Recombinant bovine chymosin expression in microalgae Chlamydomonas reinhardtii chloroplast: A step towards algal biomanufacturing of dairy enzymes.

Chymosin is the major proteolytic enzyme for cheese manufacture, where it plays an important role in the co-precipitation of milk casein. Traditional extraction of chymosin from the abomasum of young ruminants is associated with high limitations, including low yield, high production cost, and ethical issues of animal slaughter. In this study, we report on a recombinant strategy towards the production of bioactive Bos taurus chymosin in the chloroplasts of the microalga Chlamydomonas reinhardtii. The cym gene encoding preprocymosin was inserted into the chloroplast genome by the glass bead-mediated DNA transformation procedure. Successful integration and expression of the transgene were confirmed by spot test analysis, polymerase chain reaction (PCR), western blot, and enzyme-linked immunosorbent assay (ELISA). The functional activities of the recombinant enzyme were checked by the standard milk clotting assay. The engineered microalgal strains produced chymosin with an average concentration of 90 mg/kg fresh weight, i.e., 1.6% of the total soluble protein. These results show that chloroplast-engineered C. reinhardtii is a promising, sustainable, and animal-free platform for the efficient production of the industrially relevant chymosin.