In the type III CRISPR–Cas immune response of prokaryotes, infection triggers the production of cyclic oligoadenylates that bind and activate proteins that contain a CARF domain^1,2. Many type III loci are associated with proteins in which the CRISPR-associated Rossman fold (CARF) domain is fused to a restriction endonuclease-like domain^3,4. However, with the exception of the well-characterized Csm6 and Csx1 ribonucleases^5,6, whether and how these inducible effectors provide defence is not known. Here we investigated a type III CRISPR accessory protein, which we name cyclic-oligoadenylate-activated single-stranded ribonuclease and single-stranded deoxyribonuclease 1 (Card1). Card1 forms a symmetrical dimer that has a large central cavity between its CRISPR-associated Rossmann fold and restriction endonuclease domains that binds cyclic tetra-adenylate. The binding of ligand results in a conformational change comprising the rotation of individual monomers relative to each other to form a more compact dimeric scaffold, in which a manganese cation coordinates the catalytic residues and activates the cleavage of single-stranded—but not double-stranded—nucleic acids (both DNA and RNA). In vivo, activation of Card1 induces dormancy of the infected hosts to provide immunity against phage infection and plasmids. Our results highlight the diversity of strategies used in CRISPR systems to provide immunity.

在原核生物的 III 型 CRISPR-Cas 免疫反应中，感染会触发环状寡聚腺苷酸的产生，这些寡聚腺苷酸结合并激活含有 CARF 结构域的蛋白质^1,2。许多 III 型基因座与蛋白质相关，其中 CRISPR 相关罗斯曼折叠 (CARF) 结构域融合到限制性核酸内切酶样结构域^3,4。然而，除了已充分表征的 Csm6 和 Csx1 核糖核酸酶^5,6, 这些诱导效应器是否以及如何提供防御尚不清楚。在这里，我们研究了一种 III 型 CRISPR 辅助蛋白，我们将其命名为环寡腺苷酸激活的单链核糖核酸酶和单链脱氧核糖核酸酶 1 (Card1)。Card1 形成一个对称的二聚体，在其 CRISPR 相关的 Rossmann 折叠和结合环四腺苷酸的限制性核酸内切酶结构域之间有一个大的中央空腔。配体的结合导致构象变化，包括单个单体相对于彼此旋转以形成更紧凑的二聚支架，其中锰阳离子协调催化残基并激活单链（但不是双链）的切割—核酸（DNA 和 RNA）。体内，Card1 的激活诱导受感染宿主的休眠以提供针对噬菌体感染和质粒的免疫力。我们的结果突出了 CRISPR 系统中用于提供免疫力的策略的多样性。