The influence of PLD repair on the induction frequency of 6-thioguanine resistant mutants in Ehrlich ascites tumour cells after exposure to X-rays and ultraviolet light.

We have studied the influence of conditions promoting repair or expression of potentially lethal damage (PLD) on the induction of 6-thioguanine resistant mutants in plateau phase Ehrlich ascites tumour cells after exposure to either X-rays or ultraviolet light (UV). The cells were assayed either immediately after irradiation, or after a 6-8 h incubation under plateau phase conditions (in the case of exposure to X-rays) or 24 h incubation in balanced salt solution (in the case of exposure to UV light) to allow repair of PLD. For expression of PLD the cells were incubated for 4 h with 120 microM beta-araA, a treatment known to result in an exponential survival curve. For selection of the induced mutants the cells were incubated with 1.5 micrograms ml-1 6-thioguanine after a growth period of 6-12 days in fresh medium to allow expression. The results indicated a decrease in the number of mutants induced per 10(5) cells, with both radiations, when these were allowed to repair PLD, as compared with that of cells processed immediately after irradiation. An increase in the number of mutants induced was observed after postirradiation treatment of X-irradiated cells with beta-araA, but a decrease after exposure to UV light and subsequent treatment with beta-araA. We suggest therefore that potentially lethal lesions resulting in the formation of the shoulder of the survival curve are not identical with the lesions causing the induction of mutants after exposure to X-rays, although both kinds of lesions could be subgroups of the same kind of initially induced lesions.(ABSTRACT TRUNCATED AT 250 WORDS)