P170: New genotypes of genetically engineered Saccharomyces cerevisiae with heterologous xylose transporters improve xylose uptake and ethanol production

Genetically engineered Saccharomyces cerevisiae strains often show limited xylose utilization and no satisfactory strain is currently available for sustainable cellulosic ethanol production. Using the  genetically engineered S. cerevisiae strain NRRL Y-50049-YXI with a chromosomally integrated yeast codon-optimized xylose isomerase and industrial yeast background as a host, we developed six new genotypes using heterologous xylose transporter genes from Scheffersomyces stipitis namely Y-50049-YXI-XUT4, -XUT5, -XUT6, -XUT7, -RGT2, and -SUT4, respectively. While a functional YXI is necessary to enable xylose utilization for Y-50049, introduction of the heterologous xylose transporter genes further improved both growth and fermentation rates as well as biomass and ethanol accumulations for all newly developed genotypes of S. cerevisiae. Under aerobic conditions, specific growth rate improved 1.1 to 1.5-fold; biomass accumulations, 2.5- to 6-fold, and volumetric xylose uptake rates, 3.5- to 7.5-fold for the new genotypes compared to the parent Y-50049-YXI. Genotypes Y-50049-YXI-XUT7 and -RGT2 displayed the highest ethanol accumulation of 17.8-18.4 g l^-1 in a medium containing 24.3 g/L D-glucose and 32.5 g/L D-xylose: double the amount of ethanol produced by the parent strainY-50049-YXI. All new genotypes, except one, displayed improved xylose affinity as indicated by apparent xylose affinities ranged from K_m 2.47 to 8.74 g/L and maximum specific xylose uptake rates from V_max 0.26 to 0.88 g/g/h when compared with 13.26 g/L and 0.14 g/g/h, respectively, for the parent strain Y-50049-YXI.