Role of topoisomerase II in the response of mammalian cells to the action of ionizing radiation. I. Change in the mitotic cell cycle of Chinese hamster CHO K1, irradiated with x-rays, after the action of novobiocin at a high concentration]

Flow cytometry was used to study cell cycle recovery in X-irradiated Chinese hamster cells after action of novobiocin, an inhibitor of topoisomerase II. A prolonged treatment with 1 mM novobiocin (20-30 h) of intact cells results in the G2 + M delay. Novobiocin treatment of 5 Gy-irradiated cells results in a slight delay in cell exit from G1 into S phase and in a much longer G2-delay if compared with X-irradiated cells. These data allow to suggest an involvement of topoisomerase II in cell response to ionizing radiation.