Czarnota G J, Ottensmeyer F P
Ontario Cancer Institute and the Department of Medical Biophysics, University of Toronto, Toronto, Ontario M5G 2M9, Canada.
J Biol Chem. 1996 Feb 16;271(7):3677-83. doi: 10.1074/jbc.271.7.3677.
The nucleosome is the fundamental component of the eukaryotic chromosome, participating in the packaging of DNA and in the regulation of gene expression. Its numerous interactions imply a structural dynamism. Previous biophysical studies under limited sets of conditions have not been able to reconcile structural differences and transitions observed. We have determined a series of nucleosome conformations over a >10,000-fold range in salt concentration using a combination of biochemical methods, spectroscopic electron microscopy, and three-dimensional reconstruction techniques for randomly oriented single particles. This study indicates several ionic strength-dependent nucleosome conformations and also reconciles the differences between currently existing divergent models for the nucleosome. At low ionic environments, the particle appears highly elongated, becoming more compact and prolate ellipsoidal as ionic strength is increased to 10 mm NaCl. At 30 mM NaCl, the particle exhibits a spheroidal conformation. As ionic strength is increased to 150 mM NaCl, the nucleosome conformation changes and becomes oblate. Above 450 mM NaCl, the structure becomes highly elongated again. The result of this study is a unifying concept in which the three-dimensional structure of the nucleosome is inferred to be dynamic in response to ionic interactions and in accord with biochemical and genetic studies.
核小体是真核染色体的基本组成部分，参与DNA的包装和基因表达的调控。它的众多相互作用意味着结构的动态变化。以往在有限条件下进行的生物物理研究未能协调所观察到的结构差异和转变。我们使用生化方法、光谱电子显微镜以及针对随机取向的单个颗粒的三维重建技术，确定了在盐浓度超过10000倍范围内的一系列核小体构象。这项研究表明了几种依赖离子强度的核小体构象，也协调了目前存在的关于核小体的不同模型之间的差异。在低离子环境中，颗粒显得高度拉长，随着离子强度增加到10 mM NaCl，颗粒变得更加紧凑且呈长椭圆形。在30 mM NaCl时，颗粒呈现球形构象。随着离子强度增加到150 mM NaCl，核小体构象发生变化并变为扁球形。在450 mM NaCl以上，结构再次变得高度拉长。这项研究的结果是一个统一的概念，即核小体的三维结构被推断为响应离子相互作用而动态变化，并且与生化和遗传学研究一致。
