AcrIF9 tethers non-sequence specific dsDNA to the CRISPR RNA-guided surveillance complex.

Bacteria have evolved sophisticated adaptive immune systems, called CRISPR-Cas, that provide sequence-specific protection against phage infection. In turn, phages have evolved a broad spectrum of anti-CRISPRs that suppress these immune systems. Here we report structures of anti-CRISPR protein IF9 (AcrIF9) in complex with the type I-F CRISPR RNA-guided surveillance complex (Csy). In addition to sterically blocking the hybridization of complementary dsDNA to the CRISPR RNA, our results show that AcrIF9 binding also promotes non-sequence-specific engagement with dsDNA, potentially sequestering the complex from target DNA. These findings highlight the versatility of anti-CRISPR mechanisms utilized by phages to suppress CRISPR-mediated immune systems. The anti-CRISPR protein IF9 (AcrIF9) specifically inhibits the type I-F CRISPR adaptive immune system. Here, the authors present the cryo-EM structure of AcrIF9 in complex with the type I-F CRISPR RNA-guided surveillance complex (Csy) and a dsDNA bound Csy-AcrIF9 structure, and find that AcrIF9 binding to the Csy complex triggers non-sequence specific dsDNA binding to Csy-AcrIF9, which might sequester the complex from its target DNA.