8oxoG and O6mG lesion at any of the terminal guanines that promotes telomerase binding. (A) Single-molecule pull-down experiment and a plot of normalized telomerase binding vs the percent of single-molecule traces displaying an unfolded state (FRET < 0.6). Error bars represent the SD from three or more telomerase binding assays. (B) Telomerase activity on substrates containing a base lesion. 3R and 4R represent DNA constructs with three and four (TTAGGG) repeats, which serve as unfolded and folded G-quadruplex controls, respectively. Numbers indicate the position of G modification. (C) Normalized C strand binding fraction, telomerase binding, and telomerase activity obtained from C strand annealing, single-molecule pull-down, and in vitro telomerase activity assays, respectively. The number after each mutated or modified base indicates the position.