Keller W, Müller U, Eicken I, Wendel I, Zentgraf H
Cold Spring Harb Symp Quant Biol. 1978;42 Pt 1:227-44. doi: 10.1101/sqb.1978.042.01.025.
SV40 chromatin can be isolated in two forms: At moderate ionic strength (mu = 0.1-0.3) it contains histone H1 in addition to the four nucleosomal histones and has a highly condensed appearance in the electron microscope, being composed of a few closely connected large spheres [190 A (160, 220) diameter]. At high ionic strength (mu = 0.6-0.8) or after prolonged exposure to very low ionic strengths (mu less than 0.02), the compact form unfolds and the chromatin shows a typical nucleosomal morphology. Native SV40 DNA-protein complexes contain a median number of 24 nucleosomes. The number of superhelical turns does not differ in DNA obtained from the compact and the unfolded forms of chromatin. DNA-relaxing enzyme is found associated with SV40 chromatin and is capable of acting both on extraneously added circular DNA and on its own DNA in the nucleoprotein complex. Purified DNA-relaxing enzyme forms transiently nicked DNA intermediates where the enzyme can be found covalently attached to the site of the nick in the DNA. Transcriptionally active SV40 complexes undergo the same ionic-strength-dependent structural transition as that of bulk SV40 chromatin and may therefore also have a compact configuration at physiological salt concentrations.
SV40染色质可以以两种形式分离：在中等离子强度（μ = 0.1 - 0.3）下，除了四种核小体组蛋白外，它还含有组蛋白H1，并且在电子显微镜下呈现高度浓缩的外观，由几个紧密相连的大球体组成[直径为190 Å（160, 220）]。在高离子强度（μ = 0.6 - 0.8）下或长时间暴露于非常低的离子强度（μ小于0.02）后，紧密形式会展开，染色质呈现典型的核小体形态。天然的SV40 DNA - 蛋白质复合物中核小体的中位数为24个。从紧密和展开形式的染色质中获得的DNA中超螺旋圈数没有差异。发现DNA松弛酶与SV40染色质相关，并且能够作用于外源添加的环状DNA及其核蛋白复合物中的自身DNA。纯化的DNA松弛酶形成瞬时切口DNA中间体，在其中可以发现该酶共价连接到DNA中的切口位点。具有转录活性的SV40复合物与大量SV40染色质一样经历相同的离子强度依赖性结构转变，并因此在生理盐浓度下也可能具有紧密的构型。
