Single-molecule imaging reveals the kinetics of non-homologous end-joining in living cells

Abstract

Non-homologous end joining (NHEJ) is the predominant path-way that repairs DNA double-stranded breaks (DSBs) in vertebrates. The DNA termini of many DSBs must be processed to allow ligation while minimizing genetic changes that result from break repair. Emerging models propose that DNA termini are first synapsed approximately 115Å apart in one of two long-range synaptic complexes. The first long-range complex can be formed with only the KU70/80 heterodimer and DNA-PKcs while the second long-range complex also includes XRCC4, XLF, and Ligase 4. Both long-range complexes inefficiently progress to short-range synaptic complexes that juxtapose DNA ends to facilitate ligation. Here we perform singlemolecule analyses of the recruitment of Halo-tagged NHEJ factors to DSBs. Our results provide direct evidence for stepwise maturation of NHEJ complex and precisely define kinetics of core NHEJ factor binding to DSBs in living cells.