Engineering with a symbiotic plasmid for the production of phenylpyruvic acid.

Plasmid-based microbial systems have become a major avenue for the production of pharmaceutical and chemical products; however, antibiotics are often required to maintain the stability of the plasmid. To eliminate the need for antibiotics, we developed a symbiotic system between plasmids and hosts by knocking out the essential gene of on the chromosome and placing it on the same plasmid as l-amino acid dehydrogenase (); the resulting strain was named A06Δ. To increase the copy number of , different strengths of promoters were used for the expression of , resulting in the creation of a mutant A17Δ. The yield of phenylpyruvic acid (PPA) from A17Δ (4.1 ± 0.3 g L) was 1.9-fold that of A06Δ (2.1 ± 0.2 g L). Next, the stability of plasmids was tested, and results showed that the plasmids could be maintained stably for 10 transfer numbers under antibiotic-free conditions. Finally, A17Δ was used to produce PPA; the yield of PPA was 18.7 g L within 14 h.