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A comparative study of rejoining of DNA double-strand breaks in yeast irradiated with 3.5 MeV alpha-particles or with 30 MeV electrons.

Yeast cells were irradiated with 3.5 MeV alpha-particles and 30 MeV electrons, as reference radiation. The kinetics of DNA double-strand break (dsb) rejoining during incubation of cells under non-growth conditions (PLDR conditions) were measured using the neutral sedimentation technique. A monophasic kinetic was found after irradiation of cells with alpha-particles, with a dose-independent t1/2 value of about 13 h. The kinetics of rejoining of dsb induced by 30 MeV electrons was found to be biphasic, with dose-independent t1/2 values of 3.8 h for the initial and of about 11 h for the slow component. The fraction of the slow component was, however, dose-dependent. These kinetics were measured for both types of radiation at doses yielding high surviving fractions (5% up to 100%). Dsb are induced linearly with dose of both radiations. The RBE value of alpha-particles was found to be 2.5 for initial dsb. The RBE of alpha-particles increased as a consequence of dsb rejoining. This increase in RBE value suggests that DSB may be primary lesions for chromosome aberrations, cellular inactivation and oncogenic transformation of mammalian cells which all exhibit high RBE values of alpha-particles.