The CRISPR system for prokaryotic adaptive immunity provides RNA-mediated protection from viruses and mobile genetic elements. When viral RNA transcripts are detected, type III systems adopt an activated state that licenses DNA interference and synthesis of cyclic oligoadenylate (cOA). cOA activates nucleases and transcription factors that orchestrate the antiviral response. We demonstrate that cOA synthesis is subject to tight temporal control, commencing on target RNA binding, and is deactivated rapidly as target RNA is cleaved and dissociates. Mismatches in the target RNA are well tolerated and still activate the cyclase domain, except when located close to the 3' end of the target. Phosphorothioate modification reduces target RNA cleavage and stimulates cOA production. The 'RNA shredding' activity originally ascribed to type III systems may thus be a reflection of an exquisite mechanism for control of the Cas10 subunit, rather than a direct antiviral defence.
All data generated or analysed during this study are included in the manuscript and supporting files.
- Christophe Rouillon
- Malcolm F White
- Sabine Grüschow
The funders had no role in study design, data collection and interpretation, or the decision to submit the work for publication.
- Gisela Storz, National Institute of Child Health and Human Development, United States
© 2018, Rouillon et al.
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