Effect of halogenated pyrimidine 5'-mononucleotides on dTMP-permeable yeast strains and the isolation and characterization of resistant mutants.

Unlike wild-type Saccharomyces cerevisiae, yeast cells carrying the tup 7 mutation are able to take up exogenously-supplied dTMP. The tup 7 mutant was also found to be dramatically sensitive to growth inhibition by FdUMP and BrdUMP. The exclusive mode of action of FdUMP in such strains was shown to be inhibition of thymidylate synthetase. Spontaneously-arising derivatives resistant to FdUMP and BrdUMP were isolated from the tup7 strain. Genetically, these mutations were recessive and defined three complementation groups (fdr1, fdr2, and bdr2), unlinked to the tup7 locus and to each other. No resistance mutations were obtained which mapped at the structural gene for thymidylate synthetase. Biochemical analysis of cells carrying these mutations showed that in the case of fdr2 and bdr2, in addition to an inability to transport dTMP, acid and alkaline phosphatase levels were affected, indicating that phosphatase expression and 5'-mononucleotide permeability are coordinately controlled. In contrast, the fdr1 mutation and a previously identified suppressor of dTMP-permeability, sot1, affected only 5'-mononucleotide uptake and may define components of the permease responsible for dTMP entry.