RNA-based anti-CRISPRs (Racrs) interfere with the type I-F CRISPR-Cas system by mimicking the repeats found in CRISPR arrays. Here, we determined the cryo-electron microscopy (cryo-EM) structures of the type I-F crRNA-guided surveillance complex (Csy complex) from Pectobacterium atrosepticum and three RacrIF1-induced aberrant subcomplexes. Additionally, we observed that Cas7f proteins could bind to non-specific nucleic acids, forming right-handed superhelical filaments composed of different Cas7 copies. Mechanistically, RacrIF1 lacks the specific S-conformation observed in the corresponding position of the 5' handle in canonical CRISPR complexes, and it instead adopts a periodic "5 + 1" pattern. This conformation creates severe steric hindrance for Cas5f-Cas8f heterodimer and undermines their binding. Furthermore, Cas7f nonspecifically binds nucleic acids and can form infinite superhelical filaments along Racrs molecules. This oligomerization sequesters Cas6f and Cas7f from binding, therefore blocking the formation of functional CRISPR-Cas effector complexes and ultimately blocking antiviral immunity. Our study provides a structural basis underlying Racrs-mediated CRISPRs inhibition.
Replication stress is a key driver of DNA damage and genome instability. Here, we report that replication stress induces an inflammatory response in the absence of DNA damage. The DNA-sensing factor interferon-γ-inducible factor 16 (IFI16) binds nascent DNA at stalled replication forks and signals via the adaptor stimulator of interferon genes (STING) to induce activation of nuclear factor κB (NF-κB) and the production of pro-inflammatory cytokines, independently of the cytosolic DNA sensor cyclic guanosine monophosphate (GMP)-AMP synthase (cGAS). Replication stress-induced fork remodeling generates a new DNA end that is vulnerable to degradation by nucleases and is protected by a range of factors, including the tumor suppressors BRCA1 and BRCA2. IFI16 acts directly at stalled replication forks to protect nascent DNA from degradation by the nucleases MRE11, EXO1, and DNA2. Furthermore, IFI16 is required for the interferon-mediated rescue of fork protection in BRCA-deficient cells, highlighting the critical role of IFI16 in the crosstalk between innate immunity and fork protection during replication stress.

