Unwinding and Rewinding: Double Faces of Helicase?
Figure 3
Purposed model of annealing helicase function in DNA replication, DNA repair, transcription, and telomere metabolism. (a) The annealing helicases function at stalled replication forks by catalyzing fork regression through Holliday junction migration (left) or displacing nascent strands and annealing parental strands (right). The replication fork proceeds until the DNA damage is repaired or bypassed. (b) Annealing helicases function in double-stranded DNA break repair. Annealing helicases might participate in homologous recombination repair through double Holliday junction (dHJ) or synthesis-dependent strand annealing (SDSA) pathways (left) or microhomology-mediated end joining (MMEJ) or single strand annealing (SSA) pathways (right). Noncrossover (uncommon) of dHJ is not shown. (c) Annealing helicases stabilize the DNA-RNA hybrid in transcription. Cas3 promotes RNA annealing to dsDNA without ATP and unwinding RNA from dsDNA with ATP. Model is adapted by analogy from one proposed for E. coli Cas3 helicase [89]. (d) Involvement of annealing helicases in telomere maintenance. Annealing helicases may catalyze the formation of T-loop structure or sister chromatid exchange and homologous recombination-dependent DNA replication in ALT cells. See text for details.
