Lethal endotoxin (ccdB) based counterselection improved the efficiency of sequential gene editing in Escherichia coli

The CRISPR/Cas9-based technology has been used for sequential gene editing in E. coli. The plasmids carrying the sgRNA and/or Cas9 genes need to be cured after each round of editing. Curing of these plasmids, particularly the sgRNA plasmid, limits the efficiency of sequential gene editing. In this study, a lethal endotoxin (ccdB) based counterselection was established for improving the overall efficiency of sequential gene editing in E. coli. This approach was validated for sequential editing (deletion) of cstA and ppsA genes in HBUT-P2 strain (W derivative). The experimental results showed that the transformation efficiency of sgRNA plasmid (pTargetF-tcr-P_L-ccdB-N20) reached 10⁸-10⁹ cfu/μg_-DNA, resulting in a 100% and 93.75% recombination rate for cstA and ppsA gene, respectively. Upon completion of cstA gene editing, the sgRNA plasmid (pTargetF-tcr-P_L-ccdB-N20 (cstA)) was effectively cured through ccdB based counterselection at 42 °C, with a 43.75% efficiency. At the end of sequential editing of ppsA gene, both Cas9 (25A) and sgRNA (pTargetF-tcr-P_L-ccdB-N20 (ppsA)) plasmids were cured simultaneously through the sacB and ccdB based counterselections by incubating the cells on LB-sucrose (5%) plate at 42 °C, achieving a curing rate of 100% for Cas9 plasmid (25A), 37.5% for sgRNA plasmid (pTargetF-tcr-P_L-ccdB-N20 (ppsA)), and 37.5% for both Cas9 and sgRNA plasmids. Moreover, this approach was further validated through efficient site-specific insertion of the csc operon into the slmA gene in DH5α (K12 derivative) and S322 (C derivative) strains. These results demonstrated that the endotoxin (ccdB) based counterselection improved the transformation efficiency of sgRNA plasmid, the recombination rate of the editing target gene, the curing rate of sgRNA plasmid, and the overall efficiency of sequential gene editing.