Bacteria and archaea have evolved sophisticated adaptive immune systems that rely on CRISPR RNA (crRNA)-guided detection and nuclease-mediated elimination of invading nucleic acids. Here, we present the cryo-electron microscopy (cryo-EM) structure of the type I-F crRNA-guided surveillance complex (Csy complex) from Pseudomonas aeruginosa bound to a double-stranded DNA target. Comparison of this structure to previously determined structures of this complex reveals a ∼180-degree rotation of the C-terminal helical bundle on the “large” Cas8f subunit. We show that the double-stranded DNA (dsDNA)-induced conformational change in Cas8f exposes a Cas2/3 “nuclease recruitment helix” that is structurally homologous to a virally encoded anti-CRISPR protein (AcrIF3). Structural homology between Cas8f and AcrIF3 suggests that AcrIF3 is a mimic of the Cas8f nuclease recruitment helix.

细菌和古细菌已经进化出复杂的适应性免疫系统，依赖于 CRISPR RNA (crRNA) 引导的检测和核酸酶介导的入侵核酸的消除。在这里，我们展示了来自与双链 DNA 靶标结合的铜绿假单胞菌的 IF 型 crRNA 引导的监视复合物（Csy 复合物）的冷冻电镜 (cryo-EM) 结构。将该结构与之前确定的该复合物的结构进行比较，揭示了“大”Cas8f 亚基上 C 端螺旋束的约 180 度旋转。我们发现，双链 DNA (dsDNA) 诱导的 Cas8f 构象变化暴露了 Cas2/3“核酸酶募集螺旋”，该螺旋在结构上与病毒编码的抗 CRISPR 蛋白 (AcrIF3) 同源。 Cas8f 和 A​​crIF3 之间的结构同源性表明 AcrIF3 是 Cas8f 核酸酶募集螺旋的模拟物。