No abstract

No abstract

CRISPR systems have been co-opted by Tn7-like elements to direct RNA-guided transposition. Type V-K CRISPR-associated transposons rely on the concerted activities of the pseudonuclease Cas12k, the AAA+ ATPase TnsC, the Zn-finger protein TniQ, and the transposase TnsB. Here we present a cryo-electron microscopic structure of a target DNA-bound Cas12k-transposon recruitment complex comprising RNA-guided Cas12k, TniQ, TnsC and, unexpectedly, the ribosomal protein S15. Complex assembly on target DNA results in complete R-loop formation mediated by critical interactions between TniQ and the trans-activating crRNA, and is coupled with TniQ-dependent nucleation of a TnsC filament. In vivo transposition assays corroborate our structural findings, and biochemical and functional analyses of S15 supports its role as a bona fide component of the type V crRNA-guided transposition machinery. Altogether, our work uncovers key aspects of the mechanisms underpinning RNA-mediated assembly of CRISPR-associated transposons that will guide their development as programmable site- specific gene insertion tools.

Although the canonical function of CRISPR-Cas systems is to provide adaptive immunity against mobile genetic elements, type I-F, I-B and V-K systems have been adopted by Tn7-like transposons to direct RNA-guided transposon insertion. Type V-K CRISPR-associated transposons rely on the activities of the pseudonuclease Cas12k, the transposase TnsB, the AAA+ ATPase TnsC and the zinc-finger protein TniQ. However, the molecular and structural details of RNA-directed DNA transposition have remained elusive. Here we report cryo-electron microscopic structures of a Cas12k-guide RNA-target DNA complex and a DNA-bound, polymeric TnsC filament. The Cas12k complex structure reveals an intricate guide RNA architecture and critical interactions mediating RNA-guided target DNA recognition. The assembly of the TnsC helical filament is ATP-dependent and accompanied by structural remodeling of the bound DNA duplex. In vivo transposition assays corroborate key features of the structures, and biochemical experiments further show that TniQ restricts TnsC polymerization, while the TnsB transposase interacts directly with TnsC filaments to trigger their disassembly upon ATP hydrolysis. Together, these results suggest a mechanistic model whereby RNA-directed target selection by Cas12k primes TnsC polymerization and DNA remodeling, generating a recruitment platform for TnsB to catalyze site-specific transposon insertion. The present work advances our mechanistic understanding of the cross-talk between CRISPR effectors and the transposition machinery and will inform design efforts to harness CRISPR-associated transposons as programmable site-specific gene insertion tools for genome engineering applications.