Large Scale Chromatin Structure

大规模染色质结构是染色质在数百个折叠酶的规模上的高级折叠。对这种折叠知之甚少，但其结构一定会影响DNA的转录、复制和修复。为了了解更多关于这些结构，我们以前研究了400 - 2000 kb的转录活性域的光学显微镜。我们发现，几个不同的域在光学显微镜图像中表现出相似的结构，即一系列直径约0.5微米的相邻斑点或“珠”。当这些相同的结构域在转录上无活性时，仅检测到一个点或“珠”。因此，转录诱导从单珠结构到多珠结构的解凝聚。现在，我们已经研究了协会的polyermase II与这些珠状结构使用串联阵列系统中所描述的转录因子流动性项目。我们发现聚合酶也形成与上述染色质珠缔合但不直接覆盖的“珠”。虽然以前没有报道过聚合酶与染色质结构的直接比较，但聚合酶聚集的倾向已被广泛报道，并导致了这样的模型，即转录发生在这些簇或“工厂”中，通过DNA在工厂中的移动，而不是聚合酶的DNA沿着移动。该模型预测聚合酶工厂应该被最近转录的DNA包围。我们已经通过开发一种新的方法来保护细胞中脆弱的DNA结构来测试这一点，并发现与串联阵列相关的聚合酶工厂实际上被来自串联阵列的DNA包围。围绕工厂的DNA似乎是最近转录的，因为它被与最近转录的DNA相关的某种组蛋白修饰所标记。这些观察结果提供了新的支持的假设，转录发生的DNA通过一个固定的聚合酶的运动。

Large-scale chromatin structure is the higher-order folding of chromatin on the scale of several hundred kilobases. Little is known about this folding, but the structures must influence transcription, replication and repair of DNA. To learn more about these structures, we previously examined transcriptionally active domains of 400 2000 kb by light microscopy. We found that several different domains exhibit similar structures in light microscope images, namely a series of adjacent puncta or "beads" approximately 0.5 microns in diameter. When these same domains are transcriptionally inactive, only one punctum or "bead" is detected. Thus transcription induces a decondensation from single beaded to multi-beaded structures. We have now examined the association of polyermase II with these beaded structures using the tandem array system described in the transcription factor mobility project. We find that the polymerase also forms "beads" that associate with, but do not directly overlay the chromatin beads described above. Although a direct comparison of polymerase with chromatin structure has not been reported before, the tendency of polymerase to cluster has been widely reported, and has led to the model that transcription occurs in these clusters or "factories" by movement of the DNA through the factory, rather than movement along the DNA of the polymerase. This model predicts polymerase factories should be surrounded by recently transcribed DNA. We have tested this by developing a new method for preserving fragile DNA structures in cells and found that the polymerase factories associated with the tandem array are in fact surrounded by DNA from the tandem array. This DNA surrounding the factory seems to have been recently transcribed, since it is marked by a certain histone modification associated with recently transcribed DNA. These observations provide new support to the hypothesis that transcription occurs by movement of DNA through a stationary polymerase.