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 excis ion repair endonucleases formamidopyrimidine-DNA N-glycosylase (FPG) and en donuclease III (endo III). Materials and methods: Aqueous solutions of plasmid DNA were irradiated wit h Cs-137 gamma -rays in the presence of 10(-4)-10(-1) mol dm(-3) formate. A fter irradiation, aliquots were treated with FPG and/or endo III. SSB and D SB yields were then determined using gel electrophoresis. Results: Both SSB and DSB yields were found to increase after enzyme incuba tion, with the increase in the DSB yield being approximately equal to the s quare of the increase in the SSB yield. The correlation between the increas es 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 for med from two hydroxyl radical attacks.