Ghirlando Rodolfo, Felsenfeld Gary
Laboratory of Molecular Biology, National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health, Bethesda, MD 20892-0540.
Biopolymers. 2013 Apr;99(4):225-32. doi: 10.1002/bip.22157.
The structure of the 30-nm chromatin fiber has provided, over the years, an important reference in chromatin studies. Originally derived from electron microscopic studies of soluble chromatin fibers released by restriction digestion, the gross structural features of such fragments have been supported by biophysical methods such as low angle X-ray and neutron scattering, sedimentation, light scattering, and electric dichroism. Electron microscopy and sedimentation velocity measurements demonstrated that reconstituted chromatin fibers, prepared from repeating arrays of high affinity nucleosome positioning sequences, retain the same overall features as observed for native chromatin fibers. It had been suggested that the 30 nm fiber might be the form assumed in vivo by transcriptionally silent chromatin, but individual gene or genome-wide studies of chromatin released from nuclei do not reveal any such simple correlation. Furthermore, even though the 30 nm fiber has been thought to represent an intermediate in the hierarchical folding of DNA into chromosomes, most analyses of chromatin folding within the nucleus do not detect any regular extended compact structures. However, there are important exceptions in chicken erythroid cell nuclei as well as in transcribed regions that form extended loops. Localized domains within the nucleus, either at the surface of chromosome domains or constrained as a specialized kind of constitutive heterochromatin by specific DNA binding proteins, may adopt 30 nm fiber-like structures.
多年来，30纳米染色质纤维的结构一直是染色质研究中的重要参考。最初源于对限制性酶切释放的可溶性染色质纤维的电子显微镜研究，此类片段的总体结构特征已得到诸如低角度X射线和中子散射、沉降、光散射以及电二色性等生物物理方法的支持。电子显微镜和沉降速度测量表明，由高亲和力核小体定位序列的重复阵列制备的重组染色质纤维保留了与天然染色质纤维相同的总体特征。有人提出30纳米纤维可能是体内转录沉默染色质所呈现的形式，但对从细胞核中释放的染色质进行的单个基因或全基因组研究并未揭示任何此类简单的相关性。此外，尽管30纳米纤维被认为代表了DNA分层折叠成染色体过程中的一个中间阶段，但大多数对细胞核内染色质折叠的分析并未检测到任何规则的伸展致密结构。然而，鸡红细胞核以及形成伸展环的转录区域存在重要的例外情况。细胞核内的局部结构域，要么位于染色体结构域的表面，要么被特定的DNA结合蛋白限制为一种特殊类型的组成型异染色质，可能会呈现出30纳米纤维样结构。
