Distribution of messenger ribonucleic acid in polysomes and nonpolysomal particles of sea urchin embryos: translational control of actin synthesis

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Abstract
Cell-free translation and 2 dimensional gel electrophoresis were used to examine the complexities of the polysomal and cytoplasmic nonpolysomal [ribonucleoprotein (free RNP)] mRNA populations of sea urchin eggs and embryos. All mRNA species detected by this method are represented in both the polysomes and free RNPs; essentially all messages present in polysomes are also in the free RNP fraction. The cytoplasmic distribution is clearly nonrandom since some templates are relatively concentrated in the free BNPs and others are predominantly in the polysomes. The polypeptides synthesized under the direction of unfertilized egg mRNA are qualitatively indistinguishable form those made by using embryonic mRNA, indicating that the complexity of the abundant class mRNA ramains unchanged from egg through early development. Large changes in the abundancies of specific mRNAs occur, and changes are detected in the polysomal/free RNP distribution of some mRNAs through development. The differences in the relative abundancies of specific mRNAs between polysomes and free RNPs and the developmental changes that occur indicate significant cytoplasmic selection of mRNA for translation. Three different forms of actin (termed .alpha., .beta. and .gamma.) were identified among the translation products. Messages for all 3 are present in the unfertilized egg and early cleavage embryo, yet the .gamma. form is preferentially located in the polysomes and .alpha. and .beta. in the free RNPs. The relative concentrations of the 3 change greatly during development, as do their relative distributions into polysomes and free RNPs. Examinations of in vivo labeled proteins largely support the in vitro findings. The synthesis of actin mRNAs increases greatly during development, and the expression of the actin mRNAs is partly controlled at the translation level during early development.
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