The recruitment of membrane-bound mRNAs for translation in microinjected Xenopus oocytes.

Xenopus oocytes were injected with several heterologous mRNAs to investigate specific message recruitment onto polysomes. When large amounts of mRNA such as ovalbumin, which is translated on membrane-bound polysomes, are injected, most of the stable message accumulates in a postpolysomal supernatant. Conversely, when similar amounts of nonmembrane-bound mRNAs such as those which code for adenovirus protein are injected, most of the messages are found in a polysome pellet. Nonpolysomal mRNAs such as zein, ovalbumin, and lysozyme can be recruited onto membrane-bound polysomes by injected rough endoplasmic reticulum (RER). Only RER, but not Golgi apparatus, plasma membrane, or salt-washed RER has such recruiting ability. In addition, the nontranslating messengers can be recruited by injecting the salt wash of the RER. Since most nontranslating mRNAs sediment to regions less than the 80 S monosome, the step at which translational regulation takes place is probably initiation rather than elongation. The mechanism by which recruitment might occur is discussed.