Polyamine-induced compaction and aggregation of DNA--a major factor in radioprotection of chromatin under physiological conditions.

Spermine at physiological levels and ionic strength induces compaction and aggregation of SV40 DNA and minichromosomes resulting in marked radioprotection of the DNA against gamma-ray-induced formation of single-strand breaks. This phenomenon, termed the PICA effect, results in yields of single-strand breaks in DNA and minichromosomes comparable to those found with intact cells and is considered to be a major mechanism responsible for radioprotection of cellular DNA.