Initiation Does Not Limit the Rate of Globin Synthesis in Message‐Injected Xenopus Oocytes

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Abstract
Frog oocytes synthesize haemoglobin when injected with globin mRNA. Studies using this mRNA have shown that Xenopus oocytes have a finite unused translational capacity. When injected with roughly equal amounts of α‐and β‐globin mRNA, five times as many β chains as α chains are made. To help understand the underlying basis for these observations we have investigated the size of the polyribosomes which synthesize α‐and β‐globin after injecting various amounts of rabbit globin mRNA. Both α and β chains appear to be made on the same size of polyribosomes, whether or not the translational machinery of the cell is saturated.We interpret these results to mean that neither the defiency in α chain synthesis, nor the rate‐limiting component at saturation operates by limiting initiation. We suggest that the agent that limits total globin synthesis is a pre‐initiation “recruitment” or a “stabilisation” factor. The deficiency of α‐globin synthesis can best be explained by a partial block in elongation on the α globin mRNA.