Role of the 3'-poly(A) Sequence in Translational Regulation of MRNAs in Xenopus Laevis Oocytes

We investigated the effect of 3'-noncoding and poly(A) sequences on the translation and stability of membrane-bound mRNA (maize zein) and free mRNA (Xenopus beta-globin) by injecting SP6 transcripts into stage 6 Xenopus oocytes. With zein mRNA, the presence or absence of a 3'-noncoding or poly(A) sequence had little effect on mRNA stability over 24 h, and the 3'-noncoding sequence played essentially no role in mRNA translation. With short periods of incubation, e.g. 1-2 h, the presence or absence of a poly(A) tail also had little effect on zein mRNA translation; but after longer periods, translation of the poly(A-) mRNA was significantly reduced. A similar pattern of mRNA translation was observed for poly(A+) and poly(A-) Xenopus beta-globin mRNAs. These differences in translational efficiency correlated with the formation of maximally loaded polysomes (seven-eight ribosomes/mRNA) for the poly(A+) zein mRNA and a failure to form large polysomes with the poly(A-) zein mRNA. These results are consistent with a model in which the 3'-poly(A) sequence of mRNAs facilitates reinitiation of ribosomes during protein synthesis.