Structure of a type IV CRISPR-Cas effector complex

We reveal the structure of a type IV-B CRISPR effector (Csf) complex at 3.9 Å resolution using cryo-electron microscopy. The complex resembles the type III-A CRISPR Csm effector complex, but lacks subunits for RNA processing and target DNA cleavage, and is surprisingly assembled upon heterogeneous non-CRISPR RNA. These findings provide the first glimpse into the assembly and function of enigmatic type IV CRISPR systems.