Polyglutamylation of Folate Coenzymes Is Necessary for Methionine Biosynthesis and Maintenance of Intact Mitochondrial Genome inSaccharomyces cerevisiae

Hélène Cherest,Dominique Thomas,Yolande Surdin-Kerjan

doi:10.1074/jbc.275.19.14056

Abstract

One-carbon metabolism is essential to provide activated one-carbon units in the biosynthesis of methionine, purines, and thymidylate. The major forms of folates in vivo are polyglutamylated derivatives. In organisms that synthesize folate coenzymes de novo, the addition of the glutamyl side chains is achieved by the action of two enzymes, dihydrofolate synthetase and folylpolyglutamate synthetase. We report here the characterization and molecular analysis of the two glutamate-adding enzymes of Saccharomyces cerevisiae. We show that dihydrofolate synthetase catalyzing the binding of the first glutamyl side chain to dihydropteroate yielding dihydrofolate is encoded by the YMR113w gene that we propose to rename FOL3. Mutant cells bearing a fol3 mutation require folinic acid for growth and have no dihydrofolate synthetase activity. We show also that folylpolyglutamate synthetase, which catalyzes the extension of the glutamate chains of the folate coenzymes, is encoded by the MET7 gene. Folylpolyglutamate synthetase activity is required for methionine synthesis and for maintenance of mitochondrial DNA. We have tested whether two folylpolyglutamate synthetases could be encoded by the MET7 gene, by the use of alternative initiation codons. Our results show that the loss of mitochondrial functions in met7 mutant cells is not because of the absence of a mitochondrial folylpolyglutamate synthetase.

Highlights

In contrast to enteric bacteria or fungi such as Neurospora crassa and Aspergillus nidulans, Saccharomyces cerevisiae possesses a complete set of enzyme activities, which allows its growth on a large number of inorganic or organic sulfur sources
In S. cerevisiae, as well as in other eukaryotes, both the cytoplasmic and mitochondrial compartments possess a set of enzymes that catalyzes the interconversion of folate coenzymes, which differ by the oxidation state of their one-carbon unit [3]
In the present work we have brought evidence that, in S. cerevisiae dihydrofolate synthetase (DHFS) is an enzyme distinct from folylpolyglutamate synthetase (FPGS) and is encoded by the YMR113w gene that we propose to rename FOL3

Summary

Introduction

In contrast to enteric bacteria or fungi such as Neurospora crassa and Aspergillus nidulans, Saccharomyces cerevisiae possesses a complete set of enzyme activities, which allows its growth on a large number of inorganic or organic sulfur sources. Mutations impairing the conversion of homocysteine into methionine are the only ones that are expected to lead to strains that cannot grow on homocysteine or cysteine and require strictly methionine or S-adenosylmethionine for growth. In this reaction, catalyzed by homocysteine 5-methyltetrahydrofolate methyltransferase (methionine synthetase), the methyl group is supplied by 5-methyltetrahydrofolate, one of the products of the one-carbon metabolism [1]. Polyglutamylation of Folate Coenzymes in S. cerevisiae gene, which corresponds to the ORF YGL125w, has been shown to encode methylene tetrahydrofolate reductase [7, 8]. It is noteworthy that S. cerevisiae cells possess only one methionine synthetase, which functions without vitamin B12 as a cofactor

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