S1 nuclease sensitivity assay. Supercoiled plasmids pTK2 (lanes 1–2), pTK1 (lanes 3–4) and pLacZ (lanes 5–6) were treated with or without S1 nuclease prior to linearization with HindIII. DNA fragments were electrophoresed on a 0.8% agarose gel and visualized by staining with ethidium bromide. Treatment with or without S1 nuclease is indicated at the top of the gel. S1 nuclease cleavage products and a DNA ladder (in Kb) are indicated.