Cytoplasmic distributions of translatable messenger RNA species and the regulation of patterns of protein synthesis during sea urchin embryogenesis

The cytoplasmic messenger RNA population of eggs and embryos of Strongylocentrotus purpuratus was characterized by cell-free translation and two-dimensional electrophoresis. The timing and extent of changes in patterns of translation products correspond closely to changes during development in patterns of protein synthesis in vivo. The increased synthesis of most up-regulated proteins, including those identified as tissue specific, is due to concommitant increases in translatable mRNA in the cytoplasm. In a few exceptional cases, notably the mRNAs encoding actin and the heat-shock protein hsp90, stored maternal mRNAs are not rapidly recruited into polysomes upon fertilization; instead, these mRNAs remain enriched in the pool of free RNPs until they appear on polysomes and become actively translated in vivo during early cleavage. It is thus likely that selective translational activation of maternal mRNA is required for these rare examples of increased protein synthesis during early development. The free RNP pool of early embryos apparently serves as a reservoir for mRNAs later recruited into polysomes. Analysis by translation and hybridization of cloned cDNA probes to RNA blots indicates that a few mRNA species are always relatively enriched in the free RNP pool of posthatching embryos and do not serve as a store of mRNA for later translation.