The translation of polyadenylated and of non-poly-adenylated RNA obtained from lactating rat mammary gland was almost totally inhibited by 0,5 mM 7-methylguanosine-5′-phosphate in the wheat-germ cell-free system, This inhibition was maintained during the preparation of the 9S whey-protein mRNA and of the 12S and ISS casein mRNAs, Chemical decapping of these mRNAs caused a similar reduction of their activity. Although a large fraction of milk-protein mRNAs have been reported to lack 3′-polyadenylation, these results show that the mRNAs in the mammary gland do contain a 5′-terminal 7-methylguanosine cap.

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