Biobutanol - Biotechnology in China: Biofuels and Bioenergy - page 92

lignocelluloses can reduce the costs of substrates. WhenD-glucose and D-xylose
are simultaneously present in the medium, utilization of D-xylose is severely
inhibited byD-glucose [ 40 ], a phenomenon called ''carbon catabolite repres-
sion'' (CCR) [ 37 ]. To enhance efficient utilization of lignocellulosic hydroly-
sates for ABE production, Ren et al. [ 43 ] attempted to reduce or eliminate
CCR in C. acetobutylicum. The gene CAC3037 in C. acetobutylicum ATCC824
was firstly confirmed as the ccpA gene encoding CcpA by the complementary
investigation in a mutant of B. subtilis. Then CAC3037 was disrupted using
the group II intron method [ 45 ], generating mutant strain 824ccpA. Under
pH-controlled conditions, 824ccpA can use a mixture of D-xylose andD-glucose
simultaneously without CCR. When the ratio ofD-glucose to D-xylose was 1:1,
the strain of 824ccpA showed the maximal solvent titers (acetone, butanol and
ethanol are 4.94, 12.05 and 1.04 g/L, respectively), which was almost the same
solvent levels as with maize- or molasses-based fermentation by wild-type.
Disruption of ccpA offers a successful genetic modification strategy for
simultaneously utilizing the sugars in lignocellulosic materials by Clostridium,
which is essential for further exploitation of lignocellulose for the production of
solvents and biofuels. In addition, the gene talA, which encodes transaldolase in
E. coli K-12, was cloned and overexpressed in C. acetobutylicum ATCC824
[ 16 ]. The resulting strain 824-TAL showed improved ability for xylose utili-
zation and solvent production using xylose as the sole carbon source compared
with C. acetobutylicum ATCC824. It suggested that transaldolase is weak in the
pentose phosphate pathway (PPP) of C. acetobutylicum. The xylose utilization
pathway and regulons in C. acetobutylicum have been well studied by Gu et al.
via a comparative genomic approach, providing comprehensive insights into
xylose catabolism and its regulation [ 15 ].
Besides the engineering work by Chinese researchers, scientists in other
countries have also reported the metabolic engineering of other key metabolic
pathway genes involved in solvent production, such as adc, ctfAB, adhE, pta,
buk, and ctfB, transcriptional regulator solR and spo0A, genes for molecular
pumps, and chaperones such as groES, dnaKJ, hsp18, and hsp90 [ 29 ].
These studies help understanding of the physiology and metabolism of
C. acetobutylicum, based on which Chinese scientists are attempting further
metabolic engineering. Chinese researchers are also trying to solve some key
problems in butanol production, including (1) improving the capability to uti-
lize a variety of low-price and non-cereal substrates by genetic modification
and adaptation; (2) eliminating byproducts so as to obtain the sole butanol-
producing strain by metabolic engineering; (3) improving butanol tolerance
by genome-scale evolution, and (4) increasing productivity by physiological
functionality engineering.