Cas6 processes tight and relaxed repeat RNA via multiple mechanisms: A hypothesis

RNA molecules are flexible yet foldable. Proteins must cope with this structural duality when forming biologically active complexes with RNA. Recent studies of the Clustered Regularly Interspaced Short Palindromic Repeats (CRISPRs)-mediated RNA immunity illustrate some remarkable mechanisms with which proteins interact with RNA. Currently known structures of CRISPR-Cas6 endoribonucleases bound with RNA suggest a conserved protein recognition mechanism mediated by RNA stem-loops. However, a survey of CRISPR RNA reveals that many repeats either lack a productive stem-loop (Relaxed) or possess stable but inhibitory structures (Tight), which raises the question of how the enzyme processes structurally diverse RNA. In reviewing recent literature, we propose a bivalent trapping and an unwinding mechanism for CRISPR-Cas6 to interact with the Relaxed and the Tight repeat RNA, respectively. Both mechanisms aim to create an identical RNA conformation at the cleavage site for accurate processing.