Heavy-ion-induced Single- and Double-strand Breaks in ΦX-174 Replicative Form DNA

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Abstract
Damage caused by accelerated atomic nuclei was measured in ΦX-174 Replicative Form I (RF I) DNA as a function of linear-energy transfer (LET). Irradiations were performed at room temperature in a highly radioprotective broth to minimise the effects of free radicals. Most ion beams terminated in the continuously-mixed samples. By using neutral sucrose gradient sedimentation of 3H-TdR-labelled RF DNA, irradiated samples were assayed for relative proportions of RF I (supercoil), RF II (open coil), and RF III (linear) DNA configurations. A spheroplast assay was used to determine the viability of DNA samples. The efficiency of production (per rad) of single-strand break events decreased three-fold over the LET range investigated, whereas double-strand break production increased five-fold. The ratio of single-strand break events to double-strand breaks varied from about 26 : 1 for 60Co gamma-irradiation, to about 2 : 1 for oxygen-ion irradiation. Double-strand breaks accounted for less than 10 per cent of the observed loss of viability at low LET, rising to about 50 per cent at high LET. Alkaline sucrose gradient determinations of total DNA break events at low LET yielded consistently greater values than did analogous neutral sucrose determinations, suggesting the presence of induced alkali-labile bonds in irradiated DNA.

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