Proteases act on proteins or polypeptides. Historically, attention on thermophilic fungi was advanced by the finding of a strain of Mucor that produced milk-curdling activity that could substitute for chymosin (rennin), required in the manufacture of cheese. Previously, rennin was obtained from the stomach of suckling calves. The Mucor acid protease, induced in media containing wheat bran, had maximal activity at a pH of 3.7 and was stable at 55°C. The Mucor rennin gene was expressed in yeast that secreted the foreign (heterologous) protein at concentrations exceeding 150 mg/l. A recombinant mesophilic fungus, Aspergillus oryzae, produced Mucor acid protease in excess of 3 grams per liter of medium. Among the varied applications of proteases are the tenderizing of meat, manufacture of cheese, a component of detergents for the removal of proteinaceous dirt from fabric and for dehairing of leather (Johri et al., 1999).
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