FIGURE 5.

Drug-independent covalent complex formation by wild-type and mutant Top2 derivatives. The K⁺/SDS assay was used to assess quantitative levels of DNA cleavage in the absence of Top2 poisons. Cleavage reactions contained α-³²P-end-labeled pUC18 and the indicated concentrations of Top2 protein. Reactions were terminated with SDS, and protein-DNA complexes were precipitated by the addition of KCl. After washing, the levels of radioactive DNA co-precipitating with proteins were determined by scintillation counting. The percentage of α-³²P-labeled DNA recovered in the pellet is shown for different enzyme concentrations. Error bars represent the standard deviation of three independent experiments.