The Escherichia coli RuvB branch migration protein forms double hexameric rings around DNA.

The RuvB protein is induced in Escherichia coli as part of the SOS response to DNA damage. It is required for genetic recombination and the postreplication repair of DNA. In vitro, the RuvB protein promotes the branch migration of Holliday junctions and has a DNA helicase activity in reactions that require ATP hydrolysis. We have used electron microscopy, image analysis, and three-dimensional reconstruction to show that the RuvB protein, in the presence of ATP, forms a dodecamer on double-stranded DNA in which two stacked hexameric rings encircle the DNA and are oriented in opposite directions with D6 symmetry. Although helicases are ubiquitous and essential for many aspects of DNA repair, replication, and transcription, three-dimensional reconstruction of a helicase has not yet been reported, to our knowledge. The structural arrangement that is seen may be common to other helicases, such as the simian virus 40 large tumor antigen. IMAGES:

RuvB蛋白（RuvB protein）在大肠杆菌（Escherichia coli）中被诱导表达，作为应对DNA损伤的SOS应答（SOS response）的组成部分。该蛋白是基因重组与DNA复制后修复过程不可或缺的组分。体外实验表明，RuvB蛋白可促进霍利迪交叉（Holliday junctions）的分支迁移，并在依赖ATP水解的反应中展现DNA解旋酶活性。我们采用电子显微镜技术、图像分析与三维重构技术，证实ATP存在条件下，RuvB蛋白可在双链DNA上形成十二聚体结构：两个堆叠的六聚体环环绕DNA，反向排列且具有D6对称性（D6 symmetry）。尽管解旋酶（helicase）普遍存在，且对DNA修复、复制与转录的诸多环节至关重要，但据我们所知，目前尚未有关于解旋酶三维重构的研究报道。本次观察到的结构排布，可能也存在于其他解旋酶中，例如猿猴病毒40（simian virus 40）大T抗原。IMAGES: