Regulation of maternal mRNA translation in developing embryos of the surf clam Spisula solidissima.

Abstract

MESSENGER RNA not bound to ribosomes (non-polysomal mRNA) has been found in the cytoplasm of sea urchin1–3 and Spisula solidissima eggs4. The mRNA synthesised and stored during oogenesis (maternal mRNA) is utilised when protein synthesis is activated by fertilisation. Translation of maternal mRNA allows sea urchin embryos to proceed through several cellular divisions even when synthesis of embryonic mRNA is blocked by actinomycin D5–6. But, embryonic mRNA is synthesised soon after fertilisation and already translated at the early cleavage stage7. Qualitative and quantitative changes in the pattern of histone synthesis during sea urchin embryogenesis have suggested that the synthesis of these proteins is temporally regulated in the embryo and that different mRNA species are translated at successive stages of embryo development8. We report here that part of the mRNA is found unassociated with ribosomes in eight-cell embryos of S. solidissima. Some of the translation products of this non-polysomal mRNA are different from those of polysomal mRNA, but identical to translation products of maternal mRNA isolated from eggs. This suggests that part of the non-polysomal mRNA of embryos consists of maternal mRNA species which are inefficiently translated after fertilisation.