Analysis of esculin uptake by yeast expressing plant sucrose transporters. Yeast (SEY6210, except where indicated) transformed with sucrose transporter cDNAs StSUT1, AtSUC2, OsSUT1 or empty vector (pDR196) were incubated in 25 mM sodium phosphate buffer containing the fluorescent coumarin glucoside esculin. The cells were washed and fluorescence was determined at 367 nm excitation and 454 nm emission using a fluorescence plate reader. Cell density was determined by absorbance at 600 nm. Values are means for each treatment, normalized for cell density, ± SE (n = 4). (A) Effect of pH on esculin uptake. Yeast cells were incubated for 1 hr with 1 mM esculin at three pH values. (B) Effect of incubation time on esculin uptake. Yeast cells were incubated with 1 mM esculin at pH 4.0 for the indicated times. (C) Concentration dependence of esculin uptake. Yeast cells were incubated with different concentrations of esculin at pH 4.0 for 1 hr. (D) Esculin uptake by two yeast strains, SEY6210 and BY4742, transformed with sucrose transporter cDNAs. Yeast cells were incubated in 1 mM esculin at pH 4.0 for 1 hr.