DNA loop-extruding SMC complexes play crucial roles in chromosome folding and DNA immunity. Prokaryotic SMC Wadjet (JET) complexes limit the spread of plasmids through DNA cleavage, yet the mechanisms for plasmid recognition are unresolved. We show that artificial DNA circularization renders linear DNA susceptible to JET nuclease cleavage. Unlike free DNA, JET cleaves immobilized plasmid DNA at a specific site, the plasmid-anchoring point, showing that the anchor hinders DNA extrusion but not DNA cleavage. Structures of plasmid-bound JetABC reveal two presumably stalled SMC motor units that are drastically rearranged from the resting state, together entrapping a U-shaped DNA segment, which is further converted to kinked V-shaped cleavage substrate by JetD nuclease binding. Our findings uncover mechanical bending of residual unextruded DNA as molecular signature for plasmid recognition and non-self DNA elimination. We moreover elucidate key elements of SMC loop extrusion, including the motor direction and the structure of a DNA-holding state.

中文翻译：

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SMC JET 核酸酶限制质粒的结构基础

DNA 环挤压 SMC 复合物在染色体折叠和 DNA 免疫中发挥着至关重要的作用。原核 SMC Wadjet (JET) 复合物通过 DNA 切割限制质粒的传播，但质粒识别的机制尚未解决。我们发现人工 DNA 环化使线性 DNA 易于受到 JET 核酸酶切割。与游离 DNA 不同，JET 在特定位点（即质粒锚定点）切割固定的质粒 DNA，这表明锚定点会阻碍 DNA 挤出，但不会阻碍 DNA 切割。质粒结合的 Je​​tABC 的结构揭示了两个可能停滞的 SMC 运动单元，它们从静止状态彻底重新排列，共同捕获 U 形 DNA 片段，该片段通过 JetD 核酸酶结合进一步转化为扭结的 V 形切割底物。我们的研究结果揭示了残留的未挤出 DNA 的机械弯曲作为质粒识别和非自身 DNA 消除的分子特征。我们还阐明了 SMC 环挤出的关键要素，包括电机方向和 DNA 保持状态的结构。