Activation of maternal mRNA in the absence of poly(A) formation in fertilised sea urchin eggs

Abstract

THE protein synthesis which follows fertilisation of sea urchin eggs depends largely on genetic templates synthesised much earlier during oogenesis¹. In the first 2 h after fertilisation the amount of protein being made increases steadily as the preformed mRNA molecules gradually become available for translation^2,3. During this period of mRNA activation, the polyadenylic acid content of the embryo increases to more than twice the level present in unfertilised eggs^4–6. This fertilisation-stimulated polyadenylation occurs in the cytoplasm on RNA stored in the egg⁶ and is essentially complete by the time of the second cleavage. Soon after fertilisation the poly (A)-containing RNA moves from the post-ribosomal supernatant to the ribosomal-polysomal fraction of embryos^5,6, which suggests that polyadenylation is important in the activation of stored mRNA molecules. This is consistent with other reports that these tracts may play a role in the translation of viral RNAs^7,8 and eukaryotic mRNAs^9,10. We have therefore investigated whether the formation of poly (A) after fertilisation is necessary for the activation of maternal mRNA. We measured the increase in protein synthesis and active mRNA while poly (A) synthesis was repressed and found a substantial activation even when poly (A) formation could not occur.