Mechanism of Cas9 inhibition by AcrIIA11.

Mobile genetic elements evade CRISPR-Cas adaptive immunity by encoding anti-CRISPR proteins (Acrs). Acrs inactivate CRISPR-Cas systems via diverse mechanisms but generally coevolve with a narrow subset of Cas effectors that share high sequence similarity. Here, we demonstrate that AcrIIA11 inhibits Streptococcus pyogenes (Sp), Staphylococcus aureus (Sa), and Francisella novicida (Fn) Cas9s in vitro and in human cells. Single-molecule imaging reveals that AcrIIA11 hinders SaCas9 target search by reducing its diffusion on nonspecific DNA. DNA cleavage is inhibited because the AcrIIA11:SaCas9 complex binds to protospacer adjacent motif (PAM)-rich off-target sites, preventing SaCas9 from reaching its target. AcrIIA11 also greatly slows down DNA cleavage after SaCas9 reaches its target site. A negative-stain electron microscopy reconstruction of an AcrIIA11:SaCas9 RNP complex reveals that the heterodimer assembles with a 1:1 stoichiometry. Physical AcrIIA11-Cas9 interactions across type IIA and IIB Cas9s correlate with nuclease inhibition and support its broad-spectrum activity. These results add a kinetic inhibition mechanism to the phage-CRISPR arms race.