Bertin Aurélie, Mangenot Stéphanie, Renouard Madalena, Durand Dominique, Livolant Françoise
Laboratoire de Physique des Solides, CNRS UMR 8502, Université Orsay-Sud, Paris, France.
Biophys J. 2007 Nov 15;93(10):3652-63. doi: 10.1529/biophysj.107.108365. Epub 2007 Aug 10.
The degree of compaction of the eukaryotic chromatin in vivo and in vitro is highly sensitive to the ionic environment. We address the question of the effect of multivalent ions on the interactions and mutual organization of the chromatin structural units, the nucleosome core particles (NCPs). Conditions of precipitation of NCPs in the presence of 10 mM Tris buffer and various amounts of either magnesium (Mg(2+)) or spermidine (Spd(3+)) are explored, compared, and discussed in relation to theoretical models. In addition, the structure of the aggregates is analyzed by complementary techniques: freeze-fracture electron microscopy, cryoelectron microscopy, and x-ray diffraction. In Mg(2+)-NCP aggregates, NCPs tend to stack on top of one another to form columns that are not long-range organized. In the presence of Spd(3+), NCPs precipitate to form a dense isotropic phase, a disordered phase of columns, a two-dimensional columnar hexagonal phase, or a three-dimensional crystal. The more ordered phases (two-dimensional or three-dimensional hexagonal) are found close to the precipitation line, where the number of positive charges carried by cations is slightly larger than the number of available negative charges of the NCPs. All ordered phases coexist with the dense isotropic phases. Formation of hexagonal and columnar phases is prevented by an excess of polycations.
真核染色质在体内和体外的压缩程度对离子环境高度敏感。我们探讨了多价离子对染色质结构单元——核小体核心颗粒（NCPs）的相互作用和相互组织的影响问题。研究、比较并结合理论模型讨论了在10 mM Tris缓冲液以及不同量的镁离子（Mg(2+)）或亚精胺（Spd(3+)）存在下NCPs的沉淀条件。此外，通过互补技术分析聚集体的结构：冷冻断裂电子显微镜、冷冻电子显微镜和X射线衍射。在Mg(2+)-NCP聚集体中，NCPs倾向于相互堆叠形成没有长程有序排列的柱体。在Spd(3+)存在的情况下，NCPs沉淀形成致密各向同性相、无序的柱体相、二维柱状六方相或三维晶体。在接近沉淀线的地方发现了更有序的相（二维或三维六方相），此时阳离子携带的正电荷数量略大于NCPs可用的负电荷数量。所有有序相都与致密各向同性相共存。过量的聚阳离子会阻止六方相和柱状相的形成。
