Figure 7.

Catalytic activity of topoisomerase I in nuclear extracts from mitomycin C-treated cells. MCF-7 breast carcinoma cells (wt p53, A and B) and HT-29 colon carcinoma cells (mutant p53, C and D) were treated with the DNA-damaging agent mitomycin C (10 μg/ml) for 4 hours followed by a chase in drug-free media for an additional 20 hours. Different dilutions of partially purified nuclear extracts were tested for topoisomerase I activities as measured by relaxation of supercoiled DNA. The catalytic activity of untreated control cells was compared with either treated cells (A and C) or with treated cells postincubated in drug-free medium for 20 hours (B and D).