A B S T R A C T
Economical biofuel production from plant biomass requires the conversion of both cellulose and
hemicellulose in the plant cell wall. The best industrial fermentation organism, the yeast Saccharomyces
cerevisiae, has been developed to utilize xylose by heterologously expressing either a xylose reductase/
xylitol dehydrogenase (XR/XDH) pathway or a xylose isomerase (XI) pathway. Although it has been
proposed that the optimal means for fermenting xylose into biofuels would use XI instead of the XR/XDH
pathway, no clear comparison of the best publicly-available yeast strains engineered to use XR/XDH or XI
has been published. We therefore compared two of the best-performing engineered yeast strains in the
public domain—one using the XR/XDH pathway and another using XI—in anaerobic xylose
fermentations. We
find that, regardless of conditions, the strain using XR/XDH has substantially higher
productivity compared to the XI strain. By contrast, the XI strain has better yields in nearly all conditions
tested.