近日,，來自新加坡國立大學(xué)的研究者鑒別出了一種新型的雙鏈DNA結(jié)構(gòu)，雙鏈DNA結(jié)構(gòu)常常被描述為右手螺旋結(jié)構(gòu),，稱作B-DNA,。為了展現(xiàn)出其多重功能，雙鏈DNA依據(jù)不同情況會有多種結(jié)構(gòu),。例如在轉(zhuǎn)錄延伸階段會產(chǎn)生融化DNA水泡,，在轉(zhuǎn)錄調(diào)節(jié)階段會出現(xiàn)左手輪旋Z-DNA形式。
從1996年開始,，研究者寄希望于發(fā)現(xiàn)新的雙鏈DNA結(jié)構(gòu)形式,，說到S-DNA，通過拉伸B-DNA所得到,。研究者揭示了在高敏感度單鏈DNA拉脹實驗中DNA機械力學(xué)的錯綜復(fù)雜性,。研究者Yan教授和其同事發(fā)現(xiàn)了DNA的過度牽張涉及了兩個躍遷，這在躍遷動力學(xué)上非常顯著,，然而是否這種未知的DNA結(jié)構(gòu)由非滯后的躍遷產(chǎn)生目前仍然不清楚,。這項研究成果刊登在了Nucleic Acids Research上。
另一項刊登在PNAS上的研究中,，研究者Yan教授驗證了和兩種躍遷相關(guān)的熱力學(xué),，研究發(fā)現(xiàn)非滯后的躍遷和小負性熵變相關(guān)，和在滯后剝離躍遷中發(fā)現(xiàn)的大正向熵變相反,。研究結(jié)果揭示了DNA重排成為高度有序狀態(tài),。研究者同時揭示了兩種躍遷之間的選擇依賴于DNA堿基對的穩(wěn)定性。
這項研究不僅揭示了S-DNA是否存在，而且為研究S-DNA的新功能提供了新的思路,。在細胞中,，許多DNA結(jié)合蛋白利用側(cè)鏈加入來破壞DNA的主鏈，因此,，S-DNA或許也是有機體中研究的潛在的結(jié)合靶點,。（生物谷Bioon.com）
編譯自：NUS researchers identify a novel double-stranded DNA structure
Nucleic Acids Research文章鏈接
doi:10.1073/pnas.1109824109
PMC:
PMID:
Two distinct overstretched DNA structures revealed by single-molecule thermodynamics measurements
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.
