The Mycobacterium tuberculosis Ku C-terminus orchestrates LigD activity through domain-specific interactions

Bacterial non-homologous end joining (NHEJ) is a key DNA double-strand break (DSB) repair pathway that relies on the Ku-LigD complex to alleviate genomic instability. The Mycobacterium tuberculosis Ku C-terminus has been highlighted for its role in LigD recruitment to DSBs and stimulation of LigD ligase activity. However, it remains unclear how the Ku C-terminus interacts with and potentially influences other LigD activities. Here, we combine NMR spectroscopy, structural modelling, and mutational analysis to define the interaction interface between the Ku C-terminus and LigD. We identify critical residues in Ku (E246, V248, S258, K260, and N266) and the LigD polymerase (D162, V194, R198) and ligase (D522, K579, L580) domains that mediate this interaction. Functional assays reveal that Ku stimulates LigD ligase activity through contacts with both polymerase and ligase domains, while Ku attenuates template-dependent polymerase activity, contrasting previous studies in Pseudomonas aeruginosa homologs. Disrupting the Ku-LigD interface, either through Ku or LigD mutations, abolishes ligase stimulation and restores polymerase activity, highlighting a dual regulatory mechanism. Our data supports a model where Ku's C-terminal region forms a bipartite interface with LigD to balance repair activity. These findings provide mechanistic insight into the Ku-LigD repair mechanism and uncovers species-specific differences in bacterial NHEJ.