Efficiency of protein synthesis after fertilisation of sea urchin eggs

Abstract

ELUCIDATION of the mechanisms involved in the cytoplasmic control of protein synthesis is essential for an understanding of the initiation of embryogenesis. On fertilisation, echinoid eggs increase their rate of protein synthesis many-fold, independent of any increase in ribosomal or mRNA synthesis^1–4. Within 2 h of fertilisation the number of ribosomes in polyribosomes increases about 25-fold without any change in the size distribution of poly ribosomes^5,6. In addition, the polyribosome number doubles again by gastrulation. However, the total rate of protein synthesis increases 113-fold from egg to gastrulation⁷. The discrepancy between the number of ribosomes recruited into polyribosomes, an increase of ≈50 fold, and the total increase in the rate of protein synthesis, a 113-fold increase, could be explained if there were also an increase in the rate of movement of ribosomes along each message (elongation) and/or an increase in the rate of release of completed peptides (termination). We report here that the translational rate is increased about 2.5-fold after fertilisation in the sea urchin, Strongylocentrotus purpuratus. The average transit time (elongation plus termination time for an average-sized peptide) was measured for these eggs and several stages of embryos.