cGAS/DncV-like nucleotidyltransferase (CD-NTase) enzymes are immune sensors that synthesize nucleotide second messengers and initiate antiviral responses in bacterial and animal cells. Here, we discover Enterobacter cloacae CD-NTase-associated protein 4 (Cap4) as a founding member of a diverse family of >2,000 bacterial receptors that respond to CD-NTase signals. Structures of Cap4 reveal a promiscuous DNA endonuclease domain activated through ligand-induced oligomerization. Oligonucleotide recognition occurs through an appended SAVED domain that is an unexpected fusion of two CRISPR-associated Rossman fold (CARF) subunits co-opted from type III CRISPR immunity. Like a lock and key, SAVED effectors exquisitely discriminate 2′–5′- and 3′–5′-linked bacterial cyclic oligonucleotide signals and enable specific recognition of at least 180 potential nucleotide second messenger species. Our results reveal SAVED CARF family proteins as major nucleotide second messenger receptors in CBASS and CRISPR immune defense and extend the importance of linkage specificity beyond mammalian cGAS-STING signaling.

cGAS / DncV样核苷酸转移酶（CD-NTase）酶是一种免疫传感器，可合成核苷酸第二信使并在细菌和动物细胞中引发抗病毒反应。在这里，我们发现了阴沟肠杆菌CD-NTase相关蛋白4（Cap4）是对CD-NTase信号有反应的2000多种细菌受体家族的创始成员。Cap4的结构揭示了通过配体诱导的寡聚激活的混杂DNA核酸内切酶结构域。寡核苷酸识别是通过附加的SAVED域进行的，该域是从III型CRISPR免疫中选择的两个CRISPR关联的Rossman fold（CARF）亚基的意外融合。像锁和钥匙一样，SAVED效应器可以精确区分2'-5'-和3'-5'-连接的细菌环状寡核苷酸信号，并能够特异性识别至少180个潜在的核苷酸第二信使物种。