Bringing the Ends together: Cryo-EM structures of mycobacterial Ku in complex with DNA define its role in NHEJ synapsis

Non-homologous end joining (NHEJ) is the sole pathway for repairing double-strand breaks in Mycobacterium tuberculosis during dormancy, relying on mycobacterial Ku (mKu) and ligase D, with mKu as the rate-limiting factor. Despite its essential role, the lack of structural information on prokaryotic Ku has hindered understanding of the molecular mechanisms underlying bacterial two-component NHEJ machinery. Here, we present the first cryo-EM structures of mKu in DNA-bound and higher-order super-complex forms, revealing a Ku-mediated DNA synapsis mechanism unique to prokaryotes. Integrating cryo-EM with hydrogen-deuterium exchange mass spectrometry, we define key mKu-mKu dimerization, DNA-binding, and synapsis interactions essential for efficient NHEJ, bridging structure with function. Structure-guided in silico mutagenesis, coupled with electrophoretic mobility shift assays, identifies residues essential for DNA binding and synaptic assembly, which are crucial for NHEJ. Förster resonance energy transfer confirms DNA-dependent mKu oligomerization in solution, while live-cell imaging captures its spatiotemporal dynamics during DSB repair. These findings provide fundamental insights into the architecture and function of prokaryotic NHEJ, positioning mKu as a potential therapeutic target against tuberculosis and offering a framework for understanding DNA repair across bacterial species.