Strains of Saccharomyces cerevisiae have been engineered to utilize xylose by expressing either the genes for xylose reductase and xylitol dehydrogenase, or for xylose isomerase.  These strains still use xylose at sub-optimal rates for industrial fermentation.  Unlike natural xylose fermenting yeasts such as Pichia stipitis, S. cerevisiae does not contain xylose-specific transport systems.  Analysis of strains that have been adapted for enhanced growth on xylose indicates increased expression of hexose transporters and suggests that xylose transport is one of the limiting steps in xylose utilization.  Since the increase in xylose transport in these strains results from increases in hexose transporters, xylose uptake remains inhibited by glucose, thus limiting xylose and glucose co-consumption.

The goal of this study was to determine the effect of a xylose transport system on xylose/glucose co-consumption and total xylose consumption.  We expressed two heterologous transporters from Arabidopsis thaliana in xylose-utilizing S. cerevisiae strains.  Strains expressing the heterologous transporters were grown aerobically on glucose and xylose mixtures.  Sugar consumption rates and product accumulation were determined and compared to a control strain not expressing the A. thaliana xylose transport genes.  Our results from aerobic cultivation with glucose/xylose mixtures indicate that expression of the transporters increased xylose co-consumption rates (prior to glucose depletion) by up to 100%.  Increased xylose co-consumption also correlated with increased ethanol concentration, yield and productivity.  It was concluded that in these strains, xylose transport is a limiting factor for xylose utilization and that increasing xylose/glucose co-consumption is a viable strategy for improving xylose fermentation.