Purification of the mRNAs for Ewe alphaS-casein and beta-casein by immunoprecipitation of polysomes.

Specific polysomes synthesizing alphas-casein and beta-casein were immunoprecipitated from total polysomes of the lactating ewe mammary gland. The polysome - anti-casein complex was immunoprecipitated by anti-immunoglobulins. 22%, 32% and 10% of polysomes were immunoprecipitated with saturating amounts of anti-alphas-casein, anti-beta-casein and anti-chi-casein respectively. Poly(U)-Sepharose chromatography of the immunoprecipitated RNAs permitted the isolation of the corresponding poly(A)-containing RNA which migrated as one major band in polyacrylamide gel electrophoresis. As judged by the contamination with the messenger activity for one of the other caseins, the purity of the mRNA for alphas-casein as well as for beta-casein was estimated to be between 75% and 80%.