S1-sensitive sites in DNA after gamma-irradiation.

DNA from gamma-irradiated T1 bacteriophages was analyzed for "single-stranded" sites by cleavage with S1 nuclease from Aspergillus oryzae as lesion probe. The ratio of "S1-sensitive sites" to the amount of radiation-induced single-strand breaks was about one. Presumably these "denatured" sites were associated with single-strand breaks. The subsequent check for the persistence of "single-stranded" sites within the DNA molecule by thermokinetics demonstrated a strong affinity of the nuclease to its substrate, the single-stranded lesion, and a perfect excision. It is assumed that the direct absorption of radiation energy in the DNA gives rise to the formation of such bulky lesions.