An RNA-induced conformational change required for CRISPR RNA cleavage by the endoribonuclease Cse3.

Clustered regularly interspaced short palindromic repeat (CRISPR) chromosomal loci found in prokaryotes provide an adaptive immune system against bacteriophages and plasmids. CRISPR-specific endoRNases produce short RNA molecules (crRNAs) from CRISPR transcripts, which harbor sequences complementary to invasive nucleic acid elements and ensure their selective targeting by CRISPR-associated (Cas) proteins. The extreme sequence divergence of CRISPR-specific endoRNases and their RNA substrates has obscured homology-based comparison of RNA recognition and cleavage mechanisms. Here, we show that Cse3 type CRISPR-specific endoRNases bind a hairpin structure and residues downstream of the cleavage site within the repetitive segment of cognate CRISPR RNA. Cocrystal structures of Cse3-RNA complexes reveal an RNA-induced conformational change in the enzyme active site that aligns the RNA strand for site-specific cleavage. These studies provide insight into a catalytically essential RNA recognition mechanism by a large class of CRISPR-related endoRNases.