FORMATION OF NUCLEOSOMES ON POSITIVELY SUPERCOILED DNA
FORMATION OF NUCLEOSOMES ON POSITIVELY SUPERCOILED DNA
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DOI：
10.1002/j.1460-2075.1991.tb07960.x
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发表时间：
1991-02-01
期刊：
影响因子：
11.4
通讯作者：
FELSENFELD, G
中科院分区：
文献类型：
--
作者：
CLARK, DJ;FELSENFELD, G
转录RNA聚合酶被认为在前进的转录复合物前面产生正超螺旋，在后面产生负超螺旋。我们已经研究了正超螺旋可能使核小体不稳定，从而促进转录的可能性。我们发现，组蛋白八聚体绑定到正超螺旋DNA，并在复杂的放松，“经典”核小体存在。我们测试了正超螺旋DNA上的核小体处于改变的（可能是更开放的）构象，但只有在释放这种应力时才恢复到经典结构的可能性。然而，圆二色性光谱，化学交联和核心组蛋白的修饰，都表明，最初形成的复合物的正超螺旋DNA是经典的核小体。虽然这种结构是稳定的，但它们的形成需要质粒变得更加正超螺旋，导致更大的超螺旋应力。相反，负超螺旋DNA上核小体的形成缓解了超螺旋应力。在交换实验中，达到平衡，核小体转移从积极的负超螺旋DNA，从超螺旋反应的自由能预测。这表明了组装成染色质的基因的转录机制，其中八聚体从聚合酶前面的区域顺序转移到后面的区域。
A transcribing RNA polymerase is though to generate positive supercoils in front of the advancing transcription complex and negative supercoils behind. We have examined the possibility that positive supercoils might destabilize nucleosomes, facilitating transcription. We show that histone octamers bind to positively supercoiled DNA, and that after the complex is relaxed, 'classical' nucleosomes are present. We tested the possibility that nucleosomes on positively supercoiled DNA are in an altered (presumably more open) conformation, but revert to the classical structure only on release of this stress. However, circular dichroic spectra, and chemical cross-linking and modification of core histones, all suggest that the complexes initially formed on positively supercoiled DNA are classical nucleosomes. Although such structures are stable, their formation requires the plasmid to become more positively supercoiled, resulting in greater superhelical stress. In contrast, formation of nucleosomes on negatively supercoiled DNA relieves superhelical stress. In an exchange experiment in which equilibrium is achieved, nucleosomes transfer from positively to negatively supercoiled DNA, as predicted from the supercoiling free energies of the reactions. This suggests a mechanism for transcription of a gene assembled into chromatin, in which octamers are sequentially transferred from the region in front of the polymerase to the region behind.