COORDINATION OF SUCROSE UPTAKE AND RESPIRATION IN THE YEAST DEBARYOMYCES-YAMADAE

Screening in batch cultures identified Debaryomyces yamadae as a yeast that exhibits the Kluyver effect for sucrose: this disaccharide can b e respired but, even under oxygen-limited conditions, alcoholic fermen tation of sucrose does not occur. Ethanol, glycerol and arabitol were the main fermentation products during oxygen-limited growth on glucose in chemostat cultures. None of these fermentation products were produ ced in oxygen-limited chemostat cultures grown on sucrose and the frac tion of the sucrose that could not be respired remained unused in the culture medium. This absence of alcoholic fermentation was not due to repression of the key fermentative enzymes pyruvate decarboxylase and alcohol dehydrogenase. In contrast to some other yeasts that exhibit a Kluyver effect, D. yamadae did not exhibit a preference for ethanol i n batch cultures grown on mixtures of ethanol and sucrose. Sucrose met abolism in D. yamadae involves intracellular hydrolysis by an alpha-gl ucosidase. Incubation of weakly buffered cell suspensions with sucrose led to a rapid transient alkalinization, indicating the presence of a sucrose-proton symport system. The apparent substrate saturation cons tant of the sucrose-uptake system was 0.2 mmol l(-1). Sucrose-dependen t alkalinization rates were much lower in samples from oxygen-limited cultures than in samples from aerobic cultures. Transient responses of D. yamadae to oxygen limitation were investigated by applying a sudde n decrease in the oxygen feed to aerobic sugar-limited chemostat cultu res. In glucose-grown cultures, this led to alcoholic fermentation and no significant accumulation of sugar occurred after the switch. In su crose-limited cultures, sugar accumulation occurred instantaneously af ter the switch, and ethanol formation was virtually absent. The result s indicate that the Kluyver effect for sucrose in D. yamadae, i.e. the adjustment of the glycolytic flux to the cells' respiratory capacity, is effected by rapid down-regulation of the capacity of the sucrose c arrier under oxygen-limited conditions.