Figure 5.

Extra-helical loop structure stimulates WRN in initiating 3′→5′ exonucleolytic reaction from a single-strand nick. Reaction products from incubation of synthetic oligonucleotide substrates (10 nM) and WRN (2.5 nM) were resolved in denaturing gels. Lanes from left to right are: No WRN control (S), and 0, 5, 10, 20, 30, 40, 50 and 60 min incubation times. (A) DNA loop structure stimulates WRN in degradation of 3′-recessed termini in a time-dependent reaction. Degradation of the 3′-recessed, 5′-³²P-labeled loop-bearing strand proceeds step-wise from the 3′-terminus, slowing 5 bases from the loop at position 35. (B) WRN degrades 5′-³²P-labeled loop-bearing strand from the 3′-terminus within a single-strand nick. Step-wise degradation proceeds from the 3′-terminus, slowing down 5 bases from the loop, at position 35. (C) In contrast, without the stimulation by an adjacent loop structure, WRN 3′→5′ exonuclease is much less efficient in degradation of DNA from a single-strand nick.