Structural & Biochemical Insights into CRISPR-transposon systems

Image kindly provided by Irma Querques.

Several known CRISPR systems are not part of bacterial defence systems but are actually used by transposons for RNA guided transposon insertion. The Jinek lab using Cryo-EM determined two structures to gain further insights into type V CRISPR-transposon systems. They complemented their structural work with biochemical experiments. Their findings have been published in Nature in an article entitled "Target site selection and remodelling by type V CRISPR-transposon systems".

Canonical CRISPR-Cas systems provide adaptive immunity against mobile genetic elements1. However, type I-F, I-B and V-K systems have been adopted by Tn7-like transposons to direct RNA-guided transposon insertion2-7. Type V-K CRISPR-associated transposons rely on the pseudonuclease Cas12k, the transposase TnsB, the AAA+ ATPase TnsC and the zinc-finger protein TniQ7, 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.

Read the Publication in Nature

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