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Ring nucleases deactivate type III CRISPR ribonucleases by degrading cyclic oligoadenylate
Januka S Athukoralage1, Christophe Rouillon1, Shirley Graham1
1Biomedical Sciences Research Complex, School of Biology, University of St Andrews, St Andrews, UK.
Nature
|September 21, 2018
Summary
Researchers discovered a novel ring nuclease that degrades cyclic oligoadenylate, a key molecule in the CRISPR bacterial immunity system. This finding explains how the antiviral state is switched off, completing our understanding of CRISPR-Cas immunity.
Area of Science:
- Microbiology and Molecular Biology
- Gene Regulation and Gene Editing Technologies
Background:
- The CRISPR-Cas system confers adaptive immunity in prokaryotes against mobile genetic elements.
- Type III CRISPR effector complexes synthesize cyclic oligoadenylate (cOA) second messengers upon target RNA binding.
- cOA activates downstream factors, inducing an antiviral state, but the system's 'off-switch' mechanism remained unknown.
Purpose of the Study:
- To identify the nuclease responsible for degrading cyclic oligoadenylate molecules.
- To elucidate the mechanism by which the CRISPR-mediated antiviral state is deactivated.
Main Methods:
- Biochemical assays to identify and characterize the activity of the cyclic oligoadenylate-degrading nuclease.
- Structural analysis of the identified nuclease, a member of the CRISPR-associated Rossman-fold family.
Main Results:
- Identification of a novel 'ring nuclease' that specifically degrades cyclic tetraadenylate (cT) rings.
- The ring nuclease operates via a metal-independent mechanism, cleaving cT into linear diadenylate species.
- This degradation process effectively switches off the CRISPR-induced antiviral state.
Conclusions:
- The discovery of ring nucleases provides crucial insight into the regulatory 'off-switch' of the CRISPR-Cas system.
- This finding completes the mechanistic understanding of Type III CRISPR immunity, highlighting a key regulatory step.
- The identified ring nuclease represents a new class of enzymes within the CRISPR-associated protein family.
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