Figure 1.

(A) Donor (Cy3) and acceptor (Cy5) labeled DNA molecules were immobilized on the surface via biotin-neutravidin interaction. (B) Fluorescence images of single 91 bp DNA molecules in corresponding donor and acceptor channels are shown before (left panels) and 20 minutes after adding high salt (1 M NaCl) buffer (right panels). Scale bar is 5 μm. (C) Histograms of FRET efficiency as a function of time (t=0 is when high salt was introduced) show the evolution of looped (high FRET) and unlooped (low FRET) populations. (D) Fraction of looped DNA (high FRET population) as a function of time, measured from the histograms in C. An exponential fit to this curve gives the looping rate R. Data are means ± SEM (N = 5).