During homologous recombination, Rad51 forms a nucleoprotein filament with single-stranded DNA (ssDNA) that undergoes strand exchange with homologous double-stranded DNA (dsDNA). Here, we use real-time analysis to show that strand exchange by fission yeast Rad51 proceeds via two distinct three-strand intermediates, C1 and C2. Both intermediates contain Rad51, but whereas the donor duplex remains intact in C1, the ssDNA strand is intertwined with the complementary strand of the donor duplex in C2. Swi5-Sfr1, an evolutionarily conserved recombination activator, facilitates the C1-C2 transition and subsequent ssDNA release from C2 to complete strand exchange in an ATP-hydrolysis-dependent manner. In contrast, Ca2+, which activates the Rad51 filament by curbing ATP hydrolysis, facilitates the C1-C2 transition but does not promote strand exchange. These results reveal that Swi5-Sfr1 and Ca2+ have different activation modes in the late synaptic phase, despite their common function in stabilizing the presynaptic filament.

中文翻译：

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在Rad51驱动的DNA链交换过程中处理了两个三链中间体。

在同源重组过程中，Rad51与单链DNA（ssDNA）形成核蛋白丝，并与同源双链DNA（dsDNA）进行链交换。在这里，我们使用实时分析来显示裂变酵母Rad51的链交换是通过两个不同的三链中间体C1和C2进行的。两种中间体均含有Rad51，但供体双链体在C1中保持完整，而ssDNA链则与供体双链体的互补链在C2中缠绕在一起。Swi5-Sfr1是一种进化上保守的重组激活剂，可促进C1-C2过渡以及随后的ssDNA从C2释放，从而以依赖于ATP水解的方式完成链交换。相反，通过抑制ATP水解激活Rad51细丝的Ca2 +促进了C1-C2过渡，但不促进链交换。