A NOVEL DNA-REPAIR RESPONSE IS INDUCED IN HUMAN-CELLS EXPOSED TO IONIZING-RADIATION AT THE G(1) S-PHASE BORDER/

We have discovered a novel DNA repair response which is induced in cel ls irradiated with gamma rays at the G(1)/S-phase border. The inductio n of this repair response occurs at a stage in the cell cycle when ove rall levels of excision repair are reduced compared to cells irradiate d in either S phase, G(2)/M phase or exponential growth. The induced r epair is characterized by the formation of very long excision repair p atches (VLERP) containing at least 150 nucleotides compared to the con stitutive repair patches that are 3-5 nucleotides. These VLERP appear to be produced in response to a DNA lesion specific to ionizing radiat ion since they were not observed in cells irradiated with UV radiation at G(1)S phase. The formation of VLERP requires both the nucleotide e xcision repair pathway, since they are absent in irradiated xeroderma pigmentosum group A cells, and the synthesis of new protein and mRNA. The time course for the induction of the VLERP shows an initial delay of 2 h, followed by a steady increase for up to 12 h after irradiation . By comparison, the production of the constitutive short repair patch es shows an initial rapid production which levels out after 4 h. (C) 1 996 by Radiation Research Society.