UV-induced mutagenesis at the hypoxanthine-guanine phosphoribosyl transferase locus in two L5178Y mouse lymphoma cell strains with different UV sensitivities.

Two strains of L5178Y mouse lymphoma cells, L5178Y-R (LY-R) and L5178Y-S (LY-S), differ markedly in their sensitivity to 254 nm UV radiation (D0 = 0.7 and 5.5 J/m2; n = 6.0 and 2.0 for LY-R and LY-S cells, respectively). In this study, the frequency of hypoxanthine-guanine-phosphoribosyl-transferase-deficient mutants was determined, using 6-thioguanine (TG) as a selective agent, in populations of LY-R and LY-S cells exposed to various fluences of UV radiation. The spontaneous mutation frequency for LY-R cells was (3.7 +/- 0.6) X 10(-5) TGr mutants per viable cell, and the UV induction rate was (2.2 +/- 0.8) X 10(-4) TGr mutants per viable cell, per J/m2. Both spontaneous and induced mutation frequencies were much lower for LY-S cells. The spontaneous mutation frequency for these cells were too low to make its measurement practicable (less than 0.0013 X 10(-5) TGr mutants per viable cell). Mutation induction rate was (4.2 +/- 2.2) X 10(-7) TGr mutants per viable cell, per J/m2. These differences in mutability do not appear to be due to gene duplication in LY-S cells, or to selective growth disadvantage of LY-S-derived TG-resistant mutants. Possible mechanisms underlying the differences in mutability of LY-R and LY-S cells are considered.