Biophysics of left-handed Z-DNA

Substantial new information in the past several years has been reported on different types of DNA structures and the possible biological consequences of these conformations. Detailed X-ray crystallographic analyses have provided a good understanding of several types of DNA structures. Biochemical studies demonstrated the concept of DNA microheterogeneity, i.e., the realization that more than one conformation can exist in a given DNA molecule. This mandates the presence of conformational junctions. Most cellular DNA is believed to exist in a right-handed structure. However, left-handed helices are stabilized by supercoiling. The energetics of the transition reveal a delicate balance between right-handed structures and left-handed helices. Several types of sequences adopt left-handed structures and several of these are known to serve as recombination hot spots. The Hha I restriction enzyme and methylase do not act on left-handed DNA, thus these agents may be used as probes for left-handed helices within suitable sequences. Furthermore, B-Z-DNA junctions contain few if any, non-paired bases at physiological superhelical densities.