ssDNA purification

ZymoClean Gel DNA Recovery Kit (Zymo Research) was used for ssDNA gel purification. Briefly, after excising the gel band containing the ssDNA product with a clean razor blade, 750 µL (3 volumes) of the provided binding buffer were added to the excised gel, and left to melt in an incubator at 45 °C for 10 min. The melted agarose gel solution was transferred to the silica-based spin columns and mounted on a collection tube, and centrifuged for 60 s at 11,000 RPM. After discarding the flow-through, 250 µL of ethanol-based DNA wash buffer were used to wash the column twice by centrifuging for 60 s at 11,000 RPM and discarding the flow-through each time. The ssDNA was recovered with 6–15 µL of elution buffer, after centrifugation for 60 s at 11,000 RPM. The concentration of recovered ssDNA was measured using a NanoDrop™ 2000 UV-Vis Spectrophotometer (Thermo Fisher Scientific Inc.). Final purified ssDNA was verified by Sanger sequencing from the 3′ end.