Isolation and characterization of mutants of Saccharomyces cerevisiae auxotrophic and conditionally auxotrophic for 5'-dTMP.

An improved method for isolation of yeast m utants auxotrophic for 5'-dTM P is presented. The procedure employs the two folic acid antagonists aminopterin and sulfanilamide (SA A). Selec­ tiveness of the procedure depends on concentration of SAA and time of incubation. 44 mutants auxotrophic and 3 conditionally auxotrophic for 5'-dTMP were isolated. All belong to one complementation group. The corresponding gene was designated TMP1. Tetrad dissection revealed its chromosomal nature. TMP1 is not closely linked to the genes ADE2„ LEU1, ARG4, ILV 2, H1S5, LYS1 and the mating type locus. With the centromere-linked genes ARG 4 and L E V I gene TMP1 exhibited second division segregation frequencies of 0.42 and 0.53 respectively, indicative of centromere-linkage. Strains auxotrophic and conditionally auxotrophic for 5'-dTMP were all respiratory deficient (petite). Genetical analysis indicates that the petite phenotype is due to loss of the rho factor in cells harbouring either tm p l or tm p l ts alleles.