Comparative studies on rejoining of DNA-strand breaks induced by X-irradiation in mammalian cell lines in vitro

Abstract Single strand breaks of the DNA in cultured mouse L cells, porcine kidney stable (PS) cells and Ehrlich ascites tumor cells were induced to almost the same degree in alkaline sucrose gradient, as a linear function of dose. However, it was shown by the alkaline sucrose gradient method and autoradiographic observations of unscheduled DNA synthesis in X-irradiated cells that these three cell lines can repair to almost the same degree the single strand breaks in DNA induced by X-irradiation. These results agree well with the fact that there are no striking differences in sensitivity to X-irradiation among these three cell lines obtained by the colony-forming method. Hamster lung (HL) cells, an X-ray resistent cell line obtained by screening from various mammalian cell lines in vitro, seem to have a higher activity of repair enzyme that joins together most of the single strand breaks of DNA induced by X-irradiation. On the other hand, we were unable to demonstrate any significant rejoining activity of damaged double strand DNA from these cell lines in the neutral sucrose gradient. These findings suggest that double strand breaks of DNA induced by X-irradiation are the lesions principally responsible for the lethal effect of ionizing radiation in mammalian cells in vitro.