Editing Z-DNA (section)

==History==
Left-handed DNA was first proposed by [[Robert Wells (biochemist)|Robert Wells]] and colleagues, as the structure of a repeating [[polymer]] of [[inosine]]–[[cytosine]].<ref>{{cite journal |last1=Mitsui |first1=Y. |last2=Langridge |first2=R. |last3=Shortle |first3=B. E. |last4=Cantor |first4=C. R. |last5=Grant |first5=R. C. |last6=Kodama |first6=M. |last7=Wells |first7=R. D. |title=Physical and enzymatic studies on poly d(I–C)·poly d(I–C), an unusual double-helical DNA |journal=Nature |volume=228 |issue=5277 |pages=1166–1169 |year=1970 |pmid=4321098 |doi=10.1038/2281166a0|bibcode=1970Natur.228.1166M |s2cid=4248932 }}</ref> They observed a "reverse" [[circular dichroism]] spectrum for such DNAs, and interpreted this incorrectly to mean that the strands wrapped around one another in a left-handed fashion. The relationship between Z-DNA and the more familiar B-DNA was indicated by the work of Pohl and Jovin,<ref>{{cite journal |last1=Pohl |first1=F. M. |last2=Jovin |first2=T. M. |title=Salt-induced co-operative conformational change of a synthetic DNA: equilibrium and kinetic studies with poly(dG-dC) |journal=Journal of Molecular Biology |volume=67 |pages=375–396 |year=1972 |pmid=5045303 |doi=10.1016/0022-2836(72)90457-3 |issue=3}}</ref> who showed that the [[ultraviolet]] circular dichroism of poly(dG-dC) was nearly inverted in [[Molarity|4&nbsp;M]] [[sodium chloride]] solution and that the structure of poly d(I–C)·poly d(I–C) was in fact a right-handed D-DNA conformation. The suspicion that this was the result of a conversion from B-DNA to Z-DNA was confirmed by examining the [[Raman spectroscopy|Raman spectra]] of these solutions and the Z-DNA crystals.<ref>{{cite journal |last1=Thamann |first1=T. J. |last2=Lord |first2=R. C. |last3=Wang |first3=A. H. |last4=Rich |first4=A. |title=High salt form of poly(dG–dC)·poly(dG–dC) is left handed Z-DNA: raman spectra of crystals and solutions |journal=Nucleic Acids Research |volume=9 |pages=5443–5457 |year=1981 |pmid=7301594 |doi=10.1093/nar/9.20.5443 |issue=20 |pmc=327531}}</ref> Subsequently, a [[crystal structure]] of "Z-DNA" was published which turned out to be the first single-crystal X-ray structure of a DNA fragment (a self-complementary DNA hexamer d(CG)<sub>3</sub>). It was resolved as a left-handed double helix with two [[Antiparallel (biochemistry)|antiparallel]] chains that were held together by Watson–Crick [[base pair]]s (see [[X-ray crystallography]]). It was solved by [[Andrew H. J. Wang]], [[Alexander Rich]], and coworkers in 1979 at [[Massachusetts Institute of Technology|MIT]].<ref name=Wang1979>{{cite journal 
|last1=Wang |first1=A. H. |last2=Quigley |first2=G. J. |last3=Kolpak |first3=F. J. |last4=Crawford |first4=J. L. |last5=van Boom |first5=J. H. |last6=van der Marel |first6=G. |last7=Rich |first7=A. |title=Molecular structure of a left-handed double helical DNA fragment at atomic resolution |journal=Nature |volume=282 |pages=680–686 |year=1979 |doi=10.1038/282680a0 |pmid=514347 |issue=5740 |bibcode=1979Natur.282..680W|s2cid=4337955 }}</ref> The crystallisation of a B- to Z-DNA junction in 2005<ref name=Ha2005>{{cite journal|last1=Ha |first1=S. C. |last2=Lowenhaupt |first2=K. |last3=Rich |first3=A. |last4=Kim |first4=Y. G. |last5=Kim |first5=K. K. |title=Crystal structure of a junction between B-DNA and Z-DNA reveals two extruded bases |journal=Nature |volume=437 |issue=7062 |pages=1183–1186 |year=2005 |doi=10.1038/nature04088 |pmid=16237447 |bibcode=2005Natur.437.1183H|s2cid=2539819 }}</ref> provided a better understanding of the potential role Z-DNA plays in cells. Whenever a segment of Z-DNA forms, there must be B–Z junctions at its two ends, interfacing it to the B-form of DNA found in the rest of the [[genome]].

In 2007, the [[RNA]] version of Z-DNA, [[Z-RNA]], was described as a transformed version of an [[A-RNA]] double helix into a left-handed helix.<ref name="Placido2007">{{cite journal|title=A left-handed RNA double helix bound by the Zalpha domain of the RNA-editing enzyme ADAR1 |last1=Placido |first1=D. |last2=Brown |first2=B. A. II|last3=Lowenhaupt |first3=K. |last4=Rich |first4=A. |last5=Athanasiadis |first5=A. |journal=Structure |volume=15 |issue=4 |pages=395–404 |year=2007 |pmid=17437712 |doi=10.1016/j.str.2007.03.001 |pmc=2082211}}</ref> The transition from A-RNA to Z-RNA, however, was already described in 1984.<ref name="Hall1984">{{cite journal|title='Z-RNA'—a left-handed RNA double helix |last1=Hall |first1=K. |last2=Cruz |first2=P. |last3=Tinoco |first3=I. Jr |last4=Jovin |first4=T. M. |last5=van de Sande |first5=J. H. |journal=Nature |volume=311 |issue=5986 |pages=584–586 |date=Oct 1984 |pmid=6482970 |doi=10.1038/311584a0 |bibcode = 1984Natur.311..584H |s2cid=4316862 }}</ref>