[7] Generation and detection of Z-DNA

Z-DNA is the only known helical conformation of DNA that is left-handed. The Z-DNA helix is longer and thinner than B-DNA, and the bases lie relatively farther from the helix axis, creating a single deep, narrow groove. This chapter discusses the techniques available for inducing the formation of Z-DNA and detecting it. The left-handed helical sense means that any torsional stress directed opposite the right-handed helical sense of B-DNA will tend to favor the formation of Z-DNA. The syn-anti alternation dictates that alternating purine–pyrimidine sequences most easily form Z-DNA because pyrimidines favor the anti orientation, whereas purines can accommodate either the syn or the anti conformation. Because of the displacement of the bases from the helix axis, Z-DNA can be stabilized by adding bulky groups at sites that are sterically hindered in B-DNA but become more accessible in the Z conformation. Similarly, certain chemicals that react at those accessible sites can be used to recognize Z-DNA. The overall conformational differences between B-DNA and Z-DNA permit the two structures to be distinguished by many nucleases and DNA-binding proteins, which generally recognize right-handed DNA, and by antibodies specifically directed against Z-DNA.