THE SYNTHESIS OF 5S RNA AND ITS REGULATION DURING EARLY SEA URCHIN DEVELOPMENT

De novo synthesis of 5S RNA and of transfer RNA (tRNA) has been demonstrated previously to occur by mid-cleavage (128-cell stage) in sea urchin embryos (24). The present study focused on determining more precisely the time of onset of activity of the genes for 5S RNA and for tRNA during sea urchin embryogenesis by preloading the GTP precursor pools of unfertilized eggs. The results showed that newly-made 5S RNA and tRNA could be detected as early as the 32-cell stage. In order to determine whether newly-synthesized 5S RNA accumulates coordinately during development with newly-made 26S (34) and 18S ribosomal RNAs (rRNAs), the relative rates of accumulation of these three RNA molecules were measured and compared at each of several stages of sea urchin embryogenesis. In contrast to the coordinated accumulation of newly-synthesized 26S and 18S rRNAs, newly-made 5S RNA accumulated in excess at the mesenchyme blastula (9-fold excess), midgastrula (5-fold excess) and prism (3-fold excess) stages. The 5S RNA/26S RNA molar ratios only approached unity in advanced (48 hr) plutei. The non-coordinated accumulation of newly-made 5S RNA with that of 26S and 18S rRNAs suggests that the accumulation of these newly-synthesized RNAs is differentially regulated during early sea urchin development.