Ambrose C, McLaughlin R, Bina M
Nucleic Acids Res. 1987 May 11;15(9):3703-21. doi: 10.1093/nar/15.9.3703.
The linking number of DNA in minichromosomes increases by 2 turns during SV40 assembly. Changes in temperature also influence the average linking number of the total intracellular forms of SV40 DNA. When the isolated minichromosomes assembled in vivo are incubated with topoisomerase I at 33 degrees C in vitro, the linking number of SV40 DNA decreases. This decrease is about: -1.1 turns for minichromosomes with an average nucleosome spacing of 198 base pairs (bp), wt776; and -0.6 turns for minichromosomes containing a shorter average nucleosome repeat (177 bp), tsC219. The difference between the average linking number of naked SV40 DNA relaxed with topoI at 33 degrees C and minichromosomes relaxed with the enzyme at the same temperature indicates that SV40 chromatin contains on the average 26 nucleosomes. However, the results of studies obtained both on DNA flexibility in chromatin and in naked DNA, and on the shape of the topoisomer distribution curves, indicate that all of the minichromosomes, regardless of their overall structure, do not contain the same number of nucleosomes; this heterogeneity may be as large as 8 nucleosomes. We find no apparent correlation between the amount of minichromosomes containing unconstrained torsional stress and the abundance of the molecules with a structure characteristic of transcriptionally active chromatin.
在SV40装配过程中，微型染色体中DNA的连环数增加2圈。温度变化也会影响SV40 DNA细胞内总形式的平均连环数。当在体内装配的分离微型染色体在体外33℃下与拓扑异构酶I孵育时，SV40 DNA的连环数会减少。对于平均核小体间距为198个碱基对（bp）的微型染色体wt776，这种减少约为 -1.1圈；对于平均核小体重复较短（177 bp）的微型染色体tsC219，减少约为 -0.6圈。在33℃下用拓扑异构酶I松弛的裸露SV40 DNA的平均连环数与在相同温度下用该酶松弛的微型染色体的平均连环数之间的差异表明，SV40染色质平均含有26个核小体。然而，关于染色质和裸露DNA中DNA柔韧性以及拓扑异构体分布曲线形状的研究结果表明，所有微型染色体，无论其整体结构如何，所含核小体数量并不相同；这种异质性可能高达8个核小体。我们发现，含有无约束扭转应力的微型染色体数量与具有转录活性染色质结构特征的分子丰度之间没有明显的相关性。
