Protein synthesis in oocytes of Xenopus laevis is not regulated by the supply of messenger RNA

The control of protein synthesis in oocytes of Xenopus laevis has been investigated by injecting oocytes with mRNA and polysomes followed by labeling with ¹⁴C-amino acid mixtures. Contrary to previous reports in which injected oocytes were labeled with ³H-histidine, injected globin mRNA is found to decrease amino acid incorporation into endogenous proteins competitively at all concentrations tested. No increase in overall amino acid incorporation is detected when more mRNA is supplied. Similar results are obtained after labeling injected oocytes with leucine, methionine, proline or valine individually. An explanation is presented for the conflicting results obtained when histidine is used as a label.When reticulocyte polysomes are injected, rather than purified globin mRNA, incorporation of amino acids into endogenous proteins remains roughly constant and overall incorporation increases. Similarly, when encephalomyocarditis viral RNA is injected together with either globin mRNA or reticulocyte polysomes, the globin mRNA causes decreased amino acid incorporation into encephalomyocarditis proteins, but the polysomes do not do so. The results demonstrate that different types of mRNA compete for a strictly limited translational capacity which is saturated in the normal oocyte. The limiting component is present in polysomes and is not message-specific. The constraint on protein synthesis in the amphibian oocyte cannot be fully explained by masked mRNA.