BiFC detection of D71A dimers, showing the occurrence of interactions at the molecular level between two D71A molecules. (A) A typical image of BiFC-D71A (D71A-YN and D71A-YC complex) spots, obtained by single-molecule TIRF imaging (a frame time of 33 ms) and identified by the image cross-correlation method (indicated by arrowheads). (B) The number densities of the observed BiFC fluorescence spots for BiFC-D71A (green) and BiFC-ACP-TM (red) were plotted as a function of the number densities of expressed molecules (ND_mol). The lines are to help the eye. (C) The lifetime of each individual fluorescent spot was observed, and their distributions are shown for YFP, BiFC-D71A, and BiFC-ACP-TM. See the text for details.