Fig. 1

Virus-encoded inhibitors of antiviral defense systems. a RNA viruses that infect various plants, insects and mammals encode proteins that inhibit essential proteins in the RNA interference pathway, such as Dicer and Argonaute (AGO), a subunit of the RNA-induced silencing complex (RISC). Furthermore, these viruses may produce proteins that will mask double-stranded RNA (dsRNA) so that Dicer activity does not occur or that will sequester any small interfering RNA (siRNA) molecules produced. b Bacteriophage P1 co-injects DarA and DarB to mask restriction sites in the phage genome. Bacteriophage T7 encodes Ocr, which mimics the DNA phosphate backbone and sequesters both the EcoKI restriction enzyme (R) and its corresponding methylase (M). Bacteriophage Lambda encodes Ral, which hyperactivates the DNA methylase to protect phage DNA before it can be restricted. c Anti-CRISPR proteins encoded by Pseudomonas prophages (AcrF1–AcrF3) bind CRISPR-RNA–effector complexes and the nuclease Cas3 to prevent them from binding and cleaving target dsDNAs