Complex formation on nicked substrates. Complex formation on nicked substrates. Plasmids containing two cer sites in direct (pSDC115) or inverted (pSDC117) repeat were singly nicked with DNase I to remove all supercoiling. After incubation with PepA (6 nM) and ArgR (10 nM), as indicated, the nick was re‐sealed with DNA ligase, thereby trapping any supercoils caused by wrapping of the two cer sites about PepA and ArgR. The reactions were stopped with SDS and proteinase K, and topoisomers were separated on a chloroquine‐containing agarose gel. Claudia Alén et al. EMBO J. 1997;16:5188-5197 © as stated in the article, figure or figure legend