Synthesis of calf prochymosin (prorennin) in Escherichia coli.

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Abstract
A gene for calf prochymosin (prorennin) was reconstructed from chemically synthesized oligodeoxyribonucleotides and cloned DNA copies of preprochymosin mRNA. This gene was inserted into a bacterial expression plasmid containing the E. coli tryptophan promoter and a bacterial ribosome binding site. Induction of transcription from the tryptophan promoter results in prochymosin synthesis at a level of up to 5% of total protein. The enzyme was purified from bacteria by extraction with urea and chromatography on DEAE-cellulose and converted to enzymatically active chymosin by acidification and neutralization. Bacterially produced chymosin is as effective in clotting milk as the natural enzyme isolated from calf stomach.