Damage clusters after gamma irradiation of a nanoparticulate plasmid DNA peptide condensate.

We have gamma-irradiated plasmid DNA in aqueous solution in the presence of submillimolar concentrations of the ligand tetra-arginine. Depending upon the ionic strength, under these conditions, the plasmid can adopt a highly compacted and aggregated form which attenuates by some two orders of magnitude the yield of damage produced by the indirect effect. The yields of DNA single- and double-strand breaks (SSB and DSB) which result are closely comparable with those produced in living cells. The radical lifetimes, diffusion distances, and track structure are expected to be similarly well reproduced. After irradiation, the aggregation was reversed by adjusting the ionic conditions. The approximate spatial distribution of the resulting DNA damage was then assayed by comparing the increases in the SSB and DSB yields produced by a subsequent incubation with limiting concentrations of the eukaryotic base excision repair enzymes formamidopyrimidine-DNA N-glycosylase (the FPG protein) and endonuclease III. Smaller increases in DSB yields were observed in the plasmid target that was irradiated in the condensed form. By modeling the spatial distribution of DNA damage, this result can be interpreted in terms of a greater extent of damage clustering.