Small CRISPR RNAs Guide Antiviral Defense in Prokaryotes

TLDR

Prokaryotes gain viral resistance by incorporating short viral DNA fragments into CRISPR arrays. The study demonstrates how Cas proteins use virus-derived CRISPR sequences to mount an antiviral response. After CRISPR transcription, Cascade cleaves the precursor RNA and, with Cas3, the resulting mature guide RNAs direct Cascade to block viral replication. The results show that generating mature guide RNAs by Cascade’s endonuclease is essential for antiviral defense.

Abstract

Prokaryotes acquire virus resistance by integrating short fragments of viral nucleic acid into clusters of regularly interspaced short palindromic repeats (CRISPRs). Here we show how virus-derived sequences contained in CRISPRs are used by CRISPR-associated (Cas) proteins from the host to mediate an antiviral response that counteracts infection. After transcription of the CRISPR, a complex of Cas proteins termed Cascade cleaves a CRISPR RNA precursor in each repeat and retains the cleavage products containing the virus-derived sequence. Assisted by the helicase Cas3, these mature CRISPR RNAs then serve as small guide RNAs that enable Cascade to interfere with virus proliferation. Our results demonstrate that the formation of mature guide RNAs by the CRISPR RNA endonuclease subunit of Cascade is a mechanistic requirement for antiviral defense.