Fig. 4.

Single-molecule FRET histograms are derived from 30–80 FRET trajectories by using the first 200 time points of each FRET trajectory at each [Mg²⁺]. Log-normal and Gaussian fits of the distributions with four population states are indicated by solid lines. The mean FRET values that correspond to each of the five histograms from [Mg²⁺] = 0 to 5 mM are 0.13, 0.53, 0.59, 0.60, and 0.85, respectively. The numbers of molecules are 21, 29, 85, 31, and 26, in order from top to bottom. The last histogram shows the comparison among the FRET experiment, donor-only control, and acceptor-only control at [Mg²⁺] = 0.4 mM, as generated from single-molecule images.