Engineering and characterization of a symbiotic selection-marker-free vector-host system for therapeutic plasmid production.

The present study aimed to develop a symbiotic selection-marker-free plasmid and host system that would allow successful plasmid maintenance and amplification for use in gene therapy. Initially, the chromosomal aspartate‑semialdehyde dehydrogenase (asd) gene was disrupted in DH10B Escherichia coli using Red recombinase‑mediated homologous recombination. This method required the use of linear DNA fragments carrying kan‑kil genes, and/or homologous extensions to the targeted locus. The resultant auxotrophic cell wall‑deficient strain (DH10BΔasd) was evaluated as a symbiotic host for amplification of the marker‑free plasmid, allowing it to supply ASD activity. In order to construct the plasmid, an asd expression cassette was inserted, under the control of the nirB promoter, into a eukaryotic expression vector, and its kanamycin resistance gene was subsequently removed. The symbiotic plasmid and host system was assessed for numerous plasmid production and stability parameters, including structure, yield, plasmid‑retention rate, and bacterial storability, under various conditions. The presence of the plasmid was subsequently confirmed by growth test, restriction enzyme mapping, and sequencing. The plasmid yield and copy number produced in the symbiotic cells, in the absence of antibiotic selection, were shown to be similar to those produced under kanamycin selection, in the cells containing the precursor plasmid and kanamycin resistance gene. Furthermore, the results of the present study demonstrated that when inoculated with <1% inoculant volume, >98% of the cells in the culture retained the plasmid regardless of the number of passages. The strain was stable when stored at ‑70˚C, with negligible viability loss over 12 months. The constructed plasmid is stable and has potential in future gene therapy, while much work is still required.