Regulation of DNA-like RNA and the apparent activation of ribosomal RNA synthesis in sea urchin embryos: quantitative measurements of newly synthesized RNA.

Summary RNA synthesis was analyzed in sea urchin embryos in order to determine the contribution which changes in synthesis of heterogeneous DNA-like RNA and of ribosomal RNA make to the changes in TNA metabolism which occur during embryogenesis. The base composition and sedimentation behavior of RNA molecules incorporating radioactivity during a short incubation of embryos with radioactive RNA precursors indicate that heterogeneous, DNA-like RNA was the predominant class of RNA synthesized in embryos at all stages of development. During longer incubations with radioactive precursors, radioactivity preferentially accumulated in ribosomal RNA. This preferential accumulation of newly synthesized rRNA was much more extensive at later than at early stages of development. However, accumulation of newly synthesized 28 S ribosomal RNA could be demonstrated as early as the first half of blastula stage when rRNA was extensively purified from DNA-like RNA by sucrose gradient sedimentation and MAK chromatography. In order to interpret these developmental changes in the relative accumulation of ribosomal and DNA-like RNA, the amounts of newly synthesized RNA accumulated during cleavage, blastula, and pluteus stages were quantitatively estimated by measurement of the incorporation of radioactive precursors into RNA and simultaneous determination of the specific activity of the nucleotide triphosphate precursor pools. The results indicated that the accumulation of newly synthesized DNA-like RNA per nucleus was quantitatively much greater at cleavage and blastula stages than at pluteus stage, whereas the accumulation of newly synthesized rRNA per nucleus at blastula and pluteus stages were similar. Therefore, decreases in the quantitative accumulation of DNA-like RNA account entirely for the change in the relative accumulation of ribosomal RNA and DNA-like RNA which occurs during development of sea urchin embryos.