The DNA relaxation activity of Escherichia coli DNA topoisomerase I in vitro was greatly inhibited by cardiolipin. Inhibition also occurred to some extent with phosphatidylglycerol from egg yolk. Analysis with synthetic phospholipid revealed that phosphatidylglycerol containing unsaturated fatty acids exhibited a strong inhibitory effect, whereas inhibition by phosphatidylglycerol containing saturated fatty acids was weak. Phosphatidylethanolamine showed no inhibitory effect. Chlorpromazine, which interacts with phospholipids, suppressed the inhibitory effect of cardiolipin. Cardiolipin and phosphatidylglycerol with unsaturated fatty acid precipitated topoisomerase I even at low concentrations, whereas phosphatidylglycerol from egg yolk and a synthetic phosphatidylglycerol containing saturated fatty acids precipitated this enzyme only at high concentrations. One-third of the total topoisomerase I in E. coli was found in the membrane fraction. Treatment of E. coli cells with chlorpromazine resulted in relaxation of plasmid DNA. This DNA relaxation was not observed in a topA mutant, suggesting that this relaxation by chlorpromazine in vivo is catalysed by topoisomerase I.

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