Natural and Engineered Guide RNA–directed Transposition with CRISPR-Associated Tn7-like Transposons

CRISPR–Cas (clustered regularly interspaced short palindromic repeats–CRISPR-associated nuclease) defense systems have been naturally coopted for guide RNA–directed transposition on multiple occasions. In all cases, cooption occurred with diverse elements related to the bacterial transposon Tn7. Tn7 tightly controls transposition; the transposase is activated only when special targets are recognized by dedicated target-site selection proteins. Tn7 and the Tn7-like elements that coopted CRISPR–Cas systems evolved complementary targeting pathways: one that recognizes a highly conserved site in the chromosome and a second pathway that targets mobile plasmids capable of cell-to-cell transfer. Tn7 and Tn7-like elements deliver a single integration into the site they recognize and also control the orientation of the integration event, providing future potential for use as programmable gene-integration tools. Early work has shown that guide RNA–directed transposition systems can be adapted to diverse hosts, even within microbial communities, suggesting great potential for engineering these systems as powerful gene-editing tools.