The effect of the glucose analogue 5-thio-~-glucose (5TG) on the yeast Saccharomyces cerevisiae
was studied. Derepression of mitochondria1 respiratory chain cytochromes, alcohol dehydrogen-
ase (isoenzyme 11), NADH dehydrogenase and maltase was inhibited by 0.5-2 ~M-~TG.
Growth rate was only slightly affected. Ethanol was efficiently produced with 2 rnw5TG in
medium initially containing 0.25 % glucose. Mutants resistant to the growth inhibitory effects of
5TG on glycerol medium showed resistance to the catabolite repressing effects of glucose. Other
mutants, known to be catabolite repression resistant, showed resistance to 5TG. The analogue
seems to inhibit derepression of glucose repressible enzymes with greater potency than glucose
itself.