Scarless Cas9 Assisted Recombineering (no-SCAR) in Escherichia coli, an Easy-to-Use System for Genome Editing.

The discovery and development of genome editing systems that leverage the site-specific DNA endonuclease system CRISPR/Cas9 has fundamentally changed the ease and speed of genome editing in many organisms. In eukaryotes, the CRISPR/Cas9 system utilizes a "guide" RNA to enable the Cas9 nuclease to make a double-strand break at a particular genome locus, which is repaired by non-homologous end joining (NHEJ) repair enzymes, often generating random mutations in the process. A specific alteration of the target genome can also be generated by supplying a DNA template in vivo with a desired mutation, which is incorporated by homology-directed repair. However, E. coli lacks robust systems for double-strand break repair. Thus, in contrast to eukaryotes, targeting E. coli chromosomal DNA with Cas9 causes cell death. However, Cas9-mediated killing of bacteria can be exploited to select against cells with a specified genotype within a mixed population. In combination with the well described λ-Red system for recombination in E. coli, we created a highly efficient system for marker-free and scarless genome editing. © 2017 by John Wiley & Sons, Inc.