Enhanced production of human epidermal growth factor under control of the <Emphasis Type="Italic">pho</Emphasis>A promoter by acetate-tolerant <Emphasis Type="Italic">Escherichia coli</Emphasis> DB15 in a chemically defined medium

The phoA expression system is an efficient one and is successfully used in foreign gene expression. In a previous study, it was found that pH during the expression phase had a significant effect on extracellular hEGF production under control of the phoA promoter by Escherichia coli DA19, an acetate-tolerant strain of E. coli DH5α, in a chemically defined medium, but the level of hEGF production was only 75.5 mg/L. E. coli DB15 is another acetate tolerant mutant of DH5α. In the present study, production of hEGF under control of the phoA promoter by DB15 was further investigated. When transition from the growth phase, where phosphate was abundant, to the expression phase where phosphate was limited, was performed based on cell density, the extracellular hEGF reached 165 mg/L, twice that when transition was based on dissolved oxygen. Furthermore, adding 0.22 g/L of CaCl₂ during the growth phase, further increased hEGF production to 228 mg/L, which is 3-fold the level produced by DA19 (pAET-8) cultured in the same medium.