Fig 1.
A model of the mechanisms of action of three anti-CRISPR proteins.
The type I-F CRISPR loci consist of direct repeat sequences (black diamonds) separated by unique spacer sequences (colored rectangles) that were derived from foreign mobile genetic elements. During the acquisition stage, a piece of DNA from an invading phage or other mobile element is captured and inserted as a spacer into a CRISPR array (yellow rectangle). Next, the CRISPR-associated (Cas) proteins are produced, and the CRISPR loci are transcribed into long pre-CRISPR-RNAs. The Csy4 endonuclease then cleaves the repeat sequences to yield mature CRISPR-RNAs (crRNAs), each containing a single spacer sequence (colored lines). The crRNA-Csy4 complex then interacts with Csy1, Csy2, and Csy3 to form the surveillance complex. AcrF1 and AcrF2 directly interact with this complex and prevent DNA binding. By contrast, AcrF3 binds to the Cas3 helicase–nuclease protein and blocks its recruitment.