PHYSIOLOGICAL-RESPONSE TO ANAEROBICITY OF GLYCEROL-3-PHOSPHATE DEHYDROGENASE MUTANTS OF SACCHAROMYCES-CEREVISIAE

Mutants of Saccharomyces cerevisiae, in which one or both of the genes encoding the two isoforms of NAD-dependent glycerol-3-phosphate dehyd rogenase had been deleted, were studied in aerobic batch cultures and in aerobic-anaerobic step change experiments. The respirofermentative growth rates under aerobic conditions with semisynthetic medium (20 g of glucose per liter) of two single mutants, gpd1 Delta and gpd2 Delta , and the parental strain (mu = 0.5 h(-1)) were almost identical, wher eas the growth rate of a double mutant, gpd1 Delta gpd2 Delta, was app roximately half that of the parental strain, Upon a step change from a erobic to anaerobic conditions in the exponential growth phase, the sp ecific carbon dioxide evolution rates (CER) of the wild-type strain an d the gpd1 Delta strain were almost unchanged. The gpd2 Delta mutant s howed an immediate, large (>50%) decrease in CER upon a change to anae robic conditions, However, after about 45 min the CER increased again, although not to the same level as under aerobic conditions, The gpd1 Delta gpd2 Delta mutant showed a drastic fermentation rate decrease up on a transition to anaerobic conditions. However, the CER values incre ased to and even exceeded the aerobic levels after the addition of ace toin, High-pressure liquid chromatographic analyses demonstrated that the added acetoin served as an acceptor of reducing equivalents by bei ng reduced to butanediol. The results clearly show the necessity of gl ycerol formation as a redox sink for S. cerevisiae under anaerobic con ditions.