Constructing an Antibiotic-Free Protein Expression System for Ovalbumin Biosynthesis in Probiotic Escherichia coli Nissle 1917

Ovalbumin (OVA) is the principal protein constituent of eggs. As an alternative to eggs, cell-cultured OVA can reduce the environmental impact of global warming and land use. Escherichia coli Nissle 1917 (EcN), a probiotic with specific endogenous cryptic plasmids that stably exist in cells without the addition of antibiotics, was chosen as the host for the efficient heterologous expression of the OVA. OVA yield reached 20 mg·L^-1 in shake flasks using the OVA expression cassette containing a tac promoter (P_tac) upstream of the OVA-coding sequences on the endogenous plasmid pMUT2. Subsequently, we improved the level of the expression of the OVA by employing a dual promoter (P_P5 combined with P_tac via a sigma factor binding site 24) and ribosome binding site (RBS) substitution. These enhancements increased the level of production of OVA in shake flasks to 30 and 42 mg·L^-1, respectively. OVA by EcNP-P28 harboring plasmid L28 equipped with both dual promoter and the strong RBS8 reached 3.70 g·L^-1 in a 3 L bioreactor. Recombinant OVA and natural OVA showed similar biochemical characteristics, including secondary structure, isoelectric point, amino acid composition, and thermal stability. This is currently the highest OVA production reported among prokaryotes. We successfully constructed an antibiotic-free heterologous protein expression system for EcN.