FIG. 3.

Effect of a gyrB mutation on DNA topology. Plasmid pGOΔ15 was transiently introduced into wild-type B31 and the gyrB mutant X32. The topology of the plasmid was determined by one-dimensional 0.8% agarose gel electrophoresis in the absence (native) or presence of 15 μM chloroquine (CQ). The more negatively supercoiled molecules migrate faster through the gel; chloroquine intercalates into the plasmid DNA, introducing positive supercoiling that retards migration of negatively supercoiled DNA (sc) and expedites migration of relaxed DNA (r). The migration of the linearized (l) form of the plasmid is not affected in the mutant. Molecular size standards are in kilobase pairs.