Livolant Françoise, Mangenot Stéphanie, Leforestier Amélie, Bertin Aurélie, Frutos Marta de, Raspaud Eric, Durand Dominique
Laboratoire de Physique des Solides, Bât 510, Université Paris Sud, 91405 Orsay Cedex, France.
Philos Trans A Math Phys Eng Sci. 2006 Oct 15;364(1847):2615-33. doi: 10.1098/rsta.2006.1843.
Nucleosome core particles correspond to the structural units of eukaryotic chromatin. They are charged colloids, 101 Angstrom in diameter and 55 Angstrom in length, formed by the coiling of a 146/147 bp DNA fragment (50 nm) around the histone protein octamer. Solutions of these particles can be concentrated, under osmotic pressure, up to the concentrations found in the nuclei of living cells. In the presence of monovalent cations (Na(+)), nucleosomes self-assemble into crystalline or liquid crystalline phases. A lamello-columnar phase is observed at 'low salt' concentrations, while a two-dimensional hexagonal phase and a three-dimensional quasi-hexagonal phase form at 'high salt' concentrations. We followed the formation of these phases from the dilute isotropic solutions to the ordered phases by combining cryoelectron microscopy and X-ray diffraction analyses. The phase diagram is presented as a function of the monovalent salt concentration and applied osmotic pressure. An alternative method to condense nucleosomes is to induce their aggregation upon addition of divalent or multivalent cations (Mg(2+), spermidine(3+) and spermine(4+)). Ordered phases are also found in the aggregates. We also discuss whether these condensed phases of nucleosomes may be relevant from a biological point of view.
核小体核心颗粒对应于真核染色质的结构单元。它们是带电胶体，直径为101埃，长度为55埃，由146/147碱基对的DNA片段（50纳米）围绕组蛋白八聚体盘绕形成。这些颗粒的溶液在渗透压作用下可以浓缩至活细胞核中发现的浓度。在单价阳离子（Na(+)）存在的情况下，核小体自组装成结晶或液晶相。在“低盐”浓度下观察到层状柱状相，而在“高盐”浓度下形成二维六方相和三维准六方相。我们通过结合冷冻电子显微镜和X射线衍射分析，跟踪了从稀各向同性溶液到有序相的这些相的形成过程。相图表示为单价盐浓度和施加渗透压的函数。另一种浓缩核小体的方法是在加入二价或多价阳离子（Mg(2+)、亚精胺(3+)和精胺(4+)）时诱导它们聚集。在聚集体中也发现了有序相。我们还讨论了从生物学角度来看，这些核小体的凝聚相是否可能具有相关性。
