Separate Ribosomal Pools in Sea Urchin Embryos: Ammonia Activates a Movement between Pools

Monoribosomes from unfertilized eggs of Strongylocentrotus purpuratus were shown to translate mRNA less efficiently than ribosomes derived from polyribosomes of embryos, as measured by globin synthesis in a ribosome-dependent rabbit reticulocyte lysate [Danilchik, M. V., & Hille, M. B. (1981) Dev. Biol. 84, 291–298], Data presented in this paper show that monoribosomes from 16-cell and blastula embryos resemble monoribosomes from unfertilized eggs in translational capacity and are less active than the ribosomes associated with polyribosomes. Thus, we find two distinct populations of ribosomes in embryos. We define the less active monoribosome population as “naive” ribosomes and the more active, functioning polysome-derived ribosomes as “experienced” ribosomes. Naive and experienced ribosomes have the same elongation rates. The relationship between ionic triggers and the conversion of monoribosomes to experienced ribosomes was studied with the Ca²⁺ ionqphore A23187, which releases intracellular Ca²⁺ stores, and NH₄C1, which alkalinizes the cytoplasm. We found that ribosomes in the monoribosome populations from A23187-activated eggs or from NH₄Cl-activated eggs resembled naive monoribosomes from unfertilized eggs in their translational activity. In contrast, ribosomes derived from the polysomes of NH₄Cl-treated eggs were as active as the experienced polysome-derived ribosomes from normal embryos. Eggs activated with A23187 did not produce polyribosomes. The presence of significant amounts of experienced ribosomes in NH₄Cl-treated eggs implicates alkalinization of the cytoplasm as a stimulus for ribosome activation, which occurs slowly during initial development.