ATP-dependent 3'-5' DNA helicase, unwinds 3'-overhangs, 3'- flaps, and splayed-arm DNA substrates but not 5'-overhangs, 5'-flap substrates, 3-way junctions or Holliday junctions. Not highly efficient in vitro (PubMed:19199647, PubMed:22615856). Requires ATP hydrolysis for helicase activity; the ATPase activity is DNA-dependent and requires a minimum of 4 single-stranded nucleotides (nt) with 6-10 nt providing all necessary interactions for full processive unwinding. The ATPase prefers ATP over CTP or GTP, is almost inactive with TTP (PubMed:19199647). DNA helicase activity requires ATP or dATP and only acts when the 3'-overhang is >20 nt. Capable of unwinding a DNA:RNA hybrid if the 3'-overhang is DNA. Also catalyzes ATP-independent annealing of complementary DNA strands; annealing requires Mg(2+) (PubMed:22615856). ATP + H2O = ADP + H(+) + phosphate ATPase activity has a small preference for Mn(2+) over Mg(2+), Ca(2+) supports ATPase activity less well. Co(2+) and Zn(2+) are inactive (PubMed:19199647). Another study shows equal activity with Mg(2+) and Mn(2+), none with Ca(2+) (PubMed:22615856). kcat is 10 sec(-1) on a ssDNA 21-mer and about 50 sec(-1) on ssDNA 5kb template. Monomer. Removal of the N-terminus decreases solubility and/or structural integrity of the protein. Belongs to the helicase family. RAD25/XPB subfamily. Truncated N-terminus.