Casδ, an evolutionary transitional CRISPR system enables efficient genome editing across animals and plants

Clustered regularly interspaced short palindromic repeats (CRISPR)-Cas (CRISPR-associated) adaptive immune systems provide sequence-specific mechanisms for targeting foreign DNA or RNA and have been widely used in genome editing and DNA detection. Type V CRISPR-Cas systems are characterized by a single RNA-guided RuvC domain-containing effector, Cas12. Here, through comprehensive mining of large-scale genomic and metagenomic data from microbial sources, we identified a new Class 2 CRISPR-Cas effector superfamily, designated Casδ, comprising three members with protein sizes ranging from 867 to 936 amino acids. Biochemical analyses revealed that Casδ-1 functions as a single RNA-guided endonuclease with specific recognition of 5'-RYR-3' protospacer-adjacent motifs, where R represents A or G, and Y represents T or C. Casδ-1 exhibits robust double-stranded DNA cleavage activity and target-dependent trans-cleavage activity. Casδ-1 mediates efficient genome editing across species, achieving up to 60% indel rates in human cells while generating homozygous knockout lines in two agriculturally important monocot species (Oryza sativa and Zea mays) through stable transformation. Structural and evolutionary analyses reveal Casδ as an evolutionary transitional nuclease bridging Cas12n and canonical type V systems, featuring a C-terminal loop that is essential for activity. Collectively, Casδ is an evolutionarily distinct, compact (<1000 aa), tracrRNA-free CRISPR system enabling versatile cross-kingdom genome editing.