Crystal structure of Cmr2 suggests a nucleotide cyclase‐related enzyme in type III CRISPR‐Cas systems

CRISPR RNAs (crRNAs) mediate sequence-specific silencing of invading viruses and plasmids in prokaryotes. The crRNA–Cmr protein complex cleaves complementary RNA. We report the crystal structure of Pyrococcus furiosus Cmr2 (Cas10), a component of this Cmr complex and the signature protein in type III CRISPR systems. The structure reveals a nucleotide cyclase domain with a set of conserved catalytic residues that associates with an unexpected deviant cyclase domain like dimeric cyclases. Additionally, two helical domains resemble the thumb domain of A-family DNA polymerase and Cmr5, respectively. Our results suggest that Cmr2 possesses novel enzymatic activity that remains to be elucidated.