Nonrandom distribution of histone mRNAs into polysomes and nonpolysomal ribonucleoprotein particles in sea urchin embryos.

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Abstract
In the early sea urchin [Strongylocentrotus purpuratus] embryo, newly synthesized cytoplasmic histone mRNA is found both on polysomes and free of ribosomes as nonpolysomal messenger ribonucleoprotein particles (free RNP). The distribution of newly synthesized histone mRNA between translating and nontranslating compartments is nonrandom and dependent on the developmental stage. Gel electrophoresis and autoradiography of polysomal and free RNP RNA from embryos at various stages show that the fraction of total newly synthesized histone mRNA that is in polysomes is greater than the fraction of total newly synthesized poly(A)+ mRNA that is in polysomes, at all stages examined. Among the 5 histone mRNA, H1 mRNA and H4 mRNA are relatively more enriched in the free RNP than are the mRNA for H2A, H2B and H3. Evidently, histone mRNA, as a class, is more efficiently utilized as a template than the average mRNA and, of the cytoplasmic histone mRNA, the mRNA for histones H2A, H2B, and possibly H3 are selected more frequently for translation than those for H1 and H4. Cell-free translations of polysomal and free RNP RNA yield different ratios of in vitro histone products, consistent with the RNA distribution data. To test the possibility that the in vivo distribution of the histone mRNA is the consequence of different intrinsic initiation capabilities of the individual mRNA, ribosome-binding assays were carried out and unequal binding abilities of the histone mRNA in the [rabbit] reticulocyte lysate were shown. A translational level component in the regulation of histone synthesis in the sea urchin embryo is indicated.