Mechanism of Increase in the Basal Rate of Protein Synthesis in the Early Cleavage Stage of the Sea Urchin***

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Abstract
The mechanism of increase in the basal rate of protein synthesis in the early. cleavage stage of the sea urchin, Pseudocentrotus depressus, was studied. In ac-cordance with previous findings, no increase in the basal rate during incubation was found in cell-free systems and on disruption of cells their rate of synthesis is appeared to be maintained. The increase in the basal rate of synthesis was parallel with the activity of ribosomes in supporting amino acid incorporation, but no cor-relation was found between the basal rate and the activity of non-particulate fraction in supporting protein synthesis. On fertilization there were sudden' parallel increases in the amount of polyribosomes in cells, the amount of informational RNA in the ribosomes, the binding capacity of mtRNA to ribosomes and the polyribosomal activity, expressed as the amount of nascent peptide on polyribosomes. Later, these all increased more gradually, and they were not correlated with the incorporation activity of intact embryos. On sedimentation the polyribosomes were distributed between 200 and 300S. Their peak was gradually displaced toward heavy regions until the end of the first cell cycle and then remained unchanged. The mechanism of increase in the basal rate, the interrelation between the polyribosomes formed in the cyclic and basal phases, and the mechanism controlling three subphases of the basal rate (the lag phase, the first phase and the succeeding phase) are discussed on the basis of these results.

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