Structural Principles of CRISPR RNA Processing.

The Cas6 superfamily, the Cas5d subclass, and the host RNase III endoribonucleases are responsible for producing small RNAs (crRNA) that function in the CRISPR-Cas immunity. The three enzymes may also interact with the crRNA-associated nucleic acid interference complexes. Recent development in structural biology of Cas6 and Cas5d and their complexes with RNA substrates has lent new insights on principles of crRNA processing and the structural basis for linking crRNA processing to interference. Both Cas6 and Cas5d are characterized by the presence of the ferredoxin-like fold, but each has unique domain arrangement and insertion elements. Cas6 proteins often interact strongly with stable RNA stem-loop structures but can also fold unstructured RNA into stem-loop structures for their cleavage. The extraordinarily simple fold, the wide range of substrates, and kinetic properties of Cas6/Cas5d make them excellent candidates for exploring molecular evolution, protein-RNA interaction, and biotechnology applications.