NHEJ is the dominant double-strand break (DSB) repair pathway. During synapsis, the two DNA ends at a DSB are brought together into physical proximity for DNA repair. Using single-molecule FRET (smFRET), the study showed that both Ku and XRCC4:DNA ligase IV (X4L4) are necessary to achieve a flexible synapsis of blunt DNA ends. The addition of XLF causes a transition to a close synaptic state and the promotion of close synapsis by XLF indicates a role that is independent of a filament structure. The study also showed that DNA-PKcs is not required for the formation of either the flexible or close synaptic states.

This study contains supplementary data tied to the publication. The supplementary data includes data that was used to assemble different figures in the publication, which contains data about Ku and X4L4 mediated noncovalent synapsis of blunt dsDNA ends, effect of DNA-PKcs on synapsis mediated by Ku plus X4L4, effect of XLF on dsDNA within synaptic complexes, effect of PAXX on two dsDNA, and time-dependent accumulation of CS complex. In addition, data about ensemble confirmation and the proposed model of NHEJ synapsis is also provided.