CATION-INDUCED TOROIDAL CONDENSATION OF DNA STUDIES WITH CO3+(NH3)6
CATION-INDUCED TOROIDAL CONDENSATION OF DNA STUDIES WITH CO3+(NH3)6
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DOI：
10.1016/0022-2836(80)90330-7
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发表时间：
1980-01-01
影响因子：
5.6
通讯作者：
BALDWIN, RL
中科院分区：
文献类型：
--
作者：
WIDOM, J;BALDWIN, RL
多胺亚精胺（3+）和精胺（4+）引起T7或γ的协同分子内缩合。噬菌体DNA，其中DNA呈紧凑的环形构象。惰性的三价金属离子络合物Co 3+（NH3）6也会引起[酵母] DNA缩合，而水溶液中的DNA缩合是由电荷为3+或更高的阳离子引起的。多胺诱导的和钴六胺诱导的缩合的DNA产物具有通过EM判断的相似的环形构象，并且两者都具有DNA B-型的圆二色性光谱，与ψ相反。Maniatis等人（1974）研究的DNA缩合物。一价和二价阳离子（Na+，Mg 2+）逆转DNA缩合。诱导（3+或4+）和逆转（1+或2+）阳离子之间的竞争遵循Manning的大气阳离子与DNA结合理论中概述的离子交换行为。当DNA磷酸盐电荷的关键部分被吸附在DNA上的阳离子中和时，DNA缩合显然可以发生。水溶液中阳离子诱导的DNA缩合可能是由阳离子交联引起的：三价或更高价阳离子对相邻螺旋的静电桥接。DNA缩合的过渡曲线通过增加光散射，使用光子计数荧光计测量。为了确保达到平衡，通过使用Na+或Mg 2+来逆转反应，在正向和反向两个方向上研究缩合。缩合的动力学在正向方向上是缓慢的，在min至h的时间范围内，并且随着DNA浓度的增加而缓慢。通过Na+或Mg 2+的缩合反应发生得更快，在s至min内，并且过渡中点基本上与DNA浓度无关。在DNA浓度低于1 μ M-磷酸盐时，缩合和去缩合的动力学在速率上相当。分子间的DNA接触可能会与分子内的凝聚竞争，并减慢其速度。在足够低的DNA浓度下，在正向或反向方向上获得转变中点的平衡数据;在较高的DNA浓度下，在反向方向上达到平衡，但在正向方向上没有达到平衡。从Na+或Mg 2+的去缩合研究中获得了缩合相图（log [Co 3+（NH 3）6]与log [Na+]或log [Mg 2 +]在过渡中点的关系图）。当Co 3+（NH3）6、亚精胺（3+）或精胺（4+）用于诱导冷凝时，这些图的斜率为+1。如Wilson和布卢姆菲尔德所示，+1的斜率与曼宁理论计算的当临界部分的DNA电荷被中和时发生的DNA缩合一致。另外两个结果，这关系到的问题的环形DNA凝聚。在高温下更容易发生冷凝。短至400个碱基对的限制性片段通过分子间缩合形成环状体，其在直径和外观上类似于由. DNA.
The polyamines spermidine (3+) and spermine (4+) cause a cooperative intramolecular condensation of T7 or .lambda. phage DNAs in which the DNA assumes a compact toroidal conformation. An inert trivalent metal ion complex, Co3+ (NH3)6, also causes [yeast] DNA condensation, and DNA condensation in aqueous solution is caused by cations of charge 3+ or more. The DNA products of polyamine-induced and of cobalt hexamine-induced condensation have similar toroidal conformations as judged by EM and both have the circular dichroism spectrum of DNA B-form, in contrast to the .psi. DNA condensates studied by Maniatis et al. (1974). Monovalent and divalent cations (Na+, Mg2+) reverse DNA condensation. Competition between inducing (3+ or 4+) and reversing (1+ or 2+) cations follows the ion-exchange behavior outlined in Manning''s theory of atmospheric cation binding to DNA. DNA condensation can apparently occur when a critical fraction of the DNA phosphate charge has been neutralized by cations adsorbed to the DNA. Cation-induced DNA condensation in aqueous solution may result from cation crosslinking:electrostatic bridging of adjacent helices by trivalent or higher valence cations. Transition curves for DNA condensation were measured by the increase in light-scattering, using a photon-counting fluorimeter. To ensure that equilibrium is reached, condensation was studied in both the forward the reverse directions, by using either Na+ or Mg2+ to reverse the reaction. The kinetics of condensation are slow in the forward direction, in the time range of min to h, and slow as the DNA concentration is increased. Reversal of condensation by Na+ or Mg2+ occurs more rapidly, in s to min, and the transition midpoints are essentially independent of DNA concentration. At DNA concentrations below 1 .mu.M-phosphate, the kinetics of condensation and of de-condensation are comparable in rate. Intermolecular DNA contacts may compete with, and slow down, intramolecular condensation. equilibrium data for transition midpoints are obtained in either the forward or reverse direction at sufficiently low DNA concentrations; at higher DNA concentrations, equilibrium is reached in the reverse but not in the forward direction. Phase diagrams for condensation (plots of log [Co3+ (NH3)6] vs. log [Na+] or log [Mg2+] at the transition midpoint) have been obtained from studies of de-condensation by Na+ or Mg2+. These plots have a slope of +1 when either Co3+ (NH3)6, spermidine (3+) or spermine (4+) is used to induce condensation. As shown by Wilson and Bloomfield, a slope of +1 is consistent with DNA condensation occurring when a critical fraction of DNA charge has been neutralized, as calculated by Manning''s theory. Two additional results are presented, which bear on the problem of toroidal DNA condensation. Condensation occurs more readily at high temperatures. Restriction fragments as short as 400 base-pairs form toroids by intermolecular condensation, which are similar in diameter and appearance to the intramolecular condensates formed by .lambda. DNA.