Exogenous thymidine 5'-monophosphate as a precursor for DNA synthesis in yeast

Summary Strain MB1015-5C of Saccharomyces cerevisiae can utilize exogenous thymidine 5-monophosphate (5-dTMP) for its DNA synthesis. Studies with either P³²] or 2-C¹⁴] labelled 5-dTMP reveal first that some of the precursor molecules are taken up intact in DNA synthesis and secondly that 3-digests of highly purified P³²] DNA yield up to 94% of all P³²] as 5-dTMP P³²]. Under the conditions used in these experiments more than 90% of the exogenously supplied 5-dTMP is broken down into orthophosphate and thymidine by an acid phosphatase. Only the orthophosphate is utilized by the yeast cells, mainly for RNA synthesis, and thymidine is not taken up. Suppression of the phosphatase activity is possible by addition of inorganic phosphate to the medium; under these conditions breakdown of 5-dTMP is suppressed but uptake and incorporation of the molecules into the DNA of strain MB1015-5C is still not very effective.