Enzymatic probes for left-handed Z-DNA.

In conclusion, one of the aspects of the DNA polymorphism observed is the formation of Z-DNA under a variety of conditions. Left-handed DNA stretches not only represent alternate structures, but also exert long-range effects due to their influence on superhelical properties on an entire supercoiled DNA as first shown six years ago [49, 50]. The examples given in this review emphasize the site-specificity of enzymes due to structural features rather than sequence itself. In this fashion, the reversible transition from B to Z DNA could modulate site-specific events on many levels of biological regulation. Considering all of the enzymes studied to date (S1, mung bean, BAL31, P1 nucleases, Hha I, BssH II, MHha I, BamH I, EcoR I, RNA polymerase, recl, recA, DNA glycosylase, O6-methylguanine-DNA methyltransferase), only the recl (and possibly the recA) protein seems to recognize and utilize left-handed DNA. A large number of questions related to the biology of Z-DNA are unanswered including: what is the DNA structure (B or Z or other) which is in physical contact with proteins; is Z-DNA recognized by proteins or are junctions the important features; do proteins revert the Z structure to B or to some other right-handed conformation; what other cofactors (perhaps chiral in nature) may be involved; what are the alternate forms of left-handed DNA; does left-handed DNA exist in vivo; what is the biological role(s) of left-handed DNA? The future of this field of investigation will be exciting indeed.