Single-stranded DNA-dependent ATP-dependent helicase. Involved in DNA non-homologous end joining (NHEJ) required for double-strand break repair. When associated with KU70, binds to double-stranded telomeric and non-telomeric DNA sequences, but not to single-stranded DNA (By similarity). Required for the maintenance of chromosome stability and normal developmental growth. Plays a role in maintaining telomere length. Acts as a negative regulator in telomere homeostasis. ATP + H2O = ADP + H(+) + phosphate Interacts with KU80. Expressed ubiquitously. The N-terminal domain (1-265) is necessary for binding to KU80. Severe developmental defects in both vegetative and reproductive organs resulting in sterile flowers. Hypersensitivity to methylmethane sulfonate (MMS). Longer telomeres. Belongs to the ku70 family.