Injected mRNA does not increase protein synthesis in unfertilized, fertilized, or ammonia-activated sea urchin eggs.

We have investigated whether the rate of protein synthesis in unfertilized and fertilization-activated sea urchin eggs is limited by the availability of mRNA by injecting eggs, zygotes, and ammonia-activated eggs with globin mRNA. Message-injected and buffer-injected cells were labeled with radioactive amino acids and the proteins separated on a polyacrylamide gel. The relative amounts of newly synthesized globin and endogenous proteins were obtained by scanning the gel fluorograph. Globin mRNA is translated poorly in Strongylocentrotus droebachiensis eggs and does not significantly increase or decrease endogenous protein synthesis. In zygotes and ammonia-activated eggs, however, globin mRNA is translated well and appears to compete with endogenous mRNAs for the limiting component of the translational machinery as it is released. Our results are consistent with the hypothesis that either ribosomes or recruitment factors are gradually activated after fertilization or ammonia treatment, that such components are the rate-limiting factor, and that they impart the typical sigmoidal increase in protein synthesis rate observed in fertilized eggs before the first cleavage.