DNA-dependent ATPase and 5'-3' DNA helicase required for the maintenance of both mitochondrial and nuclear genome stability. Efficiently unwinds G-quadruplex (G4) DNA structures and forked RNA-DNA hybrids. Appears to move along DNA in single nucleotide or base pair steps, powered by hydrolysis of 1 molecule of ATP. Processes at an unwinding rate of about 75 bp/s. Resolves G4 structures, preventing replication pausing and double-strand breaks (DSBs) at G4 motifs. Involved in the maintenance of telomeric DNA. Inhibits telomere elongation, de novo telomere formation and telomere addition to DSBs via catalytic inhibition of telomerase. Reduces the processivity of telomerase by displacing active telomerase from DNA ends. Releases telomerase by unwinding the short telomerase RNA/telomeric DNA hybrid that is the intermediate in the telomerase reaction. Involved in the maintenance of ribosomal (rDNA). Required for efficient fork arrest at the replication fork barrier within rDNA. Involved in the maintenance of mitochondrial (mtDNA). Required to maintain mtDNA under conditions that introduce dsDNA breaks in mtDNA, either preventing or repairing dsDNA breaks. May inhibit replication progression to allow time for repair. May have a general role in chromosomal replication by affecting Okazaki fragment maturation. May have a role in conjunction with DNA2 helicase/nuclease in 5'-flap extension during Okazaki fragment processing. ATP + H2O = ADP + H(+) + phosphate Mg(2+). To a lesser extent, can also use Mn(2+). Optimum pH is 7.5-9.5. Monomer in solution. DNA binding induces dimerization. Associates with mitochondrial and telomeric DNA. Binding to mtDNA is non-specific and the protein seems to coat the entire mtDNA molecule. Binds to the telomerase RNA TLC1. Interacts with the mitochondrial single-strand DNA-binding protein RIM1. Mainly concentrated in the nucleolus, and occasionally redistributes to single nuclear foci outside the nucleolus, probably sites of DNA repair. Bound to the mitochondrial inner membrane either directly or indirectly via a protein complex. Cell cycle-regulated. The nuclear isoform is present in very low amounts in G1 phase cells, but increases as cells progress through S phase, with a peak in late S/G2. The mitochondrial isoform follows a similar, but less pronounced induction pattern. The nuclear isoform is prone to APC/C-dependent degradation in G1, whereas the mitochondrial isoform is not. Phosphorylated. Undergoes RAD53-dependent phosphorylation in response to loss of mtDNA. Present with 259 molecules/cell in log phase SD medium. Produced by alternative initiation at Met-40 of isoform Mitochondrial. Belongs to the helicase family. PIF1 subfamily.