The influence of conditions affecting repair and fixation of potentially lethal damage on the induction of 6-thioguanine resistance after exposure of mammalian cells to X-rays.

We have studied the influence of postirradiation conditions resulting in repair or fixation of X-ray-induced potentially lethal damage (PLD) on the induction of 6-thioguanine-resistant mutants in plateau phase Ehrlich ascites tumour cells. For repair of PLD cells were incubated under plateau-phase conditions for 6-8 hours after irradiation. For fixation of PLD we used either a 4-h treatment with 120 microM beta-araA or a 50-min treatment in hypertonic medium (2.5 times the normal tonicity). These treatment are known to effectively reduce or eliminate the shoulder of the X-ray survival curve. The mutants were allowed to form colonies in agar medium containing 1.5 micrograms/ml 6-thioguanine, after expression times of 6-12 days. We observed a decrease in the number of mutants induced (per 10(5) cells) when the cells were allowed to repair PLD, as compared with that of cells processed immediately after irradiation, and an increase in their number after treatment either with beta-araA or in hypertonic medium. The curves obtained for the induction of mutants as a function of the radiation dose were usually upward bending. After irradiation at low dose rate we obtained an exponential survival curve and a linear induction of mutants as a function of the dose. Based on these results we suggest that potentially lethal lesions resulting in the formation of the shoulder of the survival curve are not identical with those lesions responsible for the induction of mutants.