P130: Expression of novel bacterial xylose isomerases in Saccharomyces cerevisiae

Saccharomyces yeasts are currently used for industrial production of ethanol by fermentation of glucose derived from corn grain.  These yeasts do not naturally consume the five-carbon sugars contained in the hemicellulose component of biomass feedstocks.  Enzymes from pentose-assimilating yeasts have been transferred to S. cerevisiae allowing fermentation of xylose, the most abundant biomass-derived pentose sugar.  However, efficient conversion of xylose to ethanol is limited by cellular redox imbalance resulting from different co-factor specificities of the first two enzymes commonly used for xylose utilization (i.e., the xylose reductase and xylitol dehydrogenase enzymes).  Replacing these two enzymes with a single, co-factor independent, xylose isomerase enzyme (XI) is proposed to alleviate this redox imbalance and improve xylose fermentation.  A limited number of xylose isomerases have been demonstrated to function when expressed in S. cerevisiae.  The goal of this study was to identify additional novel XI’s that function in S. cerevisiae.  Xylose isomerase genes were isolated from human and ruminant intestinal bacteria by polymerase chain reaction (PCR) amplification with degenerate primers.  These XI genes were expressed in a S. cerevisiae strain that also expressed the S. cerevisiae xylulokinase gene at elevated levels.  The best-performing XI was chromosomally-integrated and this strain was further adapted for improved xylose utilization.  Xylose isomerase activities for these new enzymes, along with analysis of xylose metabolism for the different XI-expressing S. cerevisiae strains, will be presented.