Half-life Values for DNA Double-strand Break Rejoining in Yeast Can Vary by More Than an Order of Magnitude Depending on the Irradiation Conditions

Abstract

Yeast cells in stationary phase were exposed under oxic or anoxic conditions to sparsely (30 MeV electrons) or densely (3·5 MeV α-particles) ionizing radiation. For all four experimental set-ups postirradiation treatment of cells was the same, i.e. cells were kept under oxic conditions in non-growth medium at 30°C. Double-strand break (dsb) rejoining was measured during this treatment yielding the following results: (1) half-life values ranged from < 60 min (electrons, anoxia) to 3·8 h (low doses of electrons, oxia), 7 h (α-particles, anoxia), 10 h (high doses of electrons, oxia) and 13 h (α-particles, oxia). (2) In the case of exposure of oxic cells to electrons a biphasic rejoining kinetics is observed with a dose-dependent increase of the fraction of the slow component. These results suggest that half-life values of dsb rejoining in a given cell depend on physical, chemical and biological parameters. The rejoining of dsb slows down with increasing LET, being probably due to the increasing complexity of dsb. Oxygenation of cells at the time of irradiation affects half-life values, indicating that radiation chemistry plays an important role. The biphasic rejoining kinetics observed for dsb induced by electrons in oxic cells is interpreted in terms of a dose-dependent change of chromatin structure hindering the interaction between damaged chromatin and the rejoining enzymes rather than by two chemically distinct types of dsb with differing half-life values.