Antibiotic selection for the maintenance of expression plasmids is discouraged in the production of recombinant proteins for pharmaceutical or other human uses due to the risks of antibiotic residue contamination of the final products and the release of DNA encoding antibiotic resistance into the environment. We describe the construction of expression plasmids that are instead maintained by complementation of the lgt gene encoding a (pro)lipoprotein glyceryl transferase essential for the biosynthesis of bacterial lipoprotein. Mutations in lgt are lethal in Escherichia coli and other Gramnegative organisms. The lgt gene was deleted from E. coli and complemented by the Vibrio cholerae-derived gene provided in trans on a temperature-sensitive plasmid, allowing cells to grow at 30°C but not at 37°C. A temperature-insensitive expression vector carrying the V. cholerae-derived lgt gene was constructed, whereby transformants were selected by growth at 39°C. The vector was successfully used to express two recombinant proteins, one soluble and one forming insoluble inclusion bodies. Reciprocal construction was done by deleting the lgt gene from V. cholerae and complementing the lesion with the corresponding gene from E. coli. The resulting strain was used to produce the secreted recombinant cholera toxin B subunit (CTB) protein, a component of licensed as well as newly developed oral cholera vaccines. Overall, the lgt system described here confers extreme stability on expression plasmids, and this strategy can be easily transferred to other Gram-negative species using the E. coli-derived lgt gene for complementation.
Terrinoni, M., Nordqvist, S. L., Källgård, S., Holmgren, J., & Lebens, M. (2018). A novel nonantibiotic, lgt-based selection system for stable maintenance of expression vectors in Escherichia coli and Vibrio cholerae. Applied and Environmental Microbiology, 84(4). https://doi.org/10.1128/AEM.02143-17