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Transfection of p53 into p53-null cells is accompanied by increased topoisomerase I activity. Nuclear extracts were prepared from M1 cells expressing no p53 (S6 cells) or mutant p53 (LTR13 cells at 37°C). The catalytic activity of topoisomerase I was determined by relaxation of supercoiled DNA in the presence of decreasing concentrations of nuclear extracts. S, supercoiled DNA; R, relaxed DNA.
