Vorlícková M, Kypr J
Institute of Biophysics, Czechoslovak Academy of Sciences, Brno.
J Biomol Struct Dyn. 1985 Aug;3(1):67-83. doi: 10.1080/07391102.1985.10508399.
The article reviews data indicating that poly(dA-dT).poly(dA-dT) is able of adopting three distinct double helical structures in solution, of which only the A form conforms to classical notions. The other two structures have dinucleotides as double helical repeats. At low salt concentrations poly(dA-dT).poly(dA-dT) adopts a B-type alternating conformation which is exceptionally variable. Its architecture can gradually move in the limits demarcated by the CD spectra with inverted long wavelength CD bands and the 31P NMR spectra with a very low and a 0.6 ppm separation of two resonances. Contrary to Z-DNA, the 31P NMR spectrum of the limiting alternating B conformation of poly(dA-dT).poly(dA-dT) is characterized by an upfield shift of one resonance. We attribute the exceptional conformational flexibility of the alternating B conformation to the unequal tendency of bases in the dA-dT and dT-dA steps to stack. However, by assuming the limiting alternating B conformation, the variability of the synthetic DNA is not exhausted. Specific agents make it isomerize into another conformation by a fast, two-state mechanism, which is reflected by a further deepening of the negative long wavelength CD band and a downfield shift of the 31P NMR resonance of poly(dA-dT).poly(dA-dT) that was constant in the course of the gradual alterations of the alternating B conformation. These changes are, however, qualitatively different from the way poly(dG-dC).poly(dG-dC) behaves in the course of the B-Z isomerization. Poly(dG-dC).poly(dG-dC) displays purine-pyrimidine (dGpdC) resonance in the characteristic downfield position, while the downfield resonance of poly(dA-dT).poly(dA-dT) belongs to the pyrimidine-purine (dTpdA) phosphodiester linkages. Consequently, phosphodiester linkages in the purine-pyrimidine steps play a similar role in the appearance of the Z form to the pyrimidine-purine phosphodiesters in the course of the isomerization of poly(dA-dT).poly(dA-dT). This excludes that the high-salt structures of poly(dA-dT).poly(dA-dT) and poly(dG-dC).poly(dG-dC) are members of the same conformational family. We call the high-salt conformation of poly(dA-dT).poly(dA-dT) X-DNA. It furthermore follows from the review that synthetic molecules of DNA with alternating purine-pyrimidine sequences of bases can adopt either the Z form or the X form, or even both, depending on the environmental conditions. This introduces a new dimension into the DNA double helix conformational variability. The possible biological relevance of the X form is suggested by experiments with linear molecules of natural DNA.(ABSTRACT TRUNCATED AT 400 WORDS)
本文回顾了相关数据，这些数据表明聚（dA-dT）·聚（dA-dT）在溶液中能够呈现三种不同的双螺旋结构，其中只有A形式符合经典概念。另外两种结构以二核苷酸作为双螺旋重复单元。在低盐浓度下，聚（dA-dT）·聚（dA-dT）呈现出一种异常多变的B型交替构象。其结构可以在由具有倒置长波长CD带的CD光谱和具有非常低的两个共振峰且共振峰间距为0.6 ppm的31P NMR光谱所界定的范围内逐渐变化。与Z-DNA相反，聚（dA-dT）·聚（dA-dT）的极限交替B构象的31P NMR光谱的特征是一个共振峰的高场位移。我们将交替B构象的异常构象灵活性归因于dA-dT和dT-dA步中碱基堆积倾向的不平等。然而，假设存在极限交替B构象，合成DNA的变异性并未穷尽。特定试剂会使其通过快速的双态机制异构化为另一种构象，这表现为聚（dA-dT）·聚（dA-dT）的负长波长CD带进一步加深以及31P NMR共振峰的低场位移，而在交替B构象的逐渐变化过程中该共振峰是恒定的。然而，这些变化在性质上与聚（dG-dC）·聚（dG-dC）在B-Z异构化过程中的行为方式不同。聚（dG-dC）·聚（dG-dC）在特征性的低场位置显示嘌呤-嘧啶（dGpdC）共振，而聚（dA-dT）·聚（dA-dT）的低场共振属于嘧啶-嘌呤（dTpdA）磷酸二酯键。因此，嘌呤-嘧啶步中的磷酸二酯键在Z形式的出现中所起的作用与聚（dA-dT）·聚（dA-dT）异构化过程中嘧啶-嘌呤磷酸二酯键所起的作用类似。这排除了聚（dA-dT）·聚（dA-dT）和聚（dG-dC）·聚（dG-dC）的高盐结构属于同一构象家族的可能性。我们将聚（dA-dT）·聚（dA-dT）的高盐构象称为X-DNA。此外，从该综述还可得出，具有交替嘌呤-嘧啶碱基序列的DNA合成分子可以根据环境条件采用Z形式或X形式，甚至两者兼具。这为DNA双螺旋构象变异性引入了一个新的维度。天然DNA线性分子的实验表明了X形式可能具有生物学相关性。（摘要截断于400字）
