Boles T C, White J H, Cozzarelli N R
Department of Molecular Biology, University of California, Berkeley 94720.
J Mol Biol. 1990 Jun 20;213(4):931-51. doi: 10.1016/S0022-2836(05)80272-4.
Using electron microscopy and topological methods, we have deduced an average structure for negatively supercoiled circular DNA in solution. Our data suggest that DNA has a branched plectonemic (interwound) form over the range of supercoiling tested. The length of the superhelix axis is constant at 41% of the DNA length, whereas the superhelix radius decreases essentially hyperbolically as supercoiling increases. The number of supercoils is 89% of the linking deficit. Both writhe and twist change with supercoiling, but the ratio of the change in writhe to the change in twist is fixed at 2.6:1. The extent of branching of the superhelix axis is proportional to the length of the plasmid, but is insensitive to superhelix density. The relationship between DNA flexibility constants for twisting and bending calculated using our structural data is similar to that deduced from previous studies. The extended thin form of plectonemically supercoiled DNA offers little compaction for cellular packaging, but promotes interaction between cis-acting sequence elements that may be distant in primary structure. We discuss additional biological implications of our structural data.
利用电子显微镜和拓扑学方法，我们推导了溶液中负超螺旋环状DNA的平均结构。我们的数据表明，在所测试的超螺旋范围内，DNA具有分支的纽结（相互缠绕）形式。超螺旋轴的长度恒定为DNA长度的41%，而随着超螺旋增加，超螺旋半径基本呈双曲线减小。超螺旋的数量为连接亏缺的89%。螺旋数和扭转数均随超螺旋而变化，但螺旋数变化与扭转数变化的比率固定为2.6:1。超螺旋轴的分支程度与质粒长度成正比，但对超螺旋密度不敏感。利用我们的结构数据计算得到的DNA扭转和弯曲的柔韧性常数之间的关系，与先前研究推导的关系相似。纽结状超螺旋DNA的细长扩展形式对细胞包装几乎没有压缩作用，但促进了在一级结构上可能相距较远的顺式作用序列元件之间的相互作用。我们讨论了我们结构数据的其他生物学意义。
