IDENTIFICATION AND CHARACTERIZATION OF MAE1, THE SACCHAROMYCES-CEREVISIAE STRUCTURAL GENE ENCODING MITOCHONDRIAL MALIC ENZYME

Pyruvate, a precursor for several amino acids, can be synthesized from phosphoenolpyruvate by pyruvate kinase, Nevertheless, pyk1 pyk2 mutan ts of Saccharomyces cerevisiae devoid of pyruvate kinase activity grew normally on ethanol in defined media, indicating the presence of an a lternative 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 frame YKL029c, which is homolo gous to malic enzyme genes from other organisms, abolished malic enzym e activity in extracts of glucose-grown cells. Conversely, overexpress ion of YXL029c/MAE1 from the MET25 promoter resulted in an up to 33-fo ld increase of malic enzyme activity. Growth studies with mutants demo nstrated 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 of MAE1 alone did not result in a clear phenotype. Regulation of MAE1 was studied by determining enzyme activities and MAE1 mRNA levels in wild-type cultu res and by measuring beta-galactosidase activities in a strain carryin g a MAE1::lacZ fusion, Both in shake flask cultures and in carbon-limi ted chemostat cultures, MAE1 was constitutively expressed. A three-to fourfold induction was observed during anaerobic growth on glucose, Su bcellular fractionation experiments indicated that malic enzyme in S. cerevisiae is a mitochondrial enzyme. Its regulation and localization suggest a role in the provision of intramitochondrial NADPH or pyruvat e under anaerobic growth conditions. However, since null mutants could still grow anaerobically, this function is apparently not essential.