Single-strand breaks in the DNA of rat thymocytes induced by γ rays and irradiated histidine were studied using alkaline sucrose gradient centrifugation. γ-ray-induced DNA breaks were found to be random in nature with their number increasing linearly with increasing dose. These breaks were produced with an efficiency estimated to be 84.5 eV per break and were rejoined partially during postirradiation incubation. Treatment of the cells at 2°C with irradiated histidine, however, produced the appearance of two distinct peaks of smaller DNA. The molecular weights of these species of DNA were estimated to be$3-4\times 10^{8}$ and 6× 107 daltons, respectively, compared to 6× 108 daltons in untreated cells. Increasing the radiation dose given the histidine resulted in an increase in the proportion of the 6× 107 daltons DNA species but failed to decrease further the molecular size. Posttreatment of γ-irradiated cells with irradiated histidine at 2°C induced further breaks in the DNA. This phenomenon was not observed, however, when the radiation dosage exceeded 30 krad. Rejoining of induced breaks was not observed during incubation at 37°C of irradiated histidine-treated cells. Treatment of the cells with three sulfhydryl-binding reagents produced results similar to those produced by irradiated histidine. Moreover, both irradiated histidine and sulfhydryl agents apparently inhibited the rejoining of γ-ray-induced DNA strand breaks. It is concluded that irradiated histidine and sulfhydryl-binding agents induce specific DNA breakage, possibly the dissociation of large DNA molecules into certain subunits. The possible involvement of sulfhydryl groups in the structure of cellular DNA of high molecular weight is discussed.

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