Expression of Bovine α_s1-casein cDNA in Escherichia coli

Abstract

To produce bovine milk casein in Escherichia coli cells, we constructed expression plasmids for α_s1-casein cDNA, including the DNA region encoding the signal peptide. First, the production of casein was examined with the maxicell system, by which plasmid-coded proteins could be specifically detected with high sensitivity. The casein protein was produced in UV-irradiated CSR603 cells carrying the plasmids that contained casein cDNA preceded by the tac promoter. Then, plasmid pα_s1EK for higher production of casein was constructed; casein cDNA in this plasmid was preceded by two tac promoters connected in series and followed by the terminator (rrnBT₁T₂) of the rRNA gene. Casein production in the E. coli cells of strains C600 and JM103 carrying Pα_s1EK was detected immunologically with the blotting technique. Casein was constitutive ly produced in the C600 cells, and the JM103 cells also produced it when the inducer of the lac operator was present in the culture medium. Fractionation of the cellular compartments and observations by immunocolloidal gold electron microscopy indicated that the recombinant casein was present in an insoluble form diffusely in the cytoplasm and periplasm. The major product isolated from the induced JM103 cells had the complete signal peptide, which might have been responsible for the insolubility. The bacterial hosts producing casein elongated and, furthermore, they formed linear chains by linking one after another.