Reproducible Antigen Recognition by the Type I-F CRISPR-Cas System.

CRISPR-associated proteins 1 and 2 (Cas1-2) are necessary and sufficient for new spacer acquisition in some CRISPR-Cas systems (e.g., type I-E), but adaptation in other systems (e.g., type II-A) involves the crRNA-guided surveillance complex. Here we show that the type I-F Cas1-2/3 proteins are necessary and sufficient to produce low levels of spacer acquisition, but the presence of the type I-F crRNA-guided surveillance complex (Csy) improves the efficiency of adaptation and significantly increases the fidelity of protospacer adjacent motif selection. Sequences selected for integration are preferentially derived from specific regions of extrachromosomal DNA, and patterns of spacer selection are highly reproducible between independent biological replicates. This work helps define the role of the Csy complex in I-F adaptation and reveals that actively replicating mobile genetic elements have antigenic signatures that facilitate their integration during CRISPR adaptation.