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Pyruvate, a precursor for several amino acids, can be synthesized from phosphoenolpyruvate by pyruvate kinase. Nevertheless,pyk1 pyk2 mutants ofSaccharomyces cerevisiae devoid of pyruvate kinase activity grew normally on ethanol in defined media, indicating the presence of an alternative route for pyruvate synthesis. A candidate for this role is malic enzyme, which catalyzes the oxidative decarboxylation of malate to pyruvate. Disruption of open reading frameYKL029c , which is homologous to malic enzyme genes from other organisms, abolished malic enzyme activity in extracts of glucose-grown cells. Conversely, overexpression ofYKL029c/MAE1 from theMET25 promoter resulted in an up to 33-fold increase of malic enzyme activity. Growth studies with mutants demonstrated that presence of either Pyk1p or Mae1p is required for growth on ethanol. Mutants lacking both enzymes could be rescued by addition of alanine or pyruvate to ethanol cultures. Disruption ofMAE1 alone did not result in a clear phenotype. Regulation ofMAE1 was studied by determining enzyme activities andMAE1 mRNA levels in wild-type cultures and by measuring β-galactosidase activities in a strain carrying aMAE1 ::lacZ fusion. Both in shake flask cultures and in carbon-limited chemostat cultures,MAE1 was constitutively expressed. A three- to fourfold induction was observed during anaerobic growth on glucose. Subcellular fractionation experiments indicated that malic enzyme inS. cerevisiae is a mitochondrial enzyme. Its regulation and localization suggest a role in the provision of intramitochondrial NADPH or pyruvate under anaerobic growth conditions. However, since null mutants could still grow anaerobically, this function is apparently not essential.

Identification and Characterization of MAE1 , the Saccharomyces cerevisiae Structural Gene Encoding Mitochondrial Malic Enzyme