Murine mammary gland RNA directed synthesis of casein in a heterologous cell-free protein synthesis system

A cell-free protein synthesis system derived from Ehrlich ascites tumor cell ribosomes ( S 30) plus rabbit reticulocyte tRNA was developed and the activity of the system was dependent on rabbit reticulocyte ribosomal salt (0.5 M KCl) wash factors. The exogenous mRNAs from BALB/c mouse liver and the mammary gland were translated with a high efficiency in this heterologous cell-free system. Furthermore, the RNA from the lactating mammary gland faithfully directed the synthesis of casein. The presence of mouse casein in the reaction product was identified by radioimmunoprecipitation with mouse casein anti-serum, co-electrophoresis of the reaction product and mouse casein in urea polyacrylamide gel and by electrophoresis in sodium dodecyl sulfate (SDS) polyacrylamide gel. The major portion of the lactating mammary gland RNA directed synthesis of the milk protein in the cell-free system appeared to be analogous to α S casein.