Concerted reactions. The reactions contained 0.5 nM BspMI endonuclease and 5 nM DNA in reaction buffer at 37°C. Samples were withdrawn from the reactions at various times and analysed as in Materials and Methods to determine the amounts of each form of the DNA shown in the inserts. In (a), the substrate was supercoiled pNAG2, a plasmid with two BspMI sites in inverted orientation: white circles, the intact supercoiled substrate (SC); black circles, open circle DNA cut in one strand (OC); white squares, the full-length linear form cut in both strands at one site (FLL); black squares, the mean of the two linear fragments from cutting both sites in both strands (L1/L2). In (b), the substrate was PstI-linearised pNAG2: white squares, the DNA cut solely at site 1; black circles, the DNA cut solely at site 2; black triangles, the DNA cut at both sites 1 and 2. In (c), the substrate was PstI-linearised pNAG1, a DNA with two BspMI sites in repeat orientation: white squares, the DNA cut solely at site 1; black circles, the DNA cut solely at site 2; black triangles, the DNA cut at both sites 1 and 2.