MONOSACCHARIDE TRANSPORT-SYSTEMS IN YEAST RHODOTORULA-GLUTINIS

By using D-glucose, D-xylose, D-galactose and D-fructose in the strictly aerobic yeast R. glutinis and by comparing the half-saturation constants with inhibition constants the yeast was shown to possess a single common system for D-xylose and D-galactose (Km and Ki all between 0.5 and 1.1 mM) but another distinct transport system for D-fructose. The transport of D-glucose has a special position in that glucose blocks, apparently allotopically, all the other systems observed although it uses at least 1 of them for its own transport. The different character of D-glucose uptake is underlined by its relative independence of pH (its Km is completely pH-insensitive) in contrast with all other sugars. At low concentrations, all sugars show mutual positive cooperativity in uptake, suggesting at least 2 transport sites plus possibly a modifier site on the carrier.