REGULATION OF ALCOHOLIC FERMENTATION IN BATCH AND CHEMOSTAT CULTURES OF KLUYVEROMYCES-LACTIS CBS-2359

Kluyveromyces lactis is an important industrial yeast, as well as a po pular laboratory model. There is currently no consensus in the literat ure on the physiology of this yeast, in particular with respect to aer obic alcoholic fermentation ('Crabtree effect'). This study deals with regulation of alcoholic fermentation in K. lactis CBS 2359, a propose d reference strain for molecular studies. In aerobic, glucose-limited chemostat cultures (D=0.05-0.40 h(-1)) growth was entirely respiratory , without significant accumulation of ethanol or other metabolites. Al coholic fermentation occurred in glucose-grown shake-flask cultures, b ut was absent during batch cultivation on glucose in fermenters under strictly aerobic conditions. This indicated that ethanol formation in the shake-flask cultures resulted from oxygen limitation. Indeed, when the oxygen feed to steady-state chemostat cultures (D=0.10h(-1)) was lowered, a mixed respirofermentative metabolism only occurred at very low dissolved oxygen concentrations (less than 1% of air saturation). The onset of respirofermentative metabolism as a result of oxygen limi tation was accompanied by an increase of the levels of pyruvate decarb oxylase and alcohol dehydrogenase. When aerobic, glucose-limited chemo stat cultures (D=0.10 h(-1))were pulsed with excess glucose, ethanol p roduction did not occur during the first 40 min after the pulse. Howev er, a slow aerobic ethanol formation was invariably observed after thi s period. Since alcoholic fermentation did not occur in aerobic batch cultures this is probably a transient response, caused by an imbalance d adjustment of enzyme levels during the transition from steady-state growth at mu=0.10 h(-1) to growth at mu(max),,,. It is concluded that in K. lactis, as in other Crabtree-negative yeasts, the primary enviro nmental trigger for occurrence of alcoholic fermentation is oxygen lim itation. (C) 1998 John Wiley & Sons, Ltd.