Isolation, partial characterization and translation of mRNAs for chymosin and pepsin, the two main aspartyl proteinases of bovine stomach.

Poly(A) RNA from the mucosa of the fundal region of the fourth stomach of suckling calf and adult cattle was isolated by the phenol or guanidine thiocyanate procedure. The mRNAs for chymosin and pepsin were present in the 15S fraction of poly(A) RNA. They were active both in cell-free translation systems and in oocytes of Xenopus laevis and directed the synthesis of either chymosin or pepsin precursor, depending upon the age of the donor animal. In the reticulocyte and wheat germ system only preprochymosin or prepepsinogen were synthesized. In the oocyte system only the synthesis and secretion of prochymosin or pepsinogen could be detected. Both proenzymes, prochymosin and pepsinogen, present in oocytes or secreted into the medium, were converted to active enzymes, chymosin and pepsin, respectively, at pH 3.0, as shown by their proteolytic and milk-clotting activity.