Intermolecular linking and fragmentation of DNA by beta-propiolactone, a monoalkylating carcinogen

Brief exposure to beta-propiolactone (BPL) increases the sedimentation rate of purified Escherichia coli DNA in neutral and alkaline sucrose gradients. However, when electrophoresed in polyacrylamide-agarose gels, this BPL-treated DNA moves ahead of the control. Longer incubation with BPL gives rise to two new fractions, the first one sedimenting as a heterogeneous material of 6-8S, and the second one of very high sedimentation velocity. In acrylamide-agarose gels, the first fraction is again recovered in the 6-8S area, while the second fraction does not enter the gel at all. The DNA at this stage is hyperchromic in ultraviolet light suggesting that as much as 20% may be denaturated. Coliphage lambda DNA treated briefly with BPL and spread in a protein monolayer appears under the electron microscope as a rigid, extended molecule, up to 15% longer than the control DNA, and usually in compact, folded configurations suggesting intramolecular linking. After longer exposure, localized denaturation associated with single-strand breaks is observed. The single-stranded "whiskers" then interact with other DNA molecules, creating highly complex branched networks of single- and multi-stranded DNA. The possible relevance of these observations to the mechanisms involved in carcinogenesis and mutagenesis is considered.