Gene

Genome editing

CRISPR interference

Toxin

Antitoxin

Effector

Genetics

Computational biology

Trans-activating crRNA

Biology

Cell biology

Guide RNA

CRISPR

Science

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

Toxin-antitoxin RNA pairs safeguard CRISPR-Cas systems