LAUSR

The discovery of an anti-CRISPR reveals viral escape from CRISPR immunity Explorations of the evolutionary arms race between bacteria and bacteriophages (viruses that infect bacteria) have unearthed a variety of defense mechanisms that include CRISPR-Cas (CRISPR-associated nuclease) adaptive immune systems (1). Understanding the mechanisms of CRISPR-mediated immunity, involving DNA-encoded, RNA-guided, sequence-specific targeting of invasive nucleic acids (2, 3), has spawned powerful genome engineering platforms based on diverse Cas effectors. Subsequent studies have also revealed anti-CRISPR proteins (4) that have proven valuable as control switches for Cas molecular machines. CRISPR-Cas immune systems encompass diverse families including DNA-targeting effectors such as Cascade-Cas3, Cas9, and Cas12 as well as the recently characterized RNA-targeting Cas13 ribonuclease (RNase) (5). The evolving immune arsenal in bacteria has been matched by diverse anti-CRISPRs that enable viruses to escape Cas nuclease targeting. On page 54 of this issue, Meeske et al. (6) report an anti-CRISPR mechanism that inhibits Cas13a RNase activity, with potential utility as a CRISPR effector control switch.