Abstract
Total polysomal RNA or poly A- containing RNA extracted from free polysomes of lactating ewe mammary gland was unable to direct a significant synthesis of alpha-s- and beta-casein. Polyacrylamide gel electrophoresis of poly A- containing RNA revealed that the two major peaks of mRNA isolated from membrane-bound polysomes were not observed in mRNA isolated from free polysomes. Purified 125-I anti-alpha-s-casein could bind significantly to membrane-bound polysomes only. These data suggest that casein synthesis occurs almost exclusively on membrane-bound polysomes and that this specificity may be attributed to the presence of casein mRNAs on these polysomes and their absence from free polysomes.

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