Translational capacity of living frog eggs and oocytes, as judged by messenger RNA injection

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Growing oocytes and activated eggs of Xenopus laevis have been injected with increasing amounts of 9 s haemoglobin messenger RNA extracted from rabbit reticulocytes. At low concentrations of injected RNA, there is a linear relationship between the amount of RNA injected and the amount of haemoglobin synthesized. Higher concentrations of RNA saturate the translational capacity of oocytes and eggs, so that increasing amounts of injected RNA fail to stimulate increased haemoglobin synthesis. Under these conditions, the rate of protein synthesis promoted by endogenous messenger RNA is unaffected by the injection of 9 s RNA and the over-all rate of protein synthesis by message-saturated cells is nearly doubled. At very high concentrations of injected RNA and after long periods of incubation, the injected RNA competes with endogenous messenger RNA for a translational component which limits the over-all rate of protein synthesis in injected cells. The results show that frog eggs and oocytes possess an unused capacity for translating injected messenger RNA, and that the amount of this spare capacity can be determined by the construction of messenger RNA saturation curves. The results also suggest that the normal rate of protein synthesis in these cells is limited by their content of functional messenger RNA.