Casein synthesis by mouse polysomes and their messenger ribonucleic acid extracts.

Mammary epithelial cells from lactating mice were used to compare the in vitro casein synthesis by membrane-bound and free polysomes with that of messenger ribonucleic acid extracted from free- and bound polysomes (polysomal system and a rabbit reticulocyte lysate system). Casein synthesis was detected by incorporation of hydrogen-3 labeled amino acids in the reaction mixture into synthesized casein molecules. Membrane-bound polysomes synthesized considerably more casein than free polysomes; this difference was abolished by deoxycholate treatment of free polysomes. The synthesizing activity of messenger ribonucleic acid was similar whether extracted from free- or membrane-bound polysomes. Casein synthesis may be controlled by factors on the polysomal membrane.