Programmable enzymes for large genome edits

See p.984 & p.994 A long-standing aspiration for biologists has been to develop programmable methods to rearrange long DNA sequences in genomes. Thiscapabilitywould allow kilobase-scale DNA sequences to be inserted, inverted, deleted or moved to user-specified genome locations in cells in a single step. Excitingly, the authors show that the loops can be engineered independently to program the recombinase of an IS110 family member (known as IS621) to invert, excise or insert custom DNA sequences at user-specified regions of the genome in the bacterium Escherichia coli. The structures provide mechanistic information, such as how intricate molecular gymnastics occur to enable pairing between the DBL and the donor, and between the TBL and the target; the structure of the active site that cleaves to the DNA molecules; and how hydrophobic amino-acid residues in the recombinase destabilize the donor and target DNA duplexes to promote bRNA-mediated recognition of both the donor and the target DNAs.