Toxin-antitoxin RNA pairs safeguard CRISPR-Cas systems

TLDR

Small RNAs protect CRISPR‑Cas systems, which defend microbes against genetic invaders but incur fitness costs from occasional autoimmune reactions that destabilize CRISPR loci. The authors discovered toxin‑antitoxin RNA pairs within diverse CRISPR‑Cas loci, where the antitoxin RNA mimics a CRISPR RNA to redirect the immunity effector and transcriptionally repress a toxin RNA that would otherwise arrest growth by sequestering a rare tRNA. These small RNAs establish a symbiosis with CRISPR, rendering the system addictive to the host despite its fitness cost and demonstrating that CRISPR‑Cas can function as a selfish genetic element. Li et al., Science, this issue p.

Abstract

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. Science , this issue p. eabe5601