Induction of mitotic recombination in yeast by starvation for thymine nucleotides.

Abstract

The biosynthesis of thymine nucleotides in S. cerevisiae can be inhibited either by genetic lesions in the structural gene for thymidylate synthetase (TMP1) or by drugs that prevent the methylation of dUMP to dTMP. This methylation can be blocked by folate antagonists. 5-fluoro-dUMP (FdUMP) is also an effective inhibitor in vivo. Inhibition of dTMP biosynthesis by these 3 different routes causes thymineless death. In addition to being cytotoxic, FdUMP is highly recombinagenic in yeast, but does not induce nuclear gene mutations. Provision of exogenous dTMP eliminates this induced mitotic recombination and cell killing. Similar results were obtained when a thymineless condition was provoked in cells by antifolate drugs or by dTMP deprivation in strains auxotrophic for this nucleotide. In contrast to the situation in prokaryotes, starvation for thymine nucleotides in yeast induces genetic recombination but is not mutagenic.