Messenger ribonucleoprotein particles in developing sea urchin embryos

Messenger RNA entering polysomes during early development of the sea urchin embryo consists of both oogenetic and newly transcribed sequences. Newly transcribed mRNA enters polysomes rapidly while oogenetic mRNA enters polysomes from a pool of stable, nontranslatable messenger ribonucleoprotein particles (mRNPs) derived from the unfertilized egg. Protein content may relate to differences in the regulation of newly transcribed and oogenetic mRNAs. Oogenetic poly(A)⁺ mRNA was found to be present in both polysomal and subpolysomal fractions of cleavage stage and early blastula stage embryos. This mRNA was found to be present in subpolysomal mRNPs with a density of 1.45 g/cm³ in Cs₂SO₄. Poly(A)⁺ mRNPs released from polysomes of embryos cultured in the presence of actinomycin D sedimented in a broad peak centered at 55 S and contained RNA of 21 S. The density of these particles was sensitive to the method of release; puromycin-released mRNPs had a density of 1.45 g/cm³, while EDTA caused a shift in density to 1.55 g/cm³, indicating a partial loss of protein. The results with newly synthesized mRNAs contrast sharply. Newly transcribed mRNA in subpolysomal mRNPs had a density of 1.55–1.66 g/cm³, a density approaching that of deproteinized RNA. Messenger RNA released from polysomes either by EDTA or puromycin was examined to determine the possible existence of polysomal mRNPs. When ³H]uridine-labeled mRNA was released from late cleavage stage embryo polysomes by either technique, and centrifuged on sucrose gradients, two broad peaks were found. One peak centered at 30 S contained 21 S mRNA while the other at 15 S contained 9 S histone mRNA. When these fractions were fixed with formaldehyde, they banded on Cs₂SO₄ gradients at a density of 1.60–1.66 g/cm³, very similar to that of pure RNA. We conclude that the newly transcribed mRNA may be present in stable mRNPs containing up to 10% protein in either subpolysomal or polysomal fractions. These mRNPs are clearly distinguishable from the protein-rich mRNPs containing oogenetic mRNAs.