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Background The recent CRISPR-Cas coupled with λ recombinase mediated genome recombineering has become a common laboratory practice to modify bacterial genomes. It requires supplying a template DNA with homology arms for precise genome editing. However, generation of homology arms is a time-consuming, costly and inefficient process that is often overlooked. Results In this study, we first optimized a CRISPR-Cas genome engineering protocol in the  Escherichia coli (E. coli)  BL21 strain and successfully deleted 10 kb of DNA from the genome in one round of editing. To further simplify the protocol, asymmetric homology arms were produced by PCR in a single step with two primers and then purified using a desalting column. Unlike conventional homology arms that are prepared through overlapping PCR, cloning into a plasmid or annealing synthetic DNA fragments, our method significantly both shortened the time taken and reduced the cost of homology arm preparation. To test the robustness of the optimized workflow, we successfully deleted 26 / 27 genes across the BL21 genome. Noteworthy, gRNA design is important for the CRISPR-Cas system and a general heuristic gRNA design has been proposed in this study. To apply our established protocol, we targeted 16 genes and iteratively deleted 7 genes from BL21 genome. The resulting strain increased lycopene yield by ~ threefold. Conclusions Our work has optimized the homology arms design for gene deletion in BL21. The protocol efficiently edited BL21 to improve lycopene production. The same workflow is applicable to any  E. coli  strain in which genome engineering would be useful to further increase metabolite production.

Metabolic engineering of Escherichia coli BL21 strain using simplified CRISPR-Cas9 and asymmetric homology arms recombineering