Cell-free cyclic variation of protein synthesis associated with the cell cycle in sea urchin embryos.

Abstract

Cyclic variation in protein synthesis was studied in cell-free systems from sea urchin eggs. The periodicity of the cycle observed in the 12,000 × g supernatant of fertilized eggs was inversely proportional to the concentration of protein in the incubation medium over a certain concentration range and was not affected by temperature under the experimental conditions. Addition of the 12,000×g supernatant from fertilized eggs induced cyclic variation in the 12,000 ×g supernatant from unfertilized eggs. The active factors inducing the cyclic variation were present in the postmicrosomal fraction, not in the ribosomes. Two macromolecular factors and a class of low molecular weight factors in the postmicrosomal fraction were found to be necessary for induction of cyclic variation. One of the macromolecular factor was identified as maternal messenger RNA. The low molecular weight factor was heat stable and seemed to be composed of more than one component. It could be partially replaced by a mixture of ATP and GTP. The maternal messenger RNA and the low molecular weight factor were not present in an active state in the supernatant of unfertilized eggs, but appeared in the supernatant very soon after fertilization. A fraction precipitated at 12,000×g was necessary for their appearance. Another factor, which may control the phase and amplitude of the cyclic variation, was found in the macromolecular fraction of supernatant of both fertilized and unfertilized eggs. Experiments on induction of cyclic variation in the supernatant of unfertilized eggs by addition of these factors suggested that maternal messenger RNA and the low molecular weight factor act as developers of activity for protein synthesis, while another macromolecular factor acts as a pace-maker. Regular latent fluctuation of this factor is assumed to occur leading to cyclic variation of synthesis. Cyclic variation was initiated by elevation of the activity in the supernatant of eggs on fertilization or of unfertilized eggs on homogenization. The cycle initiated in intact cells by fertilization continued in the supernatant of these cells after homogenization in a modified way. The relationship between the cell-free cycle and the cycle in intact cells is discussed.