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. Plays a role in maintaining telomere length. Acts as a negative regulator of telomerase. Binds to and recombines double-stranded T-DNA molecules