Repair of lethal, mutagenic and transforming damage induced by X-rays in golden hamster embryo cells

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Abstract
We compared the frequencies of mutation and morphological transformation in relation to the repair of potentially lethal damage (PLD) and sublethal damage (SLD) in X-irradiated golden hamster embryo (GHE) cells. The repair of PLD induced in GHE cells by X-rays was shown to be an error-free process which reduced the frequency of mutation and morphological transformation. However, if cells were irradiated with doses divided into two halves separated by various time intervals to study repair of SLD, the frequency of mutants and morphological transformants depended upon the length of the interval between the doses. If the interval was 2 h, the frequency of mutants and transformants was lower than if the total dose were delivered in a single exposure. If the interval was 4 h, these frequencies were higher than the latter. The transformation frequencies remained high even if this interval was increased to 12 h, but the frequency of mutants decreased again. Thus, enhancement of mutant or transformant frequency by a second exposure to X-rays, 4 h after the first irradiation, correlated with recovery of DNA synthesis in the X-irradiated cell populations. Studies with synchronized cells also showed that induction of mutants and transformants was highest in cells irradiated at the DNA synthesis (S) phase. These results suggest that DNA synthesis plays a critical role in the process of X-ray-induced morphological transformation and mutagenesis.