Isolation and characterization of casein mRNAs from lactating ewe mammary glands

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Abstract
RNA from bound polysomoe of lactating ewe's mammary glard directs the synthesis of the three major milk proteins (αs, β and k-caseins) in a cell-free system derived from rabbit reticulocytes. The “in vitro” product was identified by immunopre-cipitation with specific antibodies and by electrophoresis in SDS polyacrylamide gel. Each of these messengers was purified from 20 to 25 fold from total membrane-bound polysomal RNA using poly Ü-Sepharose chromatography. This purified fraction assayed in a reticulocyte cell-free system is able to direct also the synthesis of 2 minor secretory proteins (β-lactoglobulin and α-lactalbumin). The messenger RNAs purified by hybridization to poly U-Sepharose have a sedimentation coefficient of about 12 S and an apparent molecular weight of approximatively 3.5 × 105 daltons was observed by polyacrylamide gel electrophoresis under denaturing conditions. This value which correspond to about 900 nucleotides is significantly greater than the number expected for coding milk proteins.