KCS

Left-handed Z-DNA is a higher energy form of the double helix, stabilized by negative supercoiling generated by transcription or unwrapping nucleosomes. Z-DNA is also stabilized by specific protein binding; several proteins have been identified with low nanomolar binding constants. Z-DNA occurs in a dynamic state, forming due to physiological processes then relaxing to the B form. Each time, a DNA segment turns to Z-DNA, two B-Z junctions form. These have been examined extensively, but their structure was unknown. We solved the structure of a B-Z junction by X-ray crystallography at 2.6 Å resolution. A fifteen base-pair segment of DNA is stabilized at one end in the Z conformation by Z-DNA binding proteins, while the other end remains B-DNA. Continuous stacking of bases between B-DNA and Z-DNA segments is found, with the breaking of one base pair at the junction and extrusion of the bases on each side. This distinctive structural features of B-Z junction were also examined and confirmed by fluorescence spectroscopy in solution.