PURPOSE: To enable continuous obtaining of a useful physiologically active protein by carrying out transformation with a plasmid having a specific DNA fragment.

CONSTITUTION: Synthesis is performed with a DNA synthesizer to provide a DNA fragment (A) having function to code an amino acid sequence expressed by formula I and impart the character for promoting extracellular secretion of a useful physiologically active protein to Escherichia coli. The resultant ingredient (A) is then introduced into a plasmid to afford a plasmid vector (B) such as pUC119. The obtained ingredient (B) is subsequently introduced into Escherichia coli to afford Escherichia.coli HB101 (pAK191) strain (C) which is a transformant. The resultant strain (C) is then cultured in a culture medium containing bactotrypton, etc., at about 37°C to produce the useful physiologically active protein such as β-galactosidase.