Structure reveals mechanism of CRISPR RNA-guided nuclease recruitment and anti-CRISPR viral mimicry
Bacteria and archaea have evolved sophisticated adaptive immune systems that rely on CRISPR RNA (crRNA)-guided detection and nuclease-mediated elimination of invading nucleic acids. Here we present the cryo-EM structure of the type I-F CRISPR RNA-guided surveillance complex (Csy complex) from Pseudomonas aeruginosa bound to a double-stranded DNA target. Comparison of this structure to previously determined structures of this complex reveals a I180-degree rotation of the C-terminal helical bundle on the \"large\" Cas8f subunit. We show that the dsDNA-induced conformational change in Cas8f exposes a Cas2/3 \"nuclease recruitment helix\" that is structurally homologous to a virally encoded anti-CRISPR protein (AcrIF3). Structural homology between Cas8f and AcrIF3 suggests that AcrIF3 is a mimic of the Cas8f \"nuclease recruitment helix\", implying that cas genes may sometimes serve as genetic fodder for the evolution of anti-CRISPRs.