Borst P, Overdulve J P, Weijers P J, Fase-Fowler F, Van den Berg M
Biochim Biophys Acta. 1984 Feb 24;781(1-2):100-11. doi: 10.1016/0167-4781(84)90128-3.
We have isolated a closed circular duplex DNA fraction from the unicellular parasite Isospora (Toxoplasma) gondii and examined the purified DNA by electron microscopy. A major part of this circular DNA consists of 12-micron circles containing a cruciform with 0.5-micron tails. We also found 23-micron circles with the properties expected of head-to-tail dimers of the 12-micron circles. Some of these dimers have two cruciforms with 0.4-micron tails, some have one cruciform with 0.8-micron tails. When ethidium bromide was diffused into the DNA solution, circles with tails were replaced by twisted circles without tails. Direct mixing of the DNA with high ethidium bromide concentrations (5 micrograms/ml) gave rise to highly twisted circles with tails. This proves that the tailed circles are covalently continuous and indicates that ethidium bromide blocks branch migration. The 0.5-micron tails are part of a 1.7-micron palindrome, which was visualized by spreading denatured DNA under snap-back conditions. We argue that the cruciform is not present in vivo and that the 12-micron circles may represent the mitochondrial DNA of Toxoplasma.
我们从单细胞寄生虫刚地弓形虫（等孢球虫属）中分离出了一个闭环双链DNA组分，并通过电子显微镜检查了纯化后的DNA。这种环状DNA的主要部分由12微米的环状物组成，其中含有一个带有0.5微米尾巴的十字形结构。我们还发现了23微米的环状物，其具有12微米环状物首尾相连二聚体所预期的特性。其中一些二聚体有两个带有0.4微米尾巴的十字形结构，一些有一个带有0.8微米尾巴的十字形结构。当溴化乙锭扩散到DNA溶液中时，有尾巴的环状物被没有尾巴的扭曲环状物所取代。将DNA与高浓度溴化乙锭（5微克/毫升）直接混合会产生带有尾巴的高度扭曲环状物。这证明有尾巴的环状物是共价连续的，并表明溴化乙锭会阻止分支迁移。0.5微米的尾巴是一个1.7微米回文序列的一部分，通过在快速复性条件下铺展变性DNA可以观察到。我们认为十字形结构在体内不存在，并且12微米的环状物可能代表弓形虫的线粒体DNA。
