CRISPR EVOLUTION C 2 c 2 is a single-component programmable RNA-guided RNA-targeting CRISPR effector

INTRODUCTION: 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 foreignDNA. All functionally characterized CRISPR systems have been reported to target DNA,with somemulticomponent 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), suggestingRNA-guidedRNA-targeting function.