DNA strand-break yields after post-irradiation incubation with base excision repair endonucleases implicate hydroxyl radical pairs in double-strand break formation

Purpose : To determine the increases in SSB and DSB yields after post gamma-irradiation incubation of plasmid DNA with the Escherichia coli base excision repair endonucleases formamidopyrimidineDNA N -glycosylase (FPG) and endonuclease III (endo III). Materials and methods : Aqueous solutions of plasmid DNA were irradiated with 137 Cs gamma-rays in the presence of 10 -4 -10 -1 mol dm -3 formate. After irradiation, aliquots were treated with FPG and/or endo III. SSB and DSB yields were then determined using gel electrophoresis. Results : Both SSB and DSB yields were found to increase after enzyme incubation, with the increase in the DSB yield being approximately equal to the square of the increase in the SSB yield. The correlation between the increases in the SSB and DSB yields was unaffected by the scavenger concentration during irradiation. Conclusion : Under the conditions used, the majority of DSB appear to be formed from two hydroxyl radical attacks.