Structural rearrangement of the histone octamer translocates DNA

将DNA包装成染色质调节了对遗传物质的获取。染色质的基本组成部分是核小体，它包含150个碱基对（bp）的DNA，包裹在由两个组蛋白H2A、H2B、H3和H4组成的八聚体上。核小体是高度动态的，可以沿着DNA以非催化或染色质重塑酶驱动的方式移动。非催化核小体滑动是核小体的固有特性，类似于染色质重塑酶实现的滑动。在细胞中，核小体在各种染色质重塑器的帮助下定位在DNA上。染色质重塑体的突变与癌症密切相关，甚至驱动癌症，显示核小体组织在基因组稳定性中的重要性。尽管核小体重塑具有重要意义，但对组蛋白八聚体在DNA上易位的结构机制知之甚少。使用低温电镜，我们在4.3 Å和5.0 Å下解析了组蛋白八聚体和DNA不同组织的核小体的结构。我们发现组蛋白八聚体会经历构象变化，从而扭曲整个核小体的结构。组蛋白核心a-螺旋中的重排移动DNA环流，并诱导扭曲DNA的张力。我们的数据表明，组蛋白八聚体是可塑的，组蛋白八聚体的结构变化会移动DNA。核小体的这种内在可塑性被染色质重塑者所利用，也可能被其他染色质机制所利用。然而，我们目前的分辨率不足以在核苷酸水平上观察DNA易位。在这个项目中，我们的目标是提高我们当前结构的分辨率，以便能够分辨DNA中的碱基（~3.5 Å）。这将使我们能够跟踪DNA中的单核苷酸，并解释组蛋白八聚体的DNA易位机制。除了目前的结构外，我们还将组装具有不同DNA序列的核小体和促进未催化核小体滑动的组蛋白突变体。使用这些结构，我们的目标是获得额外的构象，显示组蛋白八聚体DNA易位的中间产物。

The packaging of DNA into chromatin regulates access to genetic material. The basic building block of chromatin is the nucleosome, which contains 150 base pairs (bp) of DNA wrapped around an octamer composed of two copies of histones H2A, H2B, H3, and H4. Nucleosomes are highly dynamic and can move along the DNA in an uncatalyzed or chromatin remodeling enzyme-driven way. Uncatalyzed nucleosome sliding is an intrinsic property of nucleosomes and resembles sliding achieved by chromatin remodeling enzymes. In cells, nucleosomes are positioned on DNA with the help of various chromatin remodelers. Mutations in chromatin remodelers strongly associate with or even drive cancers showing the importance of nucleosome organization in genome stability. Despite the importance of nucleosome remodeling, relatively little is known about the structural mechanisms of histone octamer translocation on the DNA.Using cryo EM we solved structures of the nucleosome with different organization of the histone octamer and DNA at 4.3 Å and 5.0 Å. We found that the histone octamer goes through conformational changes that distort the overall nucleosome structure. Rearrangements in the histone core a-helices move DNA gyres and induce strain that distorts the DNA. Our data show that the histone octamer is plastic and that structural changes in the histone octamer move DNA. This intrinsic plasticity of the nucleosome is exploited by chromatin remodelers and might be used by other chromatin machineries as well. Our current resolution is, however, not sufficient to observe DNA translocation at the nucleotide level. In this project our goal is to improve the resolution of our current structures to be able to resolve bases in DNA (~3.5 Å). This will allow us to follow single nucleotides in the DNA and will explain the mechanism of DNA translocation by the histone octamer. In addition to current constructs, we will also assemble nucleosomes with different DNA sequences and with histone mutants that promote uncatalyzed nucleosome sliding. Using these constructs we aim to obtain additional conformations showing intermediates of DNA translocation by the histone octamer.