Bacterial Expression System with Deep Repression and Activation via CRISPR-Cas9.

Incomplete repression of recombinant genes encoding toxic polypeptides can suppress cell growth even in the absence of a transcription inducer. To address this issue, we developed a CRISPR-Cas9-based genome editing approach that directly modifies the plasmid encoding the toxic peptide during Escherichia coli cultivation. The constructed plasmids contained a transcription terminator between the promoter and coding region, preventing full gene expression through abortive transcription. Upon CRISPR-Cas9 activation, this region is excised, thus restoring the functional gene. To implement this approach, we modified widely used pET-series expression plasmids by adding extra terminators in the 5'-untranslated region of the recombinant gene. Four antimicrobial peptides with strong bactericidal properties served as toxic gene products, while green fluorescent protein was used to assess the efficiency of expression repression. As a result, we developed an expression system with strong repression, which is activated by CRISPR-Cas9-mediated excision of a DNA fragment from the plasmids.