XtrD has higher affinity for xylose than glucose. (A) S. cerevisiae EBY.VW4000 containing the xrtD gene or harboring the empty expression vector were grown for 140 h at 30°C in YNB medium plus 0.2% glucose and increasing xylose concentrations. (B) Growth rate of the experiments are shown in (A). (C) Glucose and xylose substrate specificities of the XtrD transporter. Glucose and xylose specificities of XtrD were determined in yeast cells (strain EBY.VW4000) expressing the respective cDNA. Relative transport levels were determined in the absence of a competitor or in the presence of increasing concentrations of xylose (n = 3, ±, standard deviation). The results are expressed as the percentage of inhibition of the transport with radiolabeled glucose.