Research Article

Toxin-antitoxin RNA pairs safeguard CRISPR-Cas systems

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Science  30 Apr 2021:
Vol. 372, Issue 6541, eabe5601
DOI: 10.1126/science.abe5601

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Small RNAs guard CRISPR-Cas

The microbial adaptive immunity system CRISPR-Cas benefits microbes by warding off genetic invaders, but it also inflicts a fitness cost because of occasional autoimmune reactions, rendering CRISPR loci evolutionarily unstable. Li et al. identified previously unnoticed toxin-antitoxin RNA pairs embedded within diverse CRISPR-Cas loci. The antitoxin RNA mimics a CRISPR RNA and repurposes the CRISPR immunity effector to transcriptionally repress a toxin RNA that would otherwise arrest cell growth by sequestering a rare transfer RNA. These small RNAs thus form a symbiosis with CRISPR, rendering CRISPR addictive to the host despite its fitness cost. These findings reveal how CRISPR-Cas can operate as a selfish genetic element.

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