Evidence is presented that in a cell-free system from sea urchin cells during early development cyclic variation in the rate of protein synthesis is controlled by a concomitant cyclic fluctuation in the SH-content of a particular protein species.
Cyclic fluctuation of the SH-content of the TCA-soluble protein fraction was observed in homogenates of fertilized and unfertilized cells without changes in the amount of protein. The SH-cycle seemed to start on fertilization in homogenates of fertilized eggs and at the time of homogenization in homogenates of unfertilized eggs. A similar fluctuation was also observed in the SH-content of the KCl-soluble protein fraction in cell-free systems from unfertilized eggs. Stimulation of the rate of protein synthesis by the 0.6 m KCl-soluble protein fraction was proportional to the content of protein-bound SH-groups in this fraction. This stimulatory effect was found to be due to the specific action of the SH-groups of a particular protein. A series of KCl-soluble protein fractions prepared at various stages from the supernatant fractions of unfertilized eggs induced various levels of protein synthesis in the supernatant fraction of other unfertilized cells when supplemented with maternal messenger RNA and a low-molecular-weight factor, this induction being proportional to the SH-content of these fractions. The protein responsible for the stimulation was purified to a practically homogenous state by gel filtration on Sephadex. Little species specificity was found in the stimulation of protein synthesis by purified proteins. Using a sea urchin transhydrogenase preparation, reversible electron transfer was observed between this protein and Ca2+-insoluble proteins.
This seems to be the only protein species acting as a pacemaker of the cyclic variation in the rate of protein synthesis in the cell-free system by fluctuation in its SH-content, so it is suggested that cyclic protein synthesis is guided by cyclic fluctuation in the SH-content of the KCl-soluble protein.