Abstract
Repair of ionizing radiation (IR) induced DNA DSBs shows two defined components, and is dominated by a fast component that requires the activities of DNA-PK, and DNA ligase IV/XRCC4 (D-NHEJ). Cells with defects in either DNA-PK, or DNA ligase IV complex rejoin the majorityof IR-induced DSBs utilizing an alternative pathway operating with slower kinetics that could reflect HDR. Yet, the hyper-recombinogenic DT40 chicken cell line and a set of mutants defective in homologous recombination (HR) rejoin DSB with kinetics similar to the wild type even when D-NHEJ is severely compromised. Thus, increase in the HR capacity of cells, or defects in the proteins involved, fail to alter the rejoining kinetics in a way compatible with an involvement of HDR in the slow component of rejoining. We propose therefore the operation of an alternative pathway operating as a backup and termed B-NHEJ. B-NHEJ operates in vitro independently of Ku or DNA-PKcs and requires a distinct ligase. Immunofluorescence studies however indicate prolonged Rad51 foci formation, compatible with a functional HDR. A model will be presented that recapitulates genetic and biochemical results, and addresses the processing ofIR-induced DNA DSBs by D-NHEJ, B-NHEJ and possibly HDR in the context of chromatin. [J Radiat Res 44:372 (2003)]
