LAUSR

Bacterial Immunity Bacteria use the CRISPR-Cas immune system to fend off viruses; viruses use anti-CRISPR proteins to fight back. Some bacteria have acquired anti-CRISPR proteins in their genomes to avoid self-cutting. Meeske and Marraffini report the intriguing possibility that bacteria might also use RNA to inhibit CRISPR-Cas enzymes. Cas13, the effector of the type VI CRISPR-Cas system, is directed by guide RNA to cleave target RNA in cis, which further unleashes its indiscriminate trans RNA-cutting activity. However, extended complementarity between guide and target RNAs locks Cas13 into an inactive state, which prevents both cis and trans cleavage, suggesting an endogenous RNA-based mechanism to regulate Cas13. Cas13 is a powerful tool because of its ability to detect and manipulate RNA. The discovery of “anti-CRISPR RNA” has the potential to diversify and expand Cas13's applications. Mol. Cell. 71 , 791 (2018).