“Mechanism for RNA-guided immunity in bacteria, determined by cryoEM“
Gabriel Lander – Nogales Lab, HHMI/ University of California Berkeley
Bacteria and archaea acquire resistance to viruses and plasmids by integrating short fragments of foreign DNA into Clustered Regularly Interspaced Short Palindromic Repeats (CRISPRs). In E. coli, CRISPR transcripts are processed into short RNAs that are incorporated into a large surveillance complex called “Cascade” that specifically targets foreign nucleic acids. We used cryo-electron microscopy to determine the structure of Cascade before and after binding to a target substrate. At sub-nanometer resolutions, these structures reveal a seahorse-shaped architecture that displays a crRNA along a helical arrangement of six CasC subunits. Cascade engages invading nucleic acids through high-affinity base pairing interactions located near the 5’ end of the crRNA, triggering a concerted conformational change. Together, these structures explain the molecular mechanism for target recognition and reveal a structural reorganization that likely serves as a signal for subsequent destruction of the invading DNA.