Construction Of A Glucose And Xylose Co-Fermenting Industrial Saccharomyces Cerevisiae By Expression Of Codon-Optimized Fungal Xylose Isomerase

Recombinant Saccharomyces cerevisiae with the xylose reductase/xylose dehydrogenase pathway is often not well-adapted for growth on xylose, due to cofactors regeneration and balance limitations. Alternatively, several fungal xylose isomerase (xylA) genes were reported successfully expressed in S. cerevisiae. To verify the potential of such an application, and optimize the xylose isomerase expression level, a codon-optimized fungal xylA was integrated into the genome of an industrial S. cerevisiae. The host strain cannot survive in media with xylose as the sole carbon source when oxygen is limited. With xylA expression, the modified strain MYI can grow on xylose when oxygen-limited, utilizing more than 90% of xylose. With initial xylose concentration of 46.1 g/L, strain MYI produced 11.4 g/L ethanol, 0.39 g/L glycerol, and 0.078 g/L xylitol in 168 h. During co-fermentation of 20 g/L glucose and 30 g/L xylose, the final ethanol concentration reached 14.5 g/L with a yield of 0.3 g/g on consumed xylose. As expected, the final concentrations of xylitol were very low, 0.078 g/L for xylose fermentation and 0.3 g/L for co-fermentation. These results indicate that S. cerevisiae expressing codon-optimized xylose isomerase does circumvent the redox imbalance and convert xylose into xylulose. Specific ethanol productivity may be limited by single copy integration of xylA, and low activity of the endogenous xylulose kinase; further investigation and engineering may be able to overcome these limitations.