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. 1997 Mar 4;94(5):1692–1697. doi: 10.1073/pnas.94.5.1692

Figure 6.

Figure 6

The sign of crossovers in a plectonemically wound supercoiled DNA determines the sign of the trefoil produced by DNA topoisomerases. All trefoil topoisomers were made to be positively supercoiled under electrophoresis conditions. Samples run in lanes 2 and 6 were identical to that run in the lane designated Sc in Fig. 3 and contained positively supercoiled trefoils prepared from nicked trefoils formed by the action of S. cerevisiae DNA topoisomerase II on nicked unknotted DNA rings. These samples serve as markers for the positions of negative- and positive-noded trefoil topoisomers. Lanes: 1, trefoil formed by treatment of positively supercoiled pCC56cos with S. cerevisiae DNA topoisomerase II and AMPPNP; 3, trefoil formed by treatment of positively supercoiled pCC56cos with T4 DNA topoisomerase; 4, topoisomers of unknotted pCC56cos; 5, trefoil formed by treatment of negatively supercoiled pCC56cos with S. cerevisiae DNA topoisomerase II and AMPPNP. nc, nicked circle; n3, nicked trefoil; lin, linear DNA.