasing
cellulase production [
94
].
Sun et al. [
17
] investigated the proteome profiling map of the cellulases
secreted by T. reesei Rut C-30 using two-dimensional gel electrophoresis. CBH I
and CBH II were found to represent about 37% of the total extracellular proteins,
and the CBH II concentration produced with nonpretreated rice straw powder was
about threefold higher than that with alkali-treated straw. This interesting result
suggests that the synthesis of CBH II is controlled by other factors aside from
cellulose. Sun et al. [
96
] reported the differences in the composition and expression
levels of P. decumbens cellulases under induced and basal conditions. The basal
cellulase in P. decumbens was demonstrated to be composed of CBH I, CBH II,
EG I, EG II, and bG, whereas two EGs were expressed only under induction
conditions. Furthermore, the basal and induced EGs from P. decumbens were
encoded by different genes.
To understand the mechanism of the two transcription repressors Cre1 and ACE I
in T. reesei, Su et al. [
98
] developed a new strategy wherein a plasmid that encodes a
chimeric transcription activator containing the DNA binding domains from Cre1 and
ACE I and the effector domain from the activator ACE II was constructed and
transformed into T. reesei. The recombinant strain had higher cellulase activity than
its parent strain and had a different colony appearance. The results also provide an
overview of the set of genes that might be regulated by Cre1 or ACE I. These results
contribute to further understanding the regulatory roles of these two repressors in
cellular pathways and provided a new method for strain improvement through
genetic manipulation.
Liu et al. [
99
] studied the differences in gene sequences of CBH I gene (cbh1)
from wild-type and mutant P. decumbens strains and found that the mutant strain
JU-A10 is a multiple mutant of the wild-type strain in the sequences upstream of
the gene. The enhanced CBH activity of the mutant may be due to a single base
mutation of the upstream sequence of cbh1, which affects the transcription regu-
lation of the mutant instead of the protein coding sequences. This discovery
suggests the critical role of the promoter region of cellulose-encoding genes,
which is helpful in constructing hyperproducing strains of P. decumbens.An
apparent abolishment of glucose repression was also identified in strain JU-A10,
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