Mechanisms of CRISPR-mediated immunity and applications beyond editing

ABSTRACT

Mammals, bacteria, and archaea have domesticated transposases (e.g., RAG1 and Cas1) to form adaptive immune systems. Bacteria and archaea acquire resistance to viruses and plasmids by preferentially integrating fragments of foreign DNA at one end of a CRISPR locus. DNA motifs upstream of the CRISPR (i.e., leader) facilitate integration at the first CRISPR repeat. But how do these upstream DNA motifs act over large distances of 130 bp, or roughly 440 A, to regulate integration allosterically? Here, we determine the structure of a 560 KDa integration complex that explains how the CRISPR leader DNA recruits Cas (i.e., Cas1-2/3) and non-Cas proteins (i.e., IHF). Cas1-2/3 and IHF cooperate to fold the genome into a successive U-shaped bend and a loop. The genomic U-bend traps foreign DNA against the integrase, whereas the genomic loop positions the leader-repeat junction at the Cas1 active site. The foreign DNA and the CRISPR repeat wrap around opposite faces of Cas2, poised for a Cas1-catalyzed strand-transfer reaction. The post-integration structure suggests that strand-transfer releases tension in the DNA loop. Therefore Cas1-2/3 may harness protein-induced DNA tension to favor the completion of the isoenergetic integration reaction. Cas1-2/3 interacts extensively with the leader and repeat without making sequence-specific contacts, and we demonstrate that protein-mediated folding of DNA drives integration into diverse sequences. These results reveal Cas1-2/3 and IHF strain DNA to enhance integration allosterically and suggest a mechanism for the de novo generation of new CRISPRs. Further, to address an urgent need for inexpensive and rapid detection of viruses, we recently repurposed a CRISPR immune signaling pathway to detect SARS-CoV-2 in patient samples. A.S-F. is a postdoctoral fellow of the Life Science Research Foundation, supported by the Simons Foundation. A.S-F. is supported by the PDEP award from the Burroughs Wellcome Fund, and by the National Institutes of Health, United States grant 1K99GM147842. This work was also supported by NSF (1828765), NIH (U24 GM129539, R35GM134867).Copyright © 2023 The American Society for Biochemistry and Molecular Biology, Inc.