Journal: Nature / Year: 2021 Title: Target site selection and remodelling by type V CRISPR-transposon systems. Authors: Irma Querques / Michael Schmitz / Seraina Oberli / Christelle Chanez / Martin Jinek / Abstract: Canonical CRISPR-Cas systems provide adaptive immunity against mobile genetic elements. However, type I-F, I-B and V-K systems have been adopted by Tn7-like transposons to direct RNA-guided ...Canonical CRISPR-Cas systems provide adaptive immunity against mobile genetic elements. However, 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 pseudonuclease Cas12k, the transposase TnsB, the AAA+ ATPase TnsC and the zinc-finger protein TniQ, but the molecular mechanism of RNA-directed DNA transposition has remained elusive. Here we report cryo-electron microscopic structures of a Cas12k-guide RNA-target DNA complex and a DNA-bound, polymeric TnsC filament from the CRISPR-associated transposon system of the photosynthetic cyanobacterium Scytonema hofmanni. The Cas12k complex structure reveals an intricate guide RNA architecture and critical interactions mediating RNA-guided target DNA recognition. TnsC helical filament assembly is ATP-dependent and accompanied by structural remodelling of the bound DNA duplex. In vivo transposition assays corroborate key features of the structures, and biochemical experiments show that TniQ restricts TnsC polymerization, while TnsB interacts directly with TnsC filaments to trigger their disassembly upon ATP hydrolysis. Together, these results suggest that RNA-directed target selection by Cas12k primes TnsC polymerization and DNA remodelling, generating a recruitment platform for TnsB to catalyse site-specific transposon insertion. Insights from this work will inform the development of CRISPR-associated transposons as programmable site-specific gene insertion tools.
Method to determine structure: SAD / Resolution: 1.3→43.24 Å / SU ML: 0.13 / Cross valid method: THROUGHOUT / σ(F): 1.29 / Phase error: 21.24 / Stereochemistry target values: ML
Rfactor
Num. reflection
% reflection
Rfree
0.1952
4319
5.04 %
Rwork
0.1881
81333
-
obs
0.1889
45126
99.74 %
Solvent computation
Shrinkage radii: 0.9 Å / VDW probe radii: 1.11 Å / Solvent model: FLAT BULK SOLVENT MODEL
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