Representative single-molecule traces from 160 ms images are shown for LeuT-H7C/R86C (left), LeuT-H7C/R86C/Y268A (center), and LeuT-H7C/R86C/R5A (right) in 200 mM KCl (a, e, i), 200 mM NaCl (b, f, j) or 200 mM NaCl and 20 µM Leu (c, g, k). Cy3 (donor) and cy5 (acceptor) fluorescence are shown in green and red respectively. FRET efficiency is shown in blue. One-dimensional histograms (d, h, l) represent the population data obtained in the presence of 200 mM KCl (gray bars), 200 mM NaCl (blue line), or 200 mM NaCl and 20 µM Leu (red line).