Departament de Bioquímica i Biologia Molecular, Facultat de Biociències, Universitat Autònoma de Barcelona, Barcelona, Spain.
NCD Beamline, ALBA Synchrotron Light Source, Cerdanyola del Vallès, Barcelona, Spain.
EMBO J. 2019 Apr 1;38(7). doi: 10.15252/embj.201899769. Epub 2019 Jan 4.
Cryo-electron tomography and small-angle X-ray scattering were used to investigate the chromatin folding in metaphase chromosomes. The tomographic 3D reconstructions show that frozen-hydrated chromatin emanated from chromosomes is planar and forms multilayered plates. The layer thickness was measured accounting for the contrast transfer function fringes at the plate edges, yielding a width of ~ 7.5 nm, which is compatible with the dimensions of a monolayer of nucleosomes slightly tilted with respect to the layer surface. Individual nucleosomes are visible decorating distorted plates, but typical plates are very dense and nucleosomes are not identifiable as individual units, indicating that they are tightly packed. Two layers in contact are ~ 13 nm thick, which is thinner than the sum of two independent layers, suggesting that nucleosomes in the layers interdigitate. X-ray scattering of whole chromosomes shows a main scattering peak at ~ 6 nm, which can be correlated with the distance between layers and between interdigitating nucleosomes interacting through their faces. These observations support a model where compact chromosomes are composed of many chromatin layers stacked along the chromosome axis.
利用冷冻电子断层扫描和小角 X 射线散射技术研究了中期染色体中的染色质折叠。断层扫描的 3D 重建显示，从染色体中伸出的冷冻水合染色质是平面的，并形成多层板。通过在板边缘处的对比传递函数条纹来测量层厚度，得到约 7.5nm 的宽度，这与稍微倾斜于层表面的单层核小体的尺寸相匹配。单个核小体可见于装饰变形的板上，但典型的板非常密集，核小体无法作为单个单元识别，表明它们紧密堆积。相互接触的两层约 13nm 厚，比两个独立层的总和还要薄，表明层内的核小体相互交错。整个染色体的 X 射线散射在约 6nm 处显示出一个主要散射峰，这可以与层之间以及通过其表面相互作用的交错核小体之间的距离相关联。这些观察结果支持了一个模型，即紧凑的染色体由沿着染色体轴堆叠的许多染色质层组成。
