Repair of DNA Double-strand Breaks Induced in Saccharomyces Cerevisiae Using Different γ-ray Dose-rates: A Pulsed-field Gel Electrophoresis Analysis

Abstract

We investigated the effects of γ-ray exposures at high dose-rate (HDR, 23·2 Gy/min) and low dose-rate (LDR, 0·47 Gy/min) on survival and the induction of DNA double-strand breaks (dsb) in a diploid wild-type (D7) and the repair-deficient mutant strain rad52/rad52 of Saccharomyces cerevisiae. Analysis by pulsed-field gel electrophoresis (PFGE) using a contour homogeneous electric field apparatus revealed that, at HDR, in the range 0–400 Gy, dsb are induced as a linear function of γ-ray dose. Liquid holding recovery in non-nutrient medium (LHR) for 48 h of wild-type cells treated at HDR, significantly increased survival and reduced the yield of dsb. Such changes did not occur in rad52/rad52 cells defective in the repair of dsb. Thus, in γ-irradiated wild-type cells, an efficient repair of dsb is taking place during LHR. Treatments of wild-type cells at LDR resulted in higher survival and an approximately two-fold lower yield of dsb than at HDR. Such a dose-rate effect was absent in rad52/rad52 cells suggesting that, in wild-type cells during LDR exposures, significant amounts of dsb can be repaired. This repair could be very much accentuated by 48-h LHR of wild-type cells treated at LDR. The relationship observed between γ-ray survival and dsb repair clearly indicates that increases in survival of wild-type cells, during LDR as compared with HDR exposures and after LHR, are strongly related to the repair of dsb.