Leveraging CRISPR-Cas9 Counter-Selection for Targeted Mutagenesis in Escherichia coli

Directed evolution using genetically diverse libraries is integral to advancing research in industrial microbial production and protein functionality enhancement. This process typically involves a step of sequence diversification and subsequent selection/screening steps for improved variants. While CRISPR-Cas9 systems are known to offer efficient and targeted modification of genes in vivo, concerns arise regarding off-target effects and the emergence of escaper cells evading Cas9 cleavage. This study investigated a strategy to leverage CRISPR-Cas9 counter-selection in Escherichia coli for targeted chromosomal mutagenesis. By designing gRNAs to target a desired region, the spontaneous mutations occurring at the targeted region will potentially disrupt Cas9 binding and thus allow the cell to avoid death caused by Cas9-induced double-stranded DNA breaks. This population of ‘escaper’ cells surviving the counter-selection will have mutations in the gRNA-targeting region at a higher frequency than their non-escaper counterparts. To optimize this counter-selection method, the design for the CRISPR-Cas9 expression system was improved, Cas9 variants with varied fidelities and activities were investigated, and the strategy of using truncated gRNAs for enhanced mutation selectivity was explored. Using the E. coli rpoB gene as a target for editing, the rifampicin-resistant mutation (caused by mutations in rpoB) frequency was increased by more than five orders of magnitude compared to the control E. coli strain without CRISPR targeting. Nanopore DNA sequencing of the mutants’ rpoB region confirmed the promising targeting efficacy of this approach. This study demonstrates a streamlined method for targeted genetic diversification in vivo, facilitating efficient protein engineering in bacterial systems.