Diverse Mechanisms of CRISPR-Cas9 Inhibition by Type II Anti-CRISPR Proteins

Clustered regularly interspaced short palindromic repeats (CRISPR)-Cas (CRISPR-associated proteins) systems provide bacteria and archaea with an adaptive immune response against invasion by mobile genetic elements like phages, plasmids, and transposons. These systems have been repurposed as very powerful biotechnological tools for gene editing applications in both bacterial and eukaryotic systems. The discovery of natural off-switches for CRISPR-Cas systems, known as anti-CRISPR proteins, provided a mechanism for controlling CRISPR-Cas activity and opened avenues for the development of more precise editing tools. In this review, we focus on the inhibitory mechanisms of anti-CRISPRs that are active against type II CRISPR-Cas systems and briefly discuss their biotechnological applications.