ABSTRACT
The glucose/xylose facilitator Gxf1 from Candida intermedia was expressed in the recombinant xylose-fermenting Saccharomyces cerevisiae strain TMB 3057. The new strain, TMB 3411, displayed approximately two times lower K (m) for xylose transport compared to a control strain not expressing Gxf1. In aerobic batch cultivation, the specific growth rate was significantly higher at low xylose concentration, 4 g/L, when Gxf1 was expressed, whereas it remained unchanged at high xylose concentration, 40 g/L. Similarly, in aerobic-xylose-limited chemostat culture, the Gxf1-expressing strain consumed more xylose than the control strain at low dilution rates (low xylose concentration), whereas the situation was reversed at higher dilution rates (high xylose concentration). Also, under anaerobic conditions, the Gxf1-expressing strain showed faster xylose uptake and ethanol formation at low substrate concentrations. The results are discussed in relation to previous observations, which suggested that transport controlled xylose utilization in recombinant xylose-utilizing S. cerevisiae only at low xylose concentrations.