Repges Rudolf, Beuck Christine, Weinhold Elmar, Raabe Gerhard, Fleischhauer Jorg
Institute of Organic Chemistry, RWTH Aachen University, Landoltweg 1, D-52074 Aachen, Germany.
Chirality. 2008 Sep;20(9):978-84. doi: 10.1002/chir.20540.
A combination of experimental and theoretical circular dichroism (CD) spectroscopy was used to study local deformations of DNA caused by binding of the base flipping DNA methyltransferase M.TaqI. To selectively study the structural changes within the DNA, we replaced single guanine residues at six different positions in duplex DNA with 6-thioguanine (s(6)G), which absorbs at 342 nm where unmodified DNA and the enzyme are transparent. The shape and the transition wavelength of a CD signal around 340 nm in the spectra of the free DNA and the M.TaqI-bound DNA were found to depend on the position of the s(6)G probe. Theoretical rotational strengths were calculated employing the matrix method which is frequently used to model the CD of large biomolecules. The only chromophores in these calculations were the nucleic acid bases. Comparison of the measured and the calculated CD spectra showed that the applied computational method qualitatively reproduces the dominant band observed around 340 nm in all cases. From our results we conclude that the spectral changes observed upon binding of the enzyme to the DNA are indeed predominantly due to structural changes within the DNA and not to other effects caused by the presence of the enzyme.
采用实验性与理论性圆二色光谱（CD）相结合的方法，研究碱基翻转DNA甲基转移酶M.TaqI与DNA结合所引起的DNA局部变形。为了选择性地研究DNA内部的结构变化，我们用6-硫鸟嘌呤（s(6)G）取代了双链DNA中六个不同位置的单个鸟嘌呤残基，s(6)G在342 nm处有吸收，而未修饰的DNA和该酶在此处是透明的。发现游离DNA和与M.TaqI结合的DNA光谱中340 nm附近CD信号的形状和跃迁波长取决于s(6)G探针的位置。采用常用于模拟大型生物分子CD的矩阵法计算理论旋转强度。这些计算中唯一的发色团是核酸碱基。测量的CD光谱与计算的CD光谱比较表明，所应用的计算方法在所有情况下都能定性地重现340 nm附近观察到的主要谱带。从我们的结果可以得出结论，酶与DNA结合时观察到的光谱变化确实主要是由于DNA内部的结构变化，而不是由酶的存在引起的其他效应。
