Genomic integrity of T1 DNA after gamma-and ultraviolet irradiation.

T1 DNA, gamma-irradiated in the phage particle or irradiated with ultraviolet light was checked for structural integrity by kinetics of melting and reannealing. gamma-Irradiated DNA differed in all thermokinetic properties by a factor of 3-4 from DNA degraded by mechanical or enzymatical treatments. Ultraviolet irradiation caused much smaller effects than gamma-irradiation. Considering the frequency of pyrimidine dimers in relation to the gamma-ray induced lesions, strong evidence can be derived, that in addition to single base damages, local denatured regions are produced by gamma-irradiation. Such regions, formed possibly by direct absorption of radiation energy in DNA, i.e. by primary ionizations, are associated with base lesions and are passed over during reannealing.