PNAS & Nuc Aci Res：发现新型双链DNA结构

<p align="center"><img src="http://www.bioon.com/biology/UploadFiles/201208/2012080523165624.jpg" alt="" width="400" height="134" border="0" /></p>
<p align="center"><span style="font-family: 楷体_GB2312; font-size: small;">这种新型的双链DNA结构可通过机械牵拉获得。</span></p>
<p align="center"><span style="font-family: 楷体_GB2312; font-size: small;"> Credit: National University of Singapore</span></p>
近日，来自新加坡国立大学的研究者鉴别出了一种新型的双链DNA结构，双链DNA结构常常被描述为右手螺旋结构，称作B-DNA。为了展现出其多重功能，双链DNA依据不同情况会有多种结构。例如在转录延伸阶段会产生融化DNA水泡，在转录调节阶段会出现左手轮旋Z-DNA形式。

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从1996年开始，研究者寄希望于发现新的双链DNA结构形式，说到S-DNA，通过拉伸B-DNA所得到。研究者揭示了在高敏感度单链DNA拉胀实验中DNA机械力学的错综复杂性。研究者Yan教授和其同事发现了DNA的过度牵张涉及了两个跃迁，这在跃迁动力学上非常显著，然而是否这种未知的DNA结构由非滞后的跃迁产生目前仍然不清楚。这项研究成果刊登在了<em>Nucleic Acids Research</em>上。

另一项刊登在<em>PNAS</em>上的研究中，研究者Yan教授验证了和两种跃迁相关的热力学，研究发现非滞后的跃迁和小负性熵变相关，和在滞后剥离跃迁中发现的大正向熵变相反。研究结果揭示了DNA重排成为高度有序状态。研究者同时揭示了两种跃迁之间的选择依赖于DNA碱基对的稳定性。

这项研究不仅揭示了S-DNA是否存在，而且为研究S-DNA的新功能提供了新的思路。在细胞中，许多DNA结合蛋白利用侧链加入来破坏DNA的主链，因此，S-DNA或许也是有机体中研究的潜在的结合靶点。

编译自：<a title="" href="http://phys.org/news/2012-08-nus-double-stranded-dna.html" target="_blank">NUS researchers identify a novel double-stranded DNA structure</a>

<a title="" href="http://nar.oxfordjournals.org/content/39/8/3473.abstract?sid=631d3a19-99be-49e8-927a-976ad7872771" target="_blank">Nucleic Acids Research文章链接</a>
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<a title="" href="http://dx.doi.org/doi:10.1073/pnas.1109824109" target="_blank">doi:10.1073/pnas.1109824109</a>
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<br/><strong>Two distinct overstretched DNA structures revealed by single-molecule thermodynamics measurements</strong><br/>


Xinghua Zhanga,b,c, Hu Chenb, Hongxia Fub, Patrick S. Doylea,d,1, and Jie Yana,b,c,e,1

Double-stranded DNA is a dynamic molecule whose structure can change depending on conditions. While there is consensus in the literature about many structures DNA can have, the state of highly-stretched DNA is still not clear. Several groups have shown that DNA in the torsion-unconstrained B-form undergoes an “overstretching” transition at a stretching force of around 65 pN, which leads to approximately 1.7-fold elongation of the DNA contour length. Recent experiments have revealed that two distinct structural transitions are involved in the overstretching process: (i) a hysteretic “peeling” off one strand from its complementary strand, and (ii) a nonhysteretic transition that leads to an undetermined DNA structure. We report the first simultaneous determination of the entropy (ΔS) and enthalpy changes (ΔH) pertaining to these respective transitions. For the hysteretic peeling transition, we determined ΔS ∼ 20 cal/(K.mol) and ΔH ∼ 7 kcal/mol. In the case of the nonhysteretic transition, ΔS ∼ -3 cal/(K.mol) and ΔH ∼ 1 kcal/mol. Furthermore, the response of the transition force to salt concentration implies that the two DNA strands are spatially separated after the hysteretic peeling transition. In contrast, the corresponding response after the nonhysteretic transition indicated that the strands remained in close proximity. The selection between the two transitions depends on DNA base-pair stability, and it can be illustrated by a multidimensional phase diagram. Our results provide important insights into the thermodynamics of DNA overstretching and conformational structures of overstretched DNA that may play an important role in vivo.

<br/>来源：生物谷

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