Ring nucleases deactivate Type III CRISPR ribonucleases by degrading cyclic oligoadenylate

The CRISPR-Cas system provides adaptive immunity against mobile genetic elements in prokaryotes, utilising small CRISPR RNAs which direct effector complexes to degrade invading entities. Type III effector complexes were recently demonstrated to synthesise a novel second messenger, cyclic oligoadenylate (cOA), on binding target RNA. cOA in turn binds to and activates a range of downstream effector proteins including ribonucleases (Csm6/Csx1) and transcription factors via a CARF (CRISPR associated Rossman Fold) domain, inducing an antiviral state in the cell that is important for immunity. The mechanism of the “off-switch” that resets the system is not understood. Here, we report the identification of the nuclease that degrades these cOA ring molecules. The “Ring nuclease” is itself a CARF family protein with a metal independent mechanism, which cleaves cOA 4 rings to generate linear di-adenylate species and switches off the antiviral state. The identification of Ring nucleases adds an important insight to the CRISPR-Cas system.