A persistently reduced cloning efficiency occurs in many of the cloned progeny of Chinese hamster ovary (CHO) cells surviving X irradiation, a stable phenotype we have previously termed delayed reproductive death (Int. J. Radiat. Biol. 60, 483-496, 1991). We now report that this phenotype is also induced by the alkylating agent ethyl methanesulfonate (EMS), but not by irradiation with ultraviolet light. The restriction endonuclease Hinfl, which binds at G(-)ANTC DNA sequences and generates cohesive-end double-strand breaks, was also efficient in inducing delayed reproductive death. On the other hand, an X-ray-sensitive CHO mutant, xrs-5, which is defective in the repair of DNA double-strand breaks, did not show this phenotype following X irradiation. These results suggest that DNA double-strand breaks, as well as the endogenous repair processes for these breaks, are associated with the induction of the delayed reproductive death phenotype in CHO cells. The possible mechanism for the induction of delayed reproductive death by EMS is discussed.
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