Differential capacity for translation and lack of competition between mRNAs that segregate to free and membrane-bound polysomes.

The Xenopus oocyte was used to study translation, stability and subcellular compartmentation of injected heterologous mRNAs. The capacity of the oocyte to translate a membrane-free mRNA, globin, was not saturable by amounts up to 100 ng injected. In contrast, the ability of the oocyte to translate a membrane-associated mRNA, zein, saturated at 20 ng or less of injected mRNA. The hybridization of RNA from message-injected oocytes to cDNA probes revealed that approximately the same amount of globin and zein mRNAs remained stable and that this fraction was as little as 2.1% at the highest amount injected (80 ng). However, while almost all of the globin mRNA at all concentrations injected was found in a polysome pellet, most of the zein mRNA at saturating concentrations was found in a post-polysomal supernatant. The reason for the limited translation of zein mRNA appears to be due to a paucity of oocyte rough endoplasmic reticulum since the injection of dog pancreas rough endoplasmic reticulum into oocytes previously injected with zein mRNA stimulates zein synthesis. Displacement and competition experiments between four different mRNAs indicate that injected messages are distributed to two translational compartments, membrane-bound and free, and that this occurs either prior to the initiation of translation or involves message-specific factors.