Inhibition of thymidylate biosynthesis induces mitotic unequal sister chromatid recombination in Saccharomyces cerevisiae.

Inhibition of thymidylate biosynthesis has been found to induce deletion of a LEU2 insert from the ribosomal DNA gene cluster of haploid strains of Saccharomyces cerevisiae. Loss of the insert was detected phenotypically by the enhanced production of both sectored (leu(+)/leu(-)) and non-sectored (leu(-)) colonies. Hybridization patterns obtained by Southern blot analysis of DNA from the leu(+) and leu(-) sectors were consistent with the occurrence of unequal sister chromatid recombination. The induction of sectored colonies was prevented by the rad52-1 mutation but not by defects in RAD6. However, the formation of non-sectored leu(-) colonies was induced by thymidylate depletion in both rad52-1 and rad6 strains.