With LigD forms a non-homologous end joining (NHEJ) DNA repair enzyme, which repairs dsDNA breaks with reduced fidelity. Binds linear dsDNA with 5'- and 3'- overhangs but not closed circular dsDNA nor ssDNA. Recruits and stimulates the ligase activity of LigD (By similarity). Probably involved in DNA repair during spore germination. Homodimer. Interacts with LigD. Expressed in the center of the spore core, presumably with the nucleoid. Expressed in sporulating cells. Disappears after 90 minutes of spore germination. Transcriptionally regulated by SpoVT and sigma-G factor. Stationary-phase cells lacking this gene are more sensitive to ionizing radiation. Growth rate, ultraviolet light and methyl-methanesulfonate sensitivities are unaffected. The spores formed are significantly more sensitive to dry heat, high vacuum-induced desiccation, X-rays and UV radiation, but no difference in hydrogen peroxide resistance exists. Belongs to the prokaryotic Ku family.