Porschke D
Max Planck Institut Für Biophysikalische Chemie, Göttingen, FRG.
J Biomol Struct Dyn. 1986 Dec;4(3):373-89. doi: 10.1080/07391102.1986.10506356.
The long range structure of DNA restriction fragments has been analysed by electro-optical measurements. The overall rotation time constants observed in a low salt buffer with monovalent ions is shown to decrease upon addition of Mg2+ or spermine. Since the circular dichroism and also the limiting value of the linear dichroism remain almost constant under these conditions, the effect is attributed to a change of the long range structure. According to a weakly bending rod model, the persistence length decreases from about 600 A in the absence of Mg2+ or spermine to about 350 A in the presence of these ions. The persistence length measured in the presence of Mg2+ is almost independent of temperature in the range of 10 to 40 degrees C. The nature of DNA bending is analysed by measurements of bending amplitudes and time constants from dichroism decay curves. The observed absence of changes in the bending amplitudes upon addition of Mg2+ or spermine, even though addition induces changes of the persistence length by a factor of 2, is hardly consistent with simple thermal bending. The combined results, including the remarkably small temperature dependence of persistence length and bending amplitude, can be explained by the existence of two bending effects: inherent curvature of DNA dominates at low temperature, whereas thermal bending prevails at high temperature. Analysis of bending amplitudes from dichroism decay curves according to an arc model provides an approximate measure for the degree of bending in restriction fragments. The model is consistent with the observed chain length dependence of bending amplitudes and provides an approximate curvature corresponding to a radius of about 400 A. Thus the curvature observed in restriction fragments is similar to that observed for high molecular DNA condensed into toroids by addition of ions like spermine. Particularly strong bending of DNA is induced by [Co(NH3)6]3+, indicated by an apparent persistence length of 200 A and an increased bending amplitude together with a reduced limit value of the linear dichroism. This effect is attributed to the high charge density of this ion and potential site binding.
通过电光测量分析了DNA限制片段的长程结构。在含有单价离子的低盐缓冲液中观察到的整体旋转时间常数，在加入Mg2+或精胺后会降低。由于在这些条件下圆二色性以及线性二色性的极限值几乎保持不变，所以这种效应归因于长程结构的变化。根据弱弯曲杆模型，持久长度从不存在Mg2+或精胺时的约600埃减小到存在这些离子时的约350埃。在Mg2+存在下测量的持久长度在10至40摄氏度范围内几乎与温度无关。通过测量二色性衰减曲线的弯曲幅度和时间常数来分析DNA弯曲的性质。尽管加入Mg2+或精胺会使持久长度改变2倍，但观察到加入后弯曲幅度没有变化，这很难与简单的热弯曲相一致。综合结果，包括持久长度和弯曲幅度对温度的显著微弱依赖性，可以通过存在两种弯曲效应来解释：DNA的固有曲率在低温下占主导，而热弯曲在高温下占主导。根据弧模型分析二色性衰减曲线的弯曲幅度，为限制片段中的弯曲程度提供了一种近似测量方法。该模型与观察到的弯曲幅度对链长的依赖性一致，并提供了对应于约400埃半径的近似曲率。因此，在限制片段中观察到的曲率与通过加入精胺等离子将高分子量DNA浓缩成环面时观察到的曲率相似。[Co(NH3)6]3+会诱导DNA发生特别强烈的弯曲，表现为表观持久长度为200埃，弯曲幅度增加以及线性二色性的极限值降低。这种效应归因于该离子的高电荷密度和潜在的位点结合。
