A simple ligation assay to detect effects of drugs on the curvature/flexibility of DNA.

Circular DNA molecules can readily be formed from the 169 bp tyrT fragment in the presence of T4 DNA ligase. We have analyzed the formation of DNA circles in the presence of the clinically important antitumour drugs amsacrine, mitoxantrone and daunomycin. All three are intercalating agents but they affect the closure reaction differently: daunomycin and mitoxantrone progressively inhibit the formation of circles whereas at low concentrations amsacrine strongly enhances the yield of circles suggesting that this drug can increase the flexibility and/or curvature of DNA. The ligation assay described here may prove useful and widely applicable for investigating the effects of small molecules on the secondary structure of DNA.