Pentose-fermenting Escherichia coli [32] and Klebsiella
oxytoca [33] have been generated by introducing ethanologenic
genes from Zymomonas mobilis (Figure 2). At the
same time, the first xylose-fermenting S. cerevisiae strain
was generated through the introduction of genes for xylosemetabolizing
enzymes from P. stipitis [34] (Figure 2). Later
xylose-fermenting strains of S. cerevisiae were constructed
by introducing the genes encoding xylose isomerase from
the bacterium Thermus thermophilus [35] and the anaerobic
fungus Piromyces sp. [36], respectively. For xyloseusing
S. cerevisiae, high ethanol yields from xylose also
require metabolic engineering strategies to enhance the
xylose flux. This was independently demonstrated in an
SSF set-up for corn stover, where glucose and xylose were
fermented simultaneously [37], and in a recombinant
strain with increased carbon flux [38]. Z. mobilis also
efficiently produces ethanol from the hexose sugars glucose
and fructose but not from pentose sugars, although a xylose
fermenting Z. mobilis was generated by introducing a
xylose-metabolizing pathway from E. coli [39]. More
recently, the obligatory anaerobic bacterium Thermoanaerobacterium
saccharolyticum has been genetically engineered
for improved ethanolic fermentation (Joe Shaw
et al., oral presentation, Nashville, 2006).
Pentose-fermenting Escherichia coli [32] and Klebsiellaoxytoca [33] have been generated by introducing ethanologenicgenes from Zymomonas mobilis (Figure 2). At thesame time, the first xylose-fermenting S. cerevisiae strainwas generated through the introduction of genes for xylosemetabolizingenzymes from P. stipitis [34] (Figure 2). Laterxylose-fermenting strains of S. cerevisiae were constructedby introducing the genes encoding xylose isomerase fromthe bacterium Thermus thermophilus [35] and the anaerobicfungus Piromyces sp. [36], respectively. For xyloseusingS. cerevisiae, high ethanol yields from xylose alsorequire metabolic engineering strategies to enhance thexylose flux. This was independently demonstrated in anSSF set-up for corn stover, where glucose and xylose werefermented simultaneously [37], and in a recombinantstrain with increased carbon flux [38]. Z. mobilis alsoefficiently produces ethanol from the hexose sugars glucoseand fructose but not from pentose sugars, although a xylosefermenting Z. mobilis was generated by introducing axylose-metabolizing pathway from E. coli [39]. Morerecently, the obligatory anaerobic bacterium Thermoanaerobacteriumsaccharolyticum has been genetically engineeredfor improved ethanolic fermentation (Joe Shawet al., oral presentation, Nashville, 2006).
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