Protamine-induced DNA compaction but not aggregation shows effective radioprotection against double-strand breaks

Protamine, an arginine-rich protein, is essential for the compaction of sperm DNA. We performed single-molecule observations of DNA double-strand breaks caused by 60Co γ-ray irradiation and quantitatively evaluated the protective effect of protamine. It was shown that double-strand breaks were significantly protected for the tightly compact DNA, whereas there is no apparent decrease on the damage for loosely aggregated DNA molecules. Our findings suggest that the densely packed structure of DNA reduces the ability of radiolytic reactive oxygen species to access DNA and thus protects DNA from double-strand breaks.