Camptothecin inhibits DNA relaxation but not kinase activity of DNA topoisomerase I. (A) Control and SMA fibroblasts were treated with 25 μM camptothecin, and levels of phosphorylated SR proteins in the nuclear extracts were analyzed by Western blotting. The same blot was then stripped and reprobed with anti-histone 3 (H3) antibodies as a loading control. (B) DNA topoisomerase I was immunoprecipitated from camptothecin treated fibroblasts and subjected to the DNA unwinding assay. Plasmid DNA in the supercoiled form (SC) and in the relaxed form (R) is indicated. (C) DNA topoisomerase I immunoprecipitated from untreated control fibroblasts was subjected to the DNA unwinding assay in the presence of camptothecin at the indicated concentrations. All the data shown here are representative of at least two independent experiments. CPT = camptothecin, H3 = histone 3, and Topo I = DNA topoisomerase I.