Fig. 1.

Negative supercoiling induces the B-Z transition. When the DNA tether containing the GC core DNA flanked by donor (Green) and acceptor (Red) dyes is underwound by magnetic tweezers (Left), the FRET efficiency changes from a high to a low value, indicating the transition from the B-form (Blue) to the Z-form (Yellow) (Right). When the supercoiled DNA is buckled at low tension (Middle), the B-Z transition removes some plectonemes, increasing its extension by ΔL (Right). Inset: DNA constructs. A five-fragment DNA construct containing the donor and acceptor dyes in the middle and two distinct attachment sites (Dig & Bio) at the ends was used in the hybrid scheme. The dimer construct bearing the dye pair was used for FRET measurements.