Initial Characterization of 12 New Subtypes and Variants of Type V CRISPR Systems.

Type V CRISPR systems are highly diverse in sequence, mechanism, and function. Although recent efforts have greatly expanded our understanding of their evolution, the diversity of type V systems remains to be completely explored, and many clades have not been experimentally characterized. In this work, we mined metagenomic databases to identify three new subtypes and nine new variants of Cas12, the effector of Type V systems, and provide experimental and computational characterization of their Protospacer-Adjacent Motif (PAM), interference activity, loci architecture, and tracrRNA dependence. Half of the new Cas12s are found in phages or prophages. New subtypes Cas12o and Cas12p lack the canonical RuvC catalytic residues, suggesting they interfere with the target without cleavage, possibly by blocking transcription or replication. One variant, Cas12f10, displays substantial activity on PAM-less targets. Our work expands the diversity of the functionally characterized Cas12 effectors and provides some promising candidates for genome engineering tools.