CRISPR (clustered regularly interspaced short palindromic repeats)–Cas (CRISPR-associated genes) surveillance complexes are RNA-based adaptive immune systems employed by prokaryotes against invading nucleic acids from bacteriophages and plasmids.1,2 The CRISPR-derived RNAs (crRNAs) guide the Cas effector complex to target and degrade the invading nucleic acids. Recently, bioinformatics analyses have revealed the presence of CRISPR–Cas loci in bacterial Tn7-like transposons, thereby implicating a functional relationship between RNA-guided DNA targeting and transposition, with the latter representing a new role unrelated to host defense.3 Support for this concept has emerged from recent functional studies on type I-F and type V-K effectors involved in sequence-specific DNA transposition,4,5 thereby significantly broadening the potential biological applications of CRISPR–Cas technology. To complement the available functional studies, our efforts have focused on structural studies of the Vibrio cholerae Tn6677 multi-subunit type I-F CascadecrRNA–TniQ complex, whereby transposition subunit TniQ initiates DNA transposition with the eventual help of other transposition-associated proteins TnsA, TnsB and TnsC in the gene cluster (Fig. 1a).
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