The inability to disassemble Rad51 nucleoprotein filaments leads to aberrant mitosis and cell death

The proper maintenance of genetic material is essential for the survival of living organisms. One of the main safeguards of genome stability is homologous recombination involved in the faithful repair of DNA double-strand breaks, the restoration of collapsed replication forks, and the bypass of replication barriers. Homologous recombination relies on the formation of Rad51 nucleofilaments which are responsible for the homology-based interactions between DNA strands. Here we demonstrate that without the regulation of these filaments by Srs2 and Rad54, which are known to remove Rad51 from single-stranded and double-stranded DNA respectively, the filaments strongly inhibit damage-associated DNA synthesis during DNA repair. Furthermore, this regulation is essential for cell survival under normal growth conditions as in the srs2Δ rad54Δ mutants unregulated Rad51 nucleofilaments cause activation of the DNA damage checkpoint, formation of mitotic bridges and loss of genetic material. These genome instability features may stem from the problems at stalled replication forks as the lack of Srs2 and Rad54 in the presence of Rad51 nucleofilaments impedes cell recovery from replication stress. This study demonstrates that the timely and efficient disassembly of recombination machinery is essential for genome maintenance and cell survival.遗传物质的妥善维持对于活生物体的存活至关重要。维持基因组稳定性的核心保障机制之一是同源重组（homologous recombination），其参与DNA双链断裂（DNA double-strand breaks）的精准修复、停滞复制叉的重建以及复制障碍的绕过。同源重组依赖于Rad51核蛋白丝（Rad51 nucleofilaments）的形成，这类结构负责DNA链间基于同源序列的相互作用。本研究证实，若缺乏Srs2与Rad54对该类核蛋白丝的调控——已知二者可分别将Rad51从单链DNA（single-stranded DNA）与双链DNA（double-stranded DNA）上移除——则该核蛋白丝会强烈抑制DNA修复过程中与损伤相关的DNA合成。此外，该调控对于正常生长条件下的细胞存活必不可少：在srs2Δ rad54Δ突变体中，不受调控的Rad51核蛋白丝会激活DNA损伤检验点（DNA damage checkpoint）、形成有丝分裂桥（mitotic bridges）并造成遗传物质丢失。这些基因组不稳定特征可能源于停滞复制叉处的问题，当存在Rad51核蛋白丝时，Srs2与Rad54的缺失会阻碍细胞从复制应激（replication stress）中恢复。本研究表明，及时且高效地拆解重组机器（recombination machinery），对于基因组维持与细胞存活均至关重要。