DNA-dependent ATPase and 5'-3' DNA helicase required for the maintenance of both mitochondrial and nuclear genome stability. Involved in the maintenance of mitochondrial (mtDNA). Required for both repair of mitochondrial DNA and recognition of a recombinogenic signal characterized by a 26-bp palindromic at sequence in the ery region of mitochondrial DNA. May have a general role in chromosomal replication by affecting Okazaki fragment maturation. Required for the completion of S-phase. ATP + H2O = ADP + H(+) + phosphate Monomer. Mainly concentrated in the nucleolus, and occasionally redistributes to single nuclear foci outside the nucleolus, probably sites of DNA repair. Produced by alternative initiation at Met-40 of isoform Mitochondrial. Belongs to the helicase family. PIF1 subfamily.