FIG. 1.

Electrophoretic separation of plasmid pBR322. Supercoiled DNA of plasmid pBR322 was isolated from different E. coli strains as described in Materials and Methods. Topoisomers were separated in a 1.8% agarose gel containing chloroquine at 5 μg/ml in TAE buffer. Samples were run at 2.5 V/cm for 16 h. Under these conditions relaxed control DNA of pBR322 (rel), which was obtained by treatment of supercoiled DNA with calf thymus topoisomerase I (Gibco-BRL) in accordance with the manufacturer’s recommendations, runs as positively supercoiled (+ve) while negatively supercoiled topoisomers still run as negatively supercoiled (−ve). OC, open circular DNA.