REGULATION OF CARBON METABOLISM IN CHEMOSTAT CULTURES OF SACCHAROMYCES-CEREVISIAE GROWN ON MIXTURES OF GLUCOSE AND ETHANOL

Growth efficiency and regulation of key enzyme activities were studied in carbon- and energy-limited chemostat cultures of Saccharomyces cer evisiae grown on mixtures of glucose and ethanol at a fixed dilution r ate. Biomass yields on substrate carbon and oxygen could be adequately described as the net result of growth on the single substrates. Activ ities of isocitrate lyase and malate synthase were not detected in cel l-free extracts of glucose-limited cultures. However, both enzymes wer e present when the ethanol fraction in the reservoir medium exceeded t he theoretical minimum above which the glyoxylate cycle is required fo r anabolic reactions. Fructose-1,6-bisphosphatase activity was only de tectable at high ethanol fractions in the feed, when activity of this enzyme was required for synthesis of hexose phosphates. Phospho-enol-p yruvate-carboxykinase activity was not detectable in extracts from glu cose-grown cultures and increased with the ethanol fraction in the fee d. It is concluded that, during carbon-limited growth of S. cerevisiae on mixtures of glucose and ethanol, biosynthetic intermediates with-t hree or more carbon atoms are preferentially synthesized from glucose. Synthesis of the key enzymes of gluconeogenesis and the glyoxylate cy cle is adapted to the cells' requirement for these intermediates. The gluconeogenic enzymes and their physiological antagonists (pyruvate ki nase, pyruvate carboxylase and phosphofructokinase) were expressed sim ultaneously at high ethanol fractions in the feed. If futile cycling i s prevented under these conditions, this is not primarily achieved by tight control of enzyme synthesis.