Interpretation of the shape of survival curves in terms of induction and repair/misrepair of DNA double-strand breaks.

Evidence is presented that in yeast cells one DNA double-strand break (dsb) may be considered as one potentially lethal lesion (PLL). Using a temperature conditional radiosensitive diploid yeast mutant (rad 54-3) it is demonstrated that the shoulder of survival curves, for cells plated immediately, is due to repair of dsb (PLL) within a restricted time period. Split dose experiments with the mutant rad 54-3 show that the reappearance of a shoulder is observed when two conditions are met: (1) repair of dsb (PLL) during the time interval between doses and (2) repair of mainly those dsb (PLL) which are induced by the second dose on the nutrient agar plate. Irradiation of wild type yeast cells at high dose rate followed by liquid holding treatment for 72 h (delayed plating, DP) or at low dose rate show that the bending of DP-survival curves is due to the accumulation of dsb (PLL) leading to lethal lesions probably by misrepair of dsb.