CONDENSATION OF DNA BY TRIVALENT CATIONS .1. EFFECTS OF DNA LENGTH AND TOPOLOGY ON THE SIZE AND SHAPE OF CONDENSED PARTICLES
CONDENSATION OF DNA BY TRIVALENT CATIONS .1. EFFECTS OF DNA LENGTH AND TOPOLOGY ON THE SIZE AND SHAPE OF CONDENSED PARTICLES
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DOI：
10.1002/bip.360300514
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发表时间：
1990-01-01
期刊：
影响因子：
2.9
通讯作者：
BLOOMFIELD, VA
中科院分区：
文献类型：
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作者：
ARSCOTT, PG;LI, AZ;BLOOMFIELD, VA
多价阳离子对DNA的体外缩合作用可以为研究体内DNA折叠和包装的物理因素提供有用的信息。我们用电子显微镜和激光光散射技术研究了六氨合钴（Ⅱ）诱导质粒pUC 12 DNA的2700和1350碱基对（bp）片段的凝聚。浓缩的DNA呈环状和棒状。两者都存在于所有的凝聚物中，但环的比例随着碎片的增大而增大。完整的闭合环状质粒产生的颗粒比线性片段小。粒子的大小与DNA片段的长度无关。在加入冷凝剂两小时后，典型的环形为约800埃。外径（R1）为apx。400埃，并且内半径（R2）为aptx。140.这两组碎片。这些尺寸相对稳定，但R1和R2都有足够的变化，从2小时到24小时，体积增加约50%。典型的棒在2小时时为约1800埃。长，300。宽杆长度的分布与平均环形周长的分布相似。(R1+R2），棒宽分布与环宽分布（R1 -R2）相似。2700-bp片段显示出比1350-bp片段显著更高的环状体与杆状体的比率。这两种粒子都是多分子的。从上述尺寸计算的分子/颗粒的平均数，假设具有27埃的中心间距的六方堆积的B型DNA，是13 +-。4为2700-bp片段的缩合物，26 . 1350 bp片段的同源性为11。大得多的DNA的单分子凝聚物具有相似的尺寸，这表明尺寸主要由吸引力和排斥力的平衡决定，而不是由与将许多小颗粒并入较大颗粒相关的熵因子决定。由许多小颗粒合并成一个大颗粒的相似尺寸。圆环和棒的相似的尺寸和体积表明，圆环连续弯曲的自由能成本，减去在环状粒子中额外的净吸引力所获得的自由能成本，与棒中突然弯曲或扭结的自由能成本相当。虽然凝聚颗粒是多聚体，但它们独特的环形或棒状形状将它们与通常从大的柔性聚合物的多分子缔合中预期的随机聚集体区分开来。
In vitro condensation of DNA by multivalent cations can provide useful insights into the physical factors governing folding and packaging of DNA in vivo. We have made a detailed study of hexammine cobalt (II) induced condensation of 2700 and 1350 base pair (bp) fragments of plasmid pUC12 DNA by electron microscopy and laser light scattering. The condensed DNA takes the form of toroids and rods. Both are present in all condensates, but the proportion of toroids is higher with the larger fragments. The intact, closed circular plasmid produces smaller particles than the linear fragments. The size of a particle is independent of DNA fragment length. Two hours after adding the condensing agent, a typical toroid is about 800 .ANG. in diameter; the outer radius (R1) is .apprx. 400 .ANG., and the inner radius (R2) is .apprx. 140 .ANG. for both sets of fragments. These dimensions are relatively stable, but there is sufficient change in both R1 and R2 to produce approximately 50% increase in volume from 2 to 24 h. A typical rod at 2 h is about 1800 .ANG. long and 300 .ANG. wide. The distribution of rod lengths is similar to that of mean toroid circumferences .pi.(R1+R2), and the distribution of rod widths is similar to that of toroidal widths (R1 -R2). The 2700-bp fragments show a significantly higher ratio to toroids to rods than the 1350-bp fragments. Both types of particle are multimolecular. The average number of molecules/particle, calculated from the above dimensions, assuming hexagonally packed B-form DNA with a center-to-center spacing of 27 .ANG., is 13 .+-. 4 for condensates of 2700-bp fragments and 26 .+-. 11 for those of 1350-bp fragments. Monomolecular condensates of much larger DNAs have similar dimensions, suggesting that size is governed primarily by the balance of attractive and repulsive intermolecular forces rather than by the entropic factors associated with incorporation of a number of small particles into a larger one. The similar dimensions with incorporation of a number of small particles into a larger one. The similar dimensions and volumes of toroids and rods indicate that the free energy cost of continual bending in toroids, minus that gained by extra net attraction in a cyclic particle, is comparable to that of abrupt bending or kinking in rods. Although that condensed particles are multimeric, their distinct toroidal or rodlike shapes distinguish them from the random aggregates that would be generally expected from the multimolecular association of large, flexible polymers.