Department of Biological Sciences and Centre for BioImaging Sciences, National University of Singapore, 117543, Singapore.
Temasek Life Sciences Laboratory, National University of Singapore, 117604, Singapore.
Proc Natl Acad Sci U S A. 2018 Oct 23;115(43):10977-10982. doi: 10.1073/pnas.1720476115. Epub 2018 Oct 8.
Chromosomes condense during mitosis in most eukaryotes. This transformation involves rearrangements at the nucleosome level and has consequences for transcription. Here, we use cryo-electron tomography (cryo-ET) to determine the 3D arrangement of nuclear macromolecular complexes, including nucleosomes, in frozen-hydrated cells. Using 3D classification analysis, we did not find evidence that nucleosomes resembling the crystal structure are abundant. This observation and those from other groups support the notion that a subset of fission yeast nucleosomes may be partially unwrapped in vivo. In both interphase and mitotic cells, there is also no evidence of monolithic structures the size of Hi-C domains. The chromatin is mingled with two features: pockets, which are positions free of macromolecular complexes; and "megacomplexes," which are multimegadalton globular complexes like preribosomes. Mitotic chromatin is more crowded than interphase chromatin in subtle ways. Nearest-neighbor distance analyses show that mitotic chromatin is more compacted at the oligonucleosome than the dinucleosome level. Like interphase, mitotic chromosomes contain megacomplexes and pockets. This uneven chromosome condensation helps explain a longstanding enigma of mitosis: a subset of genes is up-regulated.
在大多数真核生物中，染色体在有丝分裂过程中浓缩。这种转变涉及核小体水平的重排，并对转录产生影响。在这里，我们使用冷冻电子断层扫描（cryo-ET）来确定包括核小体在内的核大分子复合物在冷冻水合细胞中的 3D 排列。通过 3D 分类分析，我们没有发现类似于晶体结构的核小体大量存在的证据。这一观察结果和其他小组的结果支持了裂殖酵母核小体的一部分可能在体内部分展开的观点。在间期和有丝分裂细胞中，也没有证据表明存在大小相当于 Hi-C 结构域的整体结构。染色质与两个特征混合在一起：口袋，即没有大分子复合物的位置；和“巨型复合物”，是类似于前核糖体的多百万道尔顿的球状复合物。有丝分裂染色质比间期染色质在细微方面更加拥挤。最近邻距离分析表明，有丝分裂染色质在寡核小体水平比二核小体水平更紧凑。与间期一样，有丝分裂染色体包含巨型复合物和口袋。这种不均匀的染色体浓缩有助于解释有丝分裂的一个长期存在的谜团：一部分基因被上调。
