Anti-CRISPR Proteins in Archaea.

Anti-CRISPR (Acr) proteins are natural inhibitors of CRISPR-Cas immune systems. To date, Acrs inhibiting types I, II, III, V, and VI CRISPR-Cas systems have been characterized. While most known Acrs are derived from bacterial phages and prophages, very few have been characterized in the domain Archaea, despite the nearly ubiquitous presence of CRISPR-Cas in archaeal cells. Here we summarize the discovery and characterization of the archaeal Acrs with the representatives encoded by a model archaeal virus, Sulfolobus islandicus rod-shaped virus 2 (SIRV2). AcrID1 inhibits subtype I-D CRISPR-Cas immunity through direct interaction with the large subunit Cas10d of the effector complex, and AcrIIIB1 inhibits subtype III-B CRISPR-Cas immunity through a mechanism interfering with middle/late gene targeting. Future development of efficient screening methods will be key to uncovering the diversity of archaeal Acrs.