Abstract
ABSTRACT The induction of the synthesis of milk proteins is accompanied by a very large increase in the number of polyribosomes in the cells of the mammary gland. The two classes of polyribosomes (free and membrane-bound) possess very different degrees of aggregation. The ratio of bound to free polyribosomes is very high in the normally lactating gland, or after stimulation by prolactin. On the other hand, it is low after inhibition of milk secretion by progesterone. The synthesis of milk proteins is almost exclusively the consequence of activity of bound polyribosomes and the rate of turnover of the RNA content of mRNP is also more rapid for free than for bound polyribosomes. mRNAs were prepared from bound polyribosomes by sucrose-gradient ultracentrifugation and by chromatography on poly U Sepharose. The fractions thus isolated were capable of directing the synthesis of casein αs, β and K in heterologous cell-free systems. Some of the mRNAs for αs casein contained a significant poly A sequence. The control of the synthesis of mammary proteins by initiation factors and tRNAs is discussed. The purification of 2 isoacceptors of Met tRNA has enabled us to show that the mammary initiator tRNA does not possess the sequence GTψC which is present in all other tRNAs.
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