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. 2019 Mar 19;116(8):1394–1405. doi: 10.1016/j.bpj.2019.03.005

Figure 1.

Figure 1

DNA elongation and untwisting by ethidium measured with magnetic tweezers using a surface-tethered DNA with an attached magnetic bead at its other end. A pair of permanent magnets allows us to stretch and to twist the DNA. Intercalation of ethidium between two adjacent basepairs increases the basepair distance by 0.33 nm and almost completely untwists the DNA double helix by an angle of ∼26° (7, 8). In magnetic tweezers, ethidium intercalation is seen on torsionally relaxed molecules as a DNA length increase (right versus left tweezers sketch). On torsionally constrained DNA molecules, ethidium intercalation locally removes part of the helical turns. This causes positive DNA supercoiling and a DNA length reduction (central tweezers sketch). When negative supercoiling is applied, the DNA can be torsionally relaxed (right tweezers sketch). This allows us to quantify the extent of ethidium-induced DNA untwisting. The structure shown at the right side represents the ethidium-CG/GC complex from (7) (Cambridge Crystallographic Data Center: ETCYGU10).