The TcaR proteins (0.3 mM) were first incubated at 30uC for 15 min in reaction buffer [20 mM Tris-HCl, pH 8.0, 150 mM KCl, 0.1 mM MgCl 2 , 0.05 mM EDTA, 12.5% glycerol, 10 mM DTT, 12 mM circular viron wX174 ssDNA (5386 nucleotides in length), 0.2 M ammonium acetate], and then chilled on ice to stop the reaction. The reaction product was diluted 100-fold with EM sample dilution buffer (2 mM MgCl 2 , 0.5 mM DTT, 10 mM HEPES pH 7.0). A droplet (4 ml) was placed for 1 min at room temperature on a copper grid (300 mesh, Pelco, USA) coated with fresh carbon. The excess buffer was then carefully blotted away from the edge of the grid with Whatman #1 filter paper (Whatman Inc., USA). After staining for 1 min with 2% uranyl acetate, excess liquid was removed and samples were dried at room temperature. Bio-transmission EM was performed with a Tecnai F20 Bio TWIN (FEI Co., Netherlands) using an acceleration voltage of 200 kV. Images were recorded with a slow scan CCD camera (Gatan MultiScan TM 600, USA) at a resolution of at least 4k64k pixels.