Changes in Nuclease Sensitivity of Mammalian Cells after Irradiation with ⁶⁰Co γ-rays

Summary

Changes in sensitivity of mouse BALB/c 3T3 cells in the plateau phase to digestion with micrococcal nuclease were examined following γ-irradiation. Immediately after irradiation, cell nuclei were more sensitive to micrococcal nuclease compared to unirradiated nuclei. However, there were no detectable changes in length of basic repeating subunits of 182 base pairs of DNA, which include the nucleosome cores consisting of approximately 140 base pairs of DNA. When the cells were incubated at 37° following irradiation, the sensitivity of cell nuclei to the nuclease first increased then decreased, reaching a similar level to unirradiated nuclei 6 h after irradiation. Both the initial increase and the subsequent decrease in sensitivity of nuclei to micrococcal nuclease were prevented when 15 µm novobiocin was present during the post-irradiation incubation, suggesting a possible involvement of type II DNA topoisomerase in repair of DNA lesions induced by γ-rays.

• γ-irradiation
• nuclease sensitivity
• chromatin organization
• novobiocin