The eukaryotic genome is compacted inside the nucleus of the cell in the form called chromatin. The fundamental unit of chromatin is the nucleosome, which contains four types of histones (H3, H4, H2A and H2B) and approximately 150 base pairs of DNA wrapped around the histone complex. The structure of the nucleosome is highly conserved across several eukaryotic species, and molecular replacement has been the primary phasing method used to solve nucleosome structures by X‐ray crystallography. However, there is currently no simple, widely applicable experimental phasing method for the nucleosome. In the present study, it is demonstrated that selenomethionine‐incorporated histones H3, H2A and H2B can be reconstituted into nucleosomes and crystallized for structural determination. Unexpectedly, it was found that the nucleosome can be phased with a relatively small number of Se atoms. The structures of nucleosome core particles containing 12 and 16 Se atoms were solved by SAD phasing at 2.5 and 2.4 Å resolution, respectively. The present study demonstrates a simple method for determining nucleosome structures by experimental phasing, which may be particularly useful for noncanonical structures that cannot be solved by molecular replacement.

中文翻译：

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硒SAD定相法测定核小体核心颗粒的结构。

真核基因组以染色质的形式被压紧在细胞核内。染色质的基本单位是核小体，它包含四种类型的组蛋白（H3，H4，H2A和H2B）和包裹在组蛋白复合物周围的大约150个碱基对的DNA。核糖体的结构在几种真核生物中高度保守，分子置换已成为通过X射线晶体学解决核糖体结构的主要定相方法。但是，目前尚无用于核小体的简单，广泛适用的实验定相方法。在本研究中，已证明可以将掺入硒代蛋氨酸的组蛋白H3，H2A和H2B重构为核小体，并进行结晶以进行结构测定。不料，已经发现核小体可以与相对少量的Se原子相定。分别通过2.5和2.4Å分辨率的SAD分相来解析包含12和16个Se原子的核小体核心颗粒的结构。本研究表明了一种通过实验定相确定核小体结构的简单方法，这对于无法通过分子置换解决的非规范结构特别有用。