Influence of the nuclear gene tmp3 on the loss of mitochondrial genes in Saccharomyces cerevisiae.

Abstract

The Saccharomyces cerevisiae tmp3 mutant is deficient in the mitochondrial enzyme complex that participates in the formation of one-carbon-group-tetrahydrofolate coenzymes, serine transhydroxymethylase, dihydrofolate reductase, and thymidylate synthetase, thus leading to multiple nutritional requirements of dTMP, adenine, histidine, and methionine. The tmp3 mutant quickly loses its mitochondrial genome even when grown on fully supplemented medium or on a high concentration of 5-formyl tetrahydrofolate, which replaces all the four requirements. A study of the loss of the mitochondrial genome by following several mitochondrial genetic markers showed that there was a preferential specific loss of a large region of the mitochondrial genome, covering mit ts983, E, Cr, and mit ts982 up to Or, and retention of the region of pr and mit tscs1297. A kinetic study showed that there was a preferentially rapid loss of the region covering the mit+ alleles ts983 to tscs9O2 at the rate of 10%o per generation.Inhibition of DNA synthesis in Saccharomyces cerevisiae cells, either by limitation of the precursor supply (4) or by deficiencies in other functions needed for its synthesis, such as cdc8 (19), results in the formation of respiratorydeficient cells.The loss of mitochondrial DNA and the formation of respiration-deficient strains in S. cerevisiae grown in the presence of antifolate drugs (5, 6, 22, 23) is due to the limitation of tetrahydrofolate formation and, hence, of 5,10-methylene tetrahydrofolate and l1-formyl tetrahydrofolate needed for the biosynthesis of both DNA and protein.A similar formation of respiratory-deficient cells by mutations leading to a limitation in the formation of thymidylate was described in mutants bearing tmpl allelic to cdc21 (4, 19) in fol mutants (15), and in tmp3 mutants (16).There are two types of mutants which are deficient in dTMP formation. The first one, tmpl, is deficient in thymidylate synthetase (2,12