The battle between bacteria and bacteriophages has led to the development of defense mechanisms such as the CRISPR-Cas system by bacteria. This system uses ribonucleoprotein complexes to detect and destroy the DNA of invading phages [1]. To counteract this defense, bacteriophages encode small repressors called anti-CRISPR (Acr) proteins. These Acr proteins interact with the effector complex, preventing either the recognition or hydrolysis of DNA [2]. CRISPR-Cas has become a valuable tool for genome editing, while Acr proteins have been used to regulate this tool in both prokaryotes and eukaryotes [3]. To date, more than 100 Acr families have been identified. However, the molecular mechanism is characterized only for a small fraction of these proteins [4].
Here, we analyse Acr proteins that inhibit the type I-F CRIPR-Cas system from Pseudomonas aeruginosa. We identified the Acr protein blocking the system in bacterial cells. Then, purified the protein and by employing biochemical methods we studied its interplay with the CRISPR-Cas components. Our findings provide insight into the molecular mechanism of this uncharacterized Acr protein.