PYRUVATE DECARBOXYLASE - AN INDISPENSABLE ENZYME FOR GROWTH OF SACCHAROMYCES-CEREVISIAE ON GLUCOSE

In Saccharomyces cerevisiae, the structural genes PDC1, PDC5 and PDC6 each encode an active pyruvate decarboxylase. Replacement mutations in these genes were introduced in a homothallic wild-type strain, using the dominant marker genes APT1 and Ti5ble. A pyruvate-decarboxylase-ne gative (Pdc(-)) mutant lacking all three PDC genes exhibited a three-f old lower growth rate in complex medium with glucose than the isogenic wild-type strain. Growth in batch cultures on complex and defined med ia with ethanol was not impaired in Pdc(-) strains. Furthermore, in et hanol-limited chemostat cultures, the biomass yield of Pdc(-) and wild -type S. cerevisiae were identical. However, Pdc(-) S. cerevisiae was unable to grow in batch cultures on a defined mineral medium with gluc ose as the sole carbon source. When aerobic, ethanol-limited chemostat cultures (D = 0 . 10 h(-1)) were switched to a feed containing glucos e as the sole carbon source, growth ceased after approximately 4 h and , consequently, the cultures washed out. The mutant was, however, able to grow in chemostat cultures on mixtures of glucose and small amount s of ethanol or acetate (5% on a carbon basis). No growth was observed when such cultures were used to inoculate batch cultures on glucose. Furthermore, when the mixed-substrate cultures were switched to a feed containing glucose as the sole carbon source, wash-out occurred. It i s concluded that the mitochondrial pyruvate dehydrogenase complex cann ot function as the sole source of acetyl-CoA during growth of S. cerev isiae on glucose, neither in batch cultures nor in glucose-limited che mostat cultures.