SURVIVAL CURVES WITH SHOULDERS - DAMAGE INTERACTION, UNSATURATED BUT DOSE-DEPENDENT REJOINING KINETICS OR INDUCIBLE REPAIR OF DNA DOUBLE-STRAND BREAKS

Double-strand breaks (DSBs) are considered as critical lesions for rad iation-induced cell killing. Two processes, identified by the rejoinin g kinetics of DNA DSBs induced in yeast at radiation doses which are a lso applied in survival studies, can lead to survival curves with shou lders: firstly, ''DSB interaction'' when repair time is unrestricted, and secondly, a biphasic and unsaturated DSB rejoining, where the frac tions of the two rejoining components are dose-dependent and repair ti me is restricted. Thus the unsaturated rejoining kinetics of DSBs is n ot in contradiction to the observation of survival curves with shoulde rs. While these two mechanisms are derived from studies on cells kept under nongrowth conditions during postirradiation incubation, experime nts are reviewed which were designed to study inducibility of DSB rejo ining. Some of these studies suggest that rejoining of DSBs under grow th conditions may include an inducible process.