Cryo-EM structure of the RfxCas13d-crRNA-off-target-RNA complex

The CRISPR-Cas system is crucial for the adaptive immune response of prokaryotes and has been widely applied for genetic engineering. Cas13d, a type VI-D CRISPR-Cas effector, functions as RNA-guided ribonuclease and has been engineered for programmable RNA editing, which is a commonly used, active, and well-characterized small type VI editor. Here, we determined cryoelectron microscopy (cryo-EM) structures of Ruminococcus flavefaciens Cas13d in a RfxCas13d-crRNA-off-target-RNA ternary complex and RfxCas13d-crRNA binary complex at 3.10 and 3.13 Å resolution. The ternary complex consists of RfxCas13d, crRNA, and a captured short off-target ssRNA at a complex state of binding proximal mismatched RNA. RfxCas13d undergoes conformational changes with or without the off-target RNA, but the catalytic sites remain unchanged. Mg²⁺ aids in stabilizing the crRNA repeat region structure, which may be crucial for RNA binding. This discovery provides the foundation for developing RfxCas13d as a mature tool and offers a framework for advancing transcriptome engineering.