Bacteria have evolved an arsenal of diverse antiphage systems with different mechanisms, including the well-described CRISPR–Cas systems. These are adaptive immune systems that use small CRISPR RNA (crRNA) molecules to guide ribonucleoprotein effector complexes to complementary sites on foreign DNA, which leads to target cleavage. In turn, bacteriophages (phages) have developed mechanisms to overcome bacterial immunity, including anti-CRISPR proteins (Acrs), which inactivate the CRISPR–Cas machinery via diverse inhibitory mechanisms. The effector complex of the type I-F CRISPR–Cas system of Pseudomonas aeruginosa is termed the Csy complex, and it comprises a crRNA and four Cas proteins (Cas5, Cas6, Cas7 and Cas8). Once the Csy complex binds to the specific viral DNA sequence, a helicase/nuclease protein known as Cas2-3 is recruited and the DNA is destroyed by its activity. So far, many Acrs have been identified that block the function of the Csy complex by impeding DNA binding or nuclease recruitment. In this new study, Trost et al. show that the anti-CRISPR protein AcrIF25 inhibits the type I-F CRISPR–Cas system by pulling apart the fully assembled Csy complex.

Next, the authors investigated the potential of AcrIF25 to bind the Csy complex. Their analysis revealed that a complex is formed between AcrIF25 and Cas7, whereas no other components of the Csy complex co-purified with that complex. The authors concluded that AcrIF25 binds to Cas7 subunits and dissociates them from the Csy complex.