Separate Elements in the 3′ Untranslated Region of the Mouse Protamine 1 mRNA Regulate Translational Repression and Activation during Murine Spermatogenesis

The mouse protamine mRNAs,Prm-1andPrm-2,are translationally repressed for several days during male germ cell differentiation. The translational delay of mousePrm-1mRNA has previously been shown to be dependent uponcis-acting elements that reside in the last 62 nucleotides of thePrm-13′ untranslated region (3′ UTR). We have previously identified a 48/50-kDa protein that binds the 3′ UTRs of bothPrm-1andPrm-2mRNAs in a sequence-specific manner, is present in cytoplasmic fractions of postmeiotic round spermatids where the protamine mRNAs are translationally silent, and is markedly reduced in elongated spermatids where the protamine mRNAs become activated for translation. Surprisingly, the binding site for this activity maps to a region of thePrm-13′ UTR not contained within the functional 62 nucleotides described above. In this report we show that the binding site for the 48/50-kDa protein can also delay translation of a reporter RNAin vivo,suggesting that the 48/50-kDa protein can repress the translation ofPrm-1mRNA during murine spermatogenesis. This observation proves that two separate regions of thePrm-13′ UTR are sufficient to repressPrm-1translation. In addition, immunocytochemistry and polysome analysis have revealed that this transgenic reporter mRNA fails to undergo proper translational activation. These results suggest that an additional region of thePrm-13′ UTR is required for proper translational activation and thatPrm-1translational repression elements can be separated from those involved in translational activation.