Toxin-antitoxin RNA pairs safeguard CRISPR-Cas systems
Ming Li, Luyao Gong, Feiyue Cheng, Haiying Yu, Dahe Zhao, Rui Wang, Tian Wang, Shengjie Zhang, Jian Zhou, Sergey Shmakov, Eugene V. Koonin, Hua Xiang
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