Regulation of casein synthesis in the rabbit mammary gland. Titration of mRNA activity for casein under prolactin and progesterone treatments.

Total polysomal RNA or poly(A)-containing RNA isolated from membrane-bound polysomes of normal lactating rabbits directed the synthesis of casein in a reticulocyte lysate. Casein was identified by immunoprecipitation followed by SDS-polyacrylamide gel electrophoresis of the immunoprecipitate. The poly(A)-containing RNA was heterogeneous with one major peak corresponding to a 12-S sedimentation coefficient as determined by polyacrylamide gel electrophoresis. Using the same procedure, mRNAs isolated from the non-secreting tissue of pseudopregnant rabbits were found not to contain the 12-S peak and were unable to direct the synthesis of casein in vitro. Prolactin injected into pseudopregnant rabbits induced the synthesis of proteins immunoprecipitable by anti-casein anti-serum and induced the simultaneous appearance of the 12-S mRNA. Progesterone injected with prolactin prevented the induction of casein synthesis and the appearance of mRNA for casein. A close relationship was established between the ability of the tissue to synthesize immunoprecipitable casein and the corresponding mRNA content of polysomes.