Cotton R W, Hamkalo B A
Nucleic Acids Res. 1981 Jan 24;9(2):445-57. doi: 10.1093/nar/9.2.445.
Monomer nucleosomes purified on isokinetic sucrose gradients are shown to dissociate into component DNA and histones at physiological ionic strength upon dilution to a DNA concentration below 20 microgram/ml. The starting material is 11S, contains 145-190 BP DNA, and equimolar amounts of the four core histones with slightly less H1. Dilution of monomers in the presence of 0.14 M NaCl results in the rapid conversion of 10-40% of the 3H thymidine labeled material from 11S to 5S (5S is coincident with the S value of monomer length DNA). The proportion of nucleosomes which dissociate increases with increasing NaCl concentration between 0.15 M and 0.35 M and decreases with increasing DNA concentration above 1 microgram/ml. Recycling 11S monomers, which remain after dissociation, through a second dilution in salt generates an equivalent proportion of 5S material as seen after the initial dilution. Thus, the dissociation does not result from special properties of a subset of nucleosomes. An equilibrium between intact monomer and free DNA and histones appears to be rapidly established under the conditions described and the dissociated DNA will reassociate with histones to form 11S monomers if conditions of high DNA concentration and low ionic strength are established.
在等速蔗糖梯度上纯化的单体核小体在生理离子强度下，当稀释至DNA浓度低于20微克/毫升时，会解离成DNA和组蛋白成分。起始材料为11S，含有145 - 190个碱基对的DNA，以及等摩尔量的四种核心组蛋白，H1的量略少。在0.14 M NaCl存在下稀释单体，会使10 - 40%的3H胸腺嘧啶标记物质从11S快速转化为5S（5S与单体长度DNA的S值一致）。在0.15 M至0.35 M之间，随着NaCl浓度的增加，解离的核小体比例增加；而当DNA浓度高于1微克/毫升时，随着DNA浓度的增加，解离比例下降。将解离后剩余的11S单体通过在盐中的二次稀释进行循环，会产生与初始稀释后相同比例的5S物质。因此，解离并非源于一部分核小体的特殊性质。在所描述的条件下，完整单体与游离DNA和组蛋白之间似乎能迅速建立平衡，并且如果建立高DNA浓度和低离子强度的条件，解离的DNA会与组蛋白重新结合形成11S单体。
