Cruse W B, Saludjian P, Biala E, Strazewski P, Prangé T, Kennard O
Cambridge Crystallographic Data Centre, United Kingdom.
Proc Natl Acad Sci U S A. 1994 May 10;91(10):4160-4. doi: 10.1073/pnas.91.10.4160.
The nonamer r(GCUUCGGC)dBrU, where dBrU is 5-bromo-2'-deoxyuridine, contains the tetraloop sequence UUCG. It crystallizes in the presence of Rh(NH3)6Cl3. In solution the oligomer is expected to form a hairpin loop but the x-ray structure analysis, to a resolution of 1.6 A, indicates an eight-base-pair A-RNA duplex containing a central block of two G.U and two C.U pairs. Self-pairs which approximate to Watson-Crick geometry are also formed in the extended crystal structure between symmetry-related BrU residues and are part of infinite double-helical stacks. The G.U pair is a wobble base pair analogous to the G.T pair found in DNA fragments. The C.U mismatch involves one hydrogen-bonded contact between the bases and a bridging water molecule which ensures a good fit of the base pair in the RNA helix. The BrU.BrU pair is held by two hydrogen bonds in an orientation which is compatible with duplex geometry. The structure observed within the crystal has some parallels with the structure of globular RNAs, and the presence of stable, noncanonical base pairs has implications for the prediction of RNA secondary structure.
九聚体r(GCUUCGGC)dBrU（其中dBrU为5-溴-2'-脱氧尿苷）包含四环序列UUCG。它在六氨合铑(III) 氯盐存在下结晶。在溶液中，该寡聚物预计会形成发夹环，但分辨率为1.6 Å的X射线结构分析表明，它是一个包含两个G.U和两个C.U碱基对中心块的八碱基对A-RNA双链体。在对称相关的BrU残基之间的扩展晶体结构中也形成了近似沃森-克里克几何形状的自配对，并且是无限双螺旋堆积的一部分。G.U碱基对是一种摆动碱基对，类似于在DNA片段中发现的G.T碱基对。C.U错配涉及碱基之间的一个氢键接触和一个桥连水分子，这确保了碱基对在RNA螺旋中的良好契合。BrU.BrU碱基对通过两个氢键以与双链体几何形状兼容的方向结合。在晶体中观察到的结构与球状RNA的结构有一些相似之处，稳定的非规范碱基对的存在对RNA二级结构的预测有影响。
