小柯机器人

美国斯隆-凯特琳纪念癌症中心Nikola P. Pavletich团队取得最新进展。他们揭示RecA–DNA突触和D环结构的链交换机制。2020年10月14日出版的国际知名学术期刊《自然》发表了这一成果。

他们报告了具有非互补和部分互补的双链DNA（dsDNA）的突触小丝以及包含10个或12个碱基对的异源双链体的RecA–D环复合物冷冻电镜结构分析。RecA的C末端结构域与dsDNA结合，并将其引导至RecA L2环，后者插入并打开双链体。RecA扩散开放区域，在第二个DNA结合位点隔离同源链，从而释放出互补链，使其与单链DNA（ssDNA）配对。在每个RecA步骤中，大约有20％的可能性使双链体开放终止，而尚未开放的dsDNA部分将与另一个C末端结构域结合。

同源链通过异源双链配对并结合ssDNA次级位点，来延长dsDNA的开放，从而抑制了这一过程。如果未遇到同源链，该机制局部限制了用于配对的ssDNA的长度，并且可能允许在供体dsDNA上形成多个广泛分离的突触，这将增加遇到同源链的可能性。这些发现为同源重组提供了关键的机制见解。

据介绍，链交换反应是同源重组的关键。它由ATP酶RecA家族催化，该酶与ssDNA和ATP形成螺旋状微丝。该微丝与供体dsDNA结合形成突触丝，突触丝寻找同源链，然后催化互补链的交换，形成新的异源双链体或（如果同源链有限）D环。对于突触丝如何形成以及寻找同源链和催化链交换仍了解甚少。

附：英文原文

Title: Mechanism of strand exchange from RecA–DNA synaptic and D-loop structures

Author: Haijuan Yang, Chun Zhou, Ankita Dhar, Nikola P. Pavletich

Issue&Volume: 2020-10-14

Abstract: The strand-exchange reaction is central to homologous recombination. It is catalysed by the RecA family of ATPases, which form a helical filament with single-stranded DNA (ssDNA) and ATP. This filament binds to a donor double-stranded DNA (dsDNA) to form synaptic filaments, which search for homology and then catalyse the exchange of the complementary strand, forming either a new heteroduplex or—if homology is limited—a D-loop1,2. How synaptic filaments form, search for homology and catalyse strand exchange is poorly understood. Here we report the cryo-electron microscopy analysis of synaptic mini-filaments with both non-complementary and partially complementary dsDNA, and structures of RecA–D-loop complexes containing a 10- or a 12-base-pair heteroduplex. The C-terminal domain of RecA binds to dsDNA and directs it to the RecA L2 loop, which inserts into and opens up the duplex. The opening propagates through RecA sequestering the homologous strand at a secondary DNA-binding site, which frees the complementary strand to sample pairing with the ssDNA. At each RecA step, there is a roughly 20% probability that duplex opening will terminate and the as-yet-unopened dsDNA portion will bind to another C-terminal domain. Homology suppresses this process, through the cooperation of heteroduplex pairing with the binding of ssDNA to the secondary site, to extend dsDNA opening. This mechanism locally limits the length of ssDNA sampled for pairing if homology is not encountered, and could allow for the formation of multiple, widely separated synapses on the donor dsDNA, which would increase the likelihood of encountering homology. These findings provide key mechanistic insights into homologous recombination.

DOI: 10.1038/s41586-020-2820-9

Source: https://www.nature.com/articles/s41586-020-2820-9

Nature：《自然》，创刊于1869年。隶属于施普林格·自然出版集团，最新IF：69.504
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