小柯机器人

瑞士大学（USI）Petr Cejka研究组取得最新进展。他们揭示了减数分裂中MLH1-MLH3（MutLγ）核酸内切酶的调控机制。该研究于2020年8月19日发表于《自然》。

他们生化重构了这种交叉途径的关键要素，显示人类支撑交叉过程的MSH4-MSH5（MutSγ），其结合分支重组中间体并与MutLγ缔合，从而稳定了联合分子结构和相邻双链DNA。MutSγ通过MutLγ内切核酸酶直接刺激DNA切割。但只有当存在MutSγ时，EXO1才能进一步刺激MutLγ活性。复制因子C（RFC）和增殖细胞核抗原（PCNA）是核酸酶的其他组成部分，从而触发了交叉反应。MutLγ不能与PCNA相互作用的酿酒酵母菌株在形成交叉时存在缺陷。

最后，MutLγ–MutSγ–EXO1–RFC–PCNA核酸酶复合物优先切割具有霍利迪连接的DNA，但没有典型的分辨酶活性。取而代之的是，它可能通过在连接点附近切割双链DNA来加工减数分裂重组中间体。由于MutLγ产生的DNA切口依赖于其辅助因子，因此MutSγ和RFC–PCNA在减数分裂重组中间体上的不对称分布可能会驱动DNA裂解。MutLγ核酸酶激活的这种模式可能解释了减数分裂染色体中霍利迪结或其前体的交叉特异性加工。

据介绍，在第一次减数分裂前期，细胞故意破坏其DNA。这些DNA断裂可通过同源重组得到修复，这有利于适当的染色体分离，并使同源染色体之间的DNA片段相互交换。依赖于MutLγ核酸酶的途径已通过未知机制参与减数分裂重组中间体向交叉的偏向加工。

附：英文原文

Title: Regulation of the MLH1–MLH3 endonuclease in meiosis

Author: Elda Cannavo, Aurore Sanchez, Roopesh Anand, Lepakshi Ranjha, Jannik Hugener, Cline Adam, Ananya Acharya, Nicolas Weyland, Xavier Aran-Guiu, Jean-Baptiste Charbonnier, Eva R. Hoffmann, Valrie Borde, Joao Matos, Petr Cejka

Issue&Volume: 2020-08-19

Abstract: During prophase of the first meiotic division, cells deliberately break their DNA1. These DNA breaks are repaired by homologous recombination, which facilitates proper chromosome segregation and enables the reciprocal exchange of DNA segments between homologous chromosomes2. A pathway that depends on the MLH1–MLH3 (MutLγ) nuclease has been implicated in the biased processing of meiotic recombination intermediates into crossovers by an unknown mechanism3–7. Here we have biochemically reconstituted key elements of this pro-crossover pathway. We show that human MSH4–MSH5 (MutSγ), which supports crossing over8, binds branched recombination intermediates and associates with MutLγ, stabilizing the ensemble at joint molecule structures and adjacent double-stranded DNA. MutSγ directly stimulates DNA cleavage by the MutLγ endonuclease. MutLγ activity is further stimulated by EXO1, but only when MutSγ is present. Replication factor C (RFC) and the proliferating cell nuclear antigen (PCNA) are additional components of the nuclease ensemble, thereby triggering crossing-over. Saccharomyces cerevisiae strains in which MutLγ cannot interact with PCNA present defects in forming crossovers. Finally, the MutLγ–MutSγ–EXO1–RFC–PCNA nuclease ensemble preferentially cleaves DNA with Holliday junctions, but shows no canonical resolvase activity. Instead, it probably processes meiotic recombination intermediates by nicking double-stranded DNA adjacent to the junction points9. As DNA nicking by MutLγ depends on its co-factors, the asymmetric distribution of MutSγ and RFC–PCNA on meiotic recombination intermediates may drive biased DNA cleavage. This mode of MutLγ nuclease activation might explain crossover-specific processing of Holliday junctions or their precursors in meiotic chromosomes4. Reconstitution of the activation of the MLH1–MLH3 endonuclease shows how crossovers are formed during meiosis.

DOI: 10.1038/s41586-020-2592-2

Source: https://www.nature.com/articles/s41586-020-2592-2

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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