Pentose-fermenting Escherichia coli [32] and Klebsiella
oxytoca [33] have been generated by introducing ethanol ogenic
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 xylose metabolizing
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).