Subcloning of prochymosin cDNA for Plac controlled expression.

Two overlapping segments of prochymosin cDNA clones (Liebscher et al., 1985) were used to construct plasmids that expressed an activable zymogen product and thus verified the integrity of the reverse transcripts. The pUC9 vector was used for the expression, under the control of the lac promoter. The expression product (a fused protein consisting of the N-terminus of beta-galactosidase, a polylinker-coded peptide and prochymosin from its 5th amino acid) displayed, upon activation by the usual procedure, the properties of calf chymosin. The active product was identified by milk-clotting tests, "caseinography" and protein electrophoresis of immunoprecipitates. The "boxing" of prochymosin cDNA in the constructed plasmids makes them a versatile source of this cDNA for other expression constructs.