亚精胺诱导DNA凝聚的有序性的AFM研究

AFM observation of the local ordering characteristics in spermidine condensed DNA

观察生物体内ＤＮＡ凝聚参与物—亚精胺对ＤＮＡ的凝缩作用将有利于深入理解ＤＮＡ在体内的紧密存储机制。本文采用原子力显微镜（ＡＦＭ）研究了在不同的ＤＮＡ浓度下亚精胺—ＤＮＡ凝聚体的形态变化。研究表明：随着体系中ＤＮＡ浓度的变化，亚精胺将诱导ＤＮＡ形成两种类型的凝聚物（类型Ⅰ和类型Ⅱ）。当λ－ＤＮＡ浓度低于１ｎｇ／μｌ，亚精胺可诱导λ－ＤＮＡ形成一种特殊的结构—环形凝聚体，其体积与单个λ－ＤＮＡ的体积近似（类型Ⅰ）；当λ－ＤＮＡ浓度介于１ｎｇ／μｌ和１０ｎｇ／μｌ之间，可观察到由许多小颗粒堆积而成的多分子凝聚的环状体（类型Ⅱ）。对类型Ⅱ的高度分析表明它可能具有分层结构，统计表明其组成颗粒在粒径上和单个类型Ⅰ凝聚体相近。这些观察暗示了亚精胺诱导ＤＮＡ凝聚可能是一个分级自组装过程。

Condensation of DNA by multivalent cations can provide useful insights into the physical factors governing the folding and packaging of DNA in vivo.In this work,local ordered structures of spermidine DNA complexes prepared from different DNA concentrations have been examined by using atomic force microscopy (AFM).Two types (Types Ⅰ and Ⅱ) of DNA condensates,significantly different in sizes,were observed.It was found that for extremely dilute solutions (DNA concentrations around 1 ng/μl or below),the DNA molecules would collapse into toroidal structures with volume equivalent to a single λ DNA (Type Ⅰ).In relatively dilute solutions (DNA concentrations between 1ng/μl to 10ng/μl),a significantly larger structure of multimolecular toroids (type Ⅱ) were formed,which were constructed by many fine particles.Measurements show that the average diameter of these fine particles were similar to the outer diameter of the monomolecular toroids observed in extremely dilute solutions and the thickness of the multimolecular toroids had a distribution of multi layers with height increments of 11nm,indicating that the multimolecular toroidal structures have lamellar characteristics.These observations suggest that DNA spermidine condensation could have multiple stages,which are very sensitive to the DNA and spermidine concentrations.