TRANSIENT RESPONSES OF CANDIDA-UTILIS TO OXYGEN LIMITATION - REGULATION OF THE KLUYVER EFFECT FOR MALTOSE

The facultatively fermentative yeast Candida utilis exhibits the Kluyv er effect for maltose: this disaccharide is respired and assimilated b ut, in contrast to glucose, it cannot be fermented. To study the mecha nism of the Kluyver effect, metabolic responses of C. utilis to a tran sition from aerobic, sugar-limited growth to oxygen-limited conditions were studied in chemostat cultures. Unexpectedly, the initial respons e of maltose-grown cultures to oxygen limitation was very similar to t hat of glucose-grown cultures. In both cases, alcoholic fermentation o ccurred after a lag phase of 1 h, during which glycerol, pyruvate and D-lactate were the main fermentation products. After ca. 10 h the beha viour of the maltose- and glucose-grown cultures diverged: ethanol dis appeared from the maltose-grown cultures, whereas fermentation continu ed in steady-state, oxygen-limited cultures grown on glucose. The disa ppearance of alcoholic fermentation in oxygen-limited chemostat cultur es growing on maltose was not due to a repression of the synthesis of pyruvate decarboxylase and alcohol dehydrogenase. The results demonstr ate that the Kluyver effect for maltose in C. utilis does not reflect an intrinsic inability of this yeast to ferment maltose, but is caused by a regulatory phenomenon that affects a key enzyme in maltose metab olism, probably the maltose carrier. The observed kinetics indicate th at this regulation occurs at the level of enzyme synthesis rather than via modification of existing enzyme activity.