FIGURE 4 The p15 plasmid DNA was topologically pure; the minor band at the top of the t = 0 column is a small amount of nicked DNA, due to damage inflicted during purification. The p15 plasmid was mixed with gyrase for 5 to 300 seconds. After 300 seconds, a sample of DNA was treated with novobiocin and incubated for another 30 minutes. Marker lanes (MW) show the p15 plasmid DNA with change in linking number (ΔLk) in increments of 1.