AcrIIA5 Inhibits a Broad Range of Cas9 Orthologs by Preventing DNA Target Cleavage.

CRISPR-Cas9 is an adaptive immune system for prokaryotes to defend against invasive genetic elements such as phages and has been used as a powerful tool for genome editing and modulation. To overcome CRISPR immunity, phages encode anti-CRISPR proteins (Acrs) to inhibit Cas9, providing an efficient "off-switch" tool for Cas9-based applications. Here, we characterized AcrIIA5, which is a Cas9 inhibitor discovered in a virulent phage of Streptococcus thermophilus. We found that AcrIIA5 is a potent and broad-spectrum inhibitor of CRISPR-Cas9, which can inhibit diverse Cas9 orthologs of type II-A, type II-B, and type II-C. AcrIIA5 inhibits Cas9 by preventing DNA target cleavage, but DNA target binding of Cas9 is unaffected. Importantly, it can affect the activity of the RuvC nuclease domain of Cas9 independent of the HNH nuclease domain. Our work expands the diversity of the inhibitory mechanisms used by Acrs and provides the guidance for developing controlling tools in Cas9-based applications.