We have discovered a novel DNA repair response which is induced in cells irradiated with γ rays at the G1/ S-phase border. The induction of this repair response occurs at a stage in the cell cycle when overall levels of excision repair are reduced compared to cells irradiated in either S phase, G2/ M phase or exponential growth. The induced repair is characterized by the formation of very long excision repair patches (VLERP) containing at least 150 nucleotides compared to the constitutive repair patches that are 3-5 nucleotides. These VLERP appear to be produced in response to a DNA lesion specific to ionizing radiation since they were not observed in cells irradiated with UV radiation at${\rm G}_{1}/{\rm S}\ \text{phase}$. The formation of VLERP requires both the nucleotide excision 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 patches shows an initial rapid production which levels out after 4 h.

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