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Thymidylate synthetase activity was measured in crude extracts of the yeastSaccharomyces cerevisiae by a sensitive radiochemical assay. Spontaneous non-conditional mutants auxotrophic for thymidine 5′-monophosphate (tmp1 ) lacked detectable thymidylate synthetase activity in cell-free extracts. In contrast, the parent strains (tup1, -2 , or-4 ), which were permeable to thymidine 5′-monophosphate, contained levels of activity similar to those found in wild-type cells. Specific activity of thymidylate synthetase in crude extracts of normal cells or of cells carryingtup mutations was essentially unaffected by the ploidy or mating type of the cells, by the medium used for growth, by the respiratory capacity of the cells, by concentrations of exogenous thymidine 5′-monophosphate as high as 50 μg/ml, or by subsequent removal of thymidine 5′-monophosphate from the medium. Extracts of a strain bearing the temperature-sensitive cell division cycle mutationcdc21 lacked detectable thymidylate synthetase activity under all conditions tested. Its parent and another mutant (cdc8 ), which arrests with the same terminal phenotype under restrictive conditions, had normal levels of the enzyme. Cells of a temperature-sensitive thymidine 5′-monophosphate auxotroph arrested with a morphology identical to thecdc21 strain at the nonpermissive temperature and contained demonstrably thermolabile thymidylate synthetase activity. Tetrad analysis and the properties of revertants showed that the thymidylate synthetase defects were a consequence of the same mutation causing, in the auxotrophs, a requirement for thymidine 5′-monophosphate and, in the conditional mutants, temperature sensitivity. Complementation tests indicated thattmp1 andcdc21 are the same locus. These results identifytmp1 as the structural gene for yeast thymidylate synthetase.

Thymidine 5′-Monophosphate-Requiring Mutants of Saccharomyces cerevisiae Are Deficient in Thymidylate Synthetase