Z-DNA
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Z-DNA is one of the many possible double helical structures of DNA. It is a left-handed double helical structure in which the double helix winds to the left in a zig-zag pattern (instead of to the right, like the more common B-DNA form). Z-DNA is thought to be one of three biologically active double helical structures along with A- and B-DNA.
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[edit] History
Z-DNA was the first single-crystal X-ray structure of a DNA fragment (a self-complementary DNA hexamer d(CG)3). It was resovled as a lefthanded double helix with two anti-parallel chains that were held together by Watson-Crick base pairs (see: x-ray crystallography). It was solved by, Andrew Wang, Alexander Rich, and co-workers in 1979 at MIT.[1] The crystallisation of a B- to Z-DNA junction in 2005[2] 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.[3]
[edit] Structure
Z-DNA is quite different from the right-handed forms. In fact, Z-DNA is often compared against B-DNA in order to illustrate the major differences. The Z-DNA helix is left-handed and has a structure that repeats every 2 base pairs. The major and minor grooves, unlike A- and B-DNA, show little difference in width. Formation of this structure is generally unfavourable, although certain conditions can promote it; such as alternating purine-pyrimidine sequence (especially poly(dGC)2), DNA supercoiling or low salt and some cations (all at physiological temperature, 37°C, and pH 7.3-7.4). Z-DNA can form a junction with B-DNA in a structure which involves the extrusion of a base pair. The Z-DNA conformation has been difficult to study because it does not exist as a stable feature of the double helix. Instead, it is a transient structure that is occasionally induced by biological activity and then quickly disappears.[4]
[edit] Predicting Z-DNA structure
It is possible to predict the likelihood of a DNA sequence forming a Z-DNA structure. An algorithm for predicting the propensity of DNA to flip from the B-form to the Z-form, ZHunt, was written by Dr. P. Shing Ho in 1984 (at MIT). This algorithm was later developed by Tracy Camp, P. Christoph Champ, Sandor Maurice, and Jeffrey M. Vargason for genome-wide mapping of Z-DNA (with P. Shing Ho as the principal investigator).[5] Z-Hunt is available at Z-Hunt online.
[edit] Biological significance
While no definitive biological significance of Z-DNA has been found, it is commonly believed to provide torsional strain relief (supercoiling) while DNA transcription occurs.[6][2] The potential to form a Z-DNA structure also correlates with regions of active transcription. A comparison of regions with a high sequence-dependent, predicted propensity to form Z-DNA in human chromosome 22 with a selected set of known gene transcription sites suggests there is a correlation.[5]
Z-DNA formed after transcription initiation in some cases may be bound by RNA modifying enzymes which then alter the sequence of the newly-formed RNA [1].
[edit] Comparison Geometries of Some DNA Forms
| Geometry attribute | A-form | B-form | Z-form |
|---|---|---|---|
| Helix sense | right-handed | right-handed | left-handed |
| Repeating unit | 1 bp | 1 bp | 2 bp |
| Rotation/bp | 32.7° | 35.9° | 60°/2 |
| bp/turn | 11 | 10.5 | 12 |
| Inclination of bp to axis | +19° | −1.2° | −9° |
| Rise/bp along axis | 2.3 Å (0.23 nm) | 3.32 Å (0.332 nm) | 3.8 Å (0.38 nm) |
| Pitch/turn of helix | 28.2 Å (2.82 nm) | 33.2 Å (3.32 nm) | 45.6 Å (4.56 nm) |
| Mean propeller twist | +18° | +16° | 0° |
| Glycosyl angle | anti | anti | C: anti, G: syn |
| Sugar pucker | C3'-endo | C2'-endo | C: C2'-endo, G: C2'-exo |
| Diameter | 23 Å (2.3 nm) | 20 Å (2.0 nm) | 18 Å (1.8 nm) |
[edit] References
- ^ Wang AHJ, Quigley GJ, Kolpak FJ, Crawford JL, van Boom JH, Van der Marel G, Rich A (1979). "Molecular structure of a left-handed double helical DNA fragment at atomic resolution". Nature (London) 282: 680-686. doi:. PMID 514347
- ^ a b Ha SC, Lowenhaupt K, Rich A, Kim YG, Kim KK (2005). "Crystal structure of a junction between B-DNA and Z-DNA reveals two extruded bases". Nature 437: 1183-1186. doi:. PMID 16237447.
- ^ Placido D, Brown BA 2nd, Lowenhaupt K, Rich A, Athanasiadis A (2007). "A left-handed RNA double helix bound by the Zalpha domain of the RNA-editing enzyme ADAR1". Structure 15 (4): 395-404. doi:. PMID 17437712.
- ^ Zhang H, Yu H, Ren J, Qu X (2006). "Reversible B/Z-DNA Transition under the Low Salt Condition and Non-B-Form PolydApolydT Selectivity by a Cubane-Like Europium-L-Aspartic Acid Complex". Biophysical Journal 90: 3203-3207. doi:10.1529/biophysj.105.078402.
- ^ a b Champ PC, Maurice S, Vargason JM, Camp T, Ho PS (2004). "Distributions of Z-DNA and nuclear factor I in human chromosome 22: a model for coupled transcriptional regulation". Nucleic Acids Res. 32 (22): 6501–10. doi:. PMID 15598822.
- ^ Rich A, Zhang S (2003). "Timeline: Z-DNA: the long road to biological function". Nature Rev Genet 4: 566–572.
[edit] Further reading
- Sinden RR (1994). DNA structure and function. Academic Press, 179-216. ISBN 0-12-645750-6
- Rich A, Zhang S (2003). Timeline: Z-DNA: the long road to biological function. Nat Rev Genet, 4:566–572.
[edit] See also
[edit] External links
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