A high-resolution (1.2 Å) crystal structure of the anti-CRISPR protein AcrIF9.

Prokaryotic adaptive immunity by CRISPR-Cas systems, which confer resistance to foreign genetic elements, has been used by bacteria to combat viruses. To cope, viruses evolved multiple anti-CRISPR proteins, which can inhibit system function through various mechanisms. Although the structures and mechanisms of several anti-CRISPR proteins have been elucidated, those of the AcrIF9 family have not yet been identified. To understand the molecular basis underlying AcrIF9 anti-CRISPR function, we determined the 1.2 Å crystal structure of AcrIF9. Structural and biochemical studies showed that AcrIF9 exists in monomeric form in solution and can directly interact with DNA using a positively charged cleft. Based on analysis of the structure, we suggest part of the anti-CRISPR molecular mechanism by AcrIF9.