Extracellular expression of human epidermal growth factor encoded by an Escherichia coli K-12 plasmid stabilized by the ytl2-incR system of Salmonella typhimurium

A plasmid stabilization system, active in high copy-number plasmids, was cloned from the large resident plasmid, pSLT, of Salmonella typhimurium. The ytl2 gene, together with a 249-bp region (termed incR) downstream of the gene, imparted > 10⁴-fold stability to a pBR322-based plasmid. The ytl2-incR region was then used to stabilize a recombinant plasmid carrying the human epidermal growth factor gene (with the Escherichia coli K-12 ompA signal sequence), behind the lacUV5 promoter. In shake flask tests to optimize expression of human epidermal growth factor, loss of recombinant plasmid was < 1% when growth (both before and after induction with isopropyl-β-D-galactopyranoside) took place even in the absence of antibiotic selection, and the specific activity of secreted human epidermal growth factor was ca 20 μg per 10⁸ cells at harvest, compared to a figure of ca 3 μg per 10⁸ cells when a comparable plasmid, but devoid of the ytl2-incR region, was employed, as outgrowth of plasmid-free cells after induction severely compromised the specific activity of the secreted product.