FOLDING OF AN IMMOBILE DNA BRANCHED JUNCTION

Holliday连接是一个四链DNA分子，其中 四股成对形成四个双螺旋臂，它们位于一个 分支点，这是遗传过程中的中心中间点 重组。自然发生的分支点的位置 假日交汇点是不稳定的，因为它的两侧有两个 (同源的)允许连接重新定位的序列对称性 通过一种称为分支迁移的异构化反应。当着大家的面 在镁离子中，连接折叠成一种结构，其中四个 两臂成对堆叠，形成两个双螺旋结构域； HTwo链采用大致螺旋结构，其他两条 Hstrands在这两个域之间交叉。在没有镁的情况下 阳离子，四臂采用四重对称排列。这个 羟基自由基裂解模式对折叠很敏感 结在有镁存在的情况下。相对于它们的线形图案 双链DNA，两个交叉链在 H-双域结构。同样，在分支上有四个核苷酸3‘ Hpoint，螺旋链显示出保护作用，因为它们 在那里相互遮挡。这两种保护措施似乎都在 缺乏镁。因此，这是一个系统， 这一关键中间体的折叠动力学是可以测量的 通过这些记者保护的差异。剂量反应 在hthe同步加速器的实验已经完成，足迹 条件H要求42个基准靶曝光100毫秒 序列。

The Holliday junction is a four-stranded DNA molecule, in which the four strands pair to form four double helical arms that flank a branch point, that is a central intermediate in the process of genetic recombination. The site of the branch point in naturally occurring Holiday junctions is unstable, because it is flanked by twofold (homologous) sequence symmetry that permits the junction to relocate through an isomerization known as branch migration. In the presence of Mg cations, the junction folds into a structure in which the four arms stack on each other pairwise, to form two double helical domains; Htwo strands adopt a roughly helical structure, and the other two Hstrands cross over between the two domains. In the absence of Mg cations, the four arms adopt a fourfold symmetric arrangement. The hydroxyl radical cleavage pattern is sensitive to the folding of the junction in the presence of Mg. Relative to their pattern in linear duplex DNA, the two crossover strands are protected from attack in the Htwo-domain structure. Likewise, four nucleotides 3' to the branch Hpoint, the helical strands demonstrate protection, because they Hocclude each other there. Both protections appear to be lessened in Hthe absence of magnesium. Hence, this is a system whereby the Hkinetics of the folding of this key intermediate is measurable through Hthese reporter protection differences. Dose response experiments at Hthe synchrotron have been completed and footprinting conditions Hrequired 100 milliseconds of exposure for a 42 base target sequence.