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Almost all archaea and about half of bacteria possess clustered regularly interspaced short palindromic repeat (CRISPR)–CRISPR-associated genes (Cas) adaptive immune systems, which protect microbes against viruses and other foreign DNA. All functionally characterized CRISPR systems have been reported to target DNA, with some multicomponent type III systems also targeting RNA. The putative class 2 type VI system, which has not been functionally characterized, encompasses the single-effector protein C2c2, which contains two Higher Eukaryotes and Prokaryotes Nucleotide-binding (HEPN) domains commonly associated with ribonucleases (RNases), suggesting RNA-guided RNA-targeting function.

Existing studies have only established a role for RNA interference, in addition to DNA interference, in the multicomponent type III-A and III-B systems. We investigated the …