Sen D, Crothers D M
Biochemistry. 1986 Apr 8;25(7):1495-503. doi: 10.1021/bi00355a004.
We have used electric dichroism to investigate the influence of multivalent cations upon the compaction of chicken erythrocyte chromatin from the unfolded, 10-nm fiber to the 30-nm solenoid and subsequent aggregation. The pattern of condensation, which consists of compaction plus aggregation, is found to be strikingly similar for a variety of cations of differing charge, including the physiologically important polyamines spermine and spermidine. With a few exceptions such as Cu2+ and Gd3+, an optimally compacted fiber with reproducible hydrodynamic properties is produced prior to the onset of aggregation. We report the concentrations of di-, tri-, and tetravalent cations required for optimal condensation; in addition, for tri- and tetravalent cations, we were able to estimate the extent of charge neutralization produced by their binding to the optimally compacted fiber. The results show that the multivalent ion concentration required for optimal compaction decreases as cationic charge increases. In addition, the effect of a mixture of dilute mono- and multivalent cations on chromatin condensation is synergistic, rather than competitive as has been found for the multivalent cation induced condensation of DNA or the B----Z conformational transition. A simple calculation indicates that the entropy of ion uptake in chromatin condensation is surprisingly constant for a range of ionic conditions; this factor may be a dominant one in determining the folding equilibrium.
我们利用电二色性研究了多价阳离子对鸡红细胞染色质从展开的10纳米纤维压缩成30纳米螺线管以及随后聚集过程的影响。发现由压缩和聚集组成的凝聚模式对于多种不同电荷的阳离子，包括生理上重要的多胺精胺和亚精胺，都惊人地相似。除了少数例外，如Cu2+和Gd3+，在聚集开始之前会产生具有可重复流体动力学性质的最佳压缩纤维。我们报告了实现最佳凝聚所需的二价、三价和四价阳离子的浓度；此外，对于三价和四价阳离子，我们能够估计它们与最佳压缩纤维结合所产生的电荷中和程度。结果表明，随着阳离子电荷增加，实现最佳压缩所需的多价离子浓度降低。此外，稀单价和多价阳离子混合物对染色质凝聚的影响是协同的，而不是像在多价阳离子诱导的DNA凝聚或B----Z构象转变中所发现的那样具有竞争性。一个简单的计算表明，在一系列离子条件下，染色质凝聚过程中离子摄取的熵惊人地恒定；这个因素可能是决定折叠平衡的一个主导因素。
