Structural basis for target DNA cleavage and guide RNA processing by CRISPR-Casλ2.

RNA-guided CRISPR-Cas nucleases are widely used as versatile genome-engineering tools. Among the diverse CRISPR-Cas effectors, CRISPR-Casλ-also referred to as Cas12n-is a recently identified miniature type V nuclease encoded in phage genomes. Given its demonstrated nuclease activity in both mammalian and plant cells, Casλ has emerged as a promising candidate for genome-editing applications. However, the precise molecular mechanisms of Casλ family enzymes remain poorly understood. In this study, we report the identification and detailed biochemical and structural characterizations of CRISPR-Casλ2. The cryo-electron microscopy structures of Casλ2 in five different functional states unveiled the dynamic domain rearrangements during its activation. Our biochemical analyses indicated that Casλ2 processes its precursor crRNA to a mature crRNA using the RuvC active site through a unique ruler mechanism, in which Casλ2 defines the spacer length of the mature crRNA. Furthermore, structural comparisons of Casλ2 with Casλ1 and CasΦ highlighted the diversity and conservation of phage-encoded type V CRISPR-Cas enzymes. Collectively, our findings augment the mechanistic understanding of diverse CRISPR-Cas nucleases and establish a framework for rational engineering of the CRISPR-Casλ-based genome-editing platform.