TRANSPORT OF HEMICELLULOSE MONOMERS IN THE XYLOSE-FERMENTING YEAST CANDIDA-SHEHATAE

Cells of Candida shehatae repressed by growth in glucose- or D-xylose-medium produced a facilitated diffusion system that transported glucose (Ks .+-. 2 mM, Vmax .+-. 2.3 mmoles g-1 h-1), D-xylose (Ks .+-. 125 mM, Vmax .+-. 22.5 mmoles g-1 h-1) and D-mannose, but neither D-galactose nor L-arabinose. Cells derepressed by starvation formed several sugar-proton symports. One proton symport accumulated 3-0-methylglucose about 400-fold and transported glucose (Ks .+-. 0.12 mM, Vmax .+-. 3.2 mmoles g-1 h-1) and D-mannose, a second proton symport transported D-xylose (Ks .+-. 1.0 mM, Vmax 1.4 mmoles g-1 h-1) and D-galactose, while L-arabinose apparently used a third proton symport. The stoicheiometry was one proton for each molecule of glucose or D-xylose transported. Substrates of one sugar proton symport inhibited non-competitively the transport of substrates of the other symports. Starvation, while inducing the sugar-proton symports, silenced the facilitated diffusion system with respect to glucose transport but not with respect to the transport of D-xylose, facilitated diffusion functioning simultaneously with the D-xylose-proton symport.