Degradation of a developmentally regulated mRNA in Xenopus embryos is controlled by the 3' region and requires the translation of another maternal mRNA.

By injecting the appropriately constructed plasmids into one-cell Xenopus embryos, we determined that the 3' region of the maternal Xenopus Eg2 mRNA confers instability on the chimeric mRNA transcribed from these plasmids. This instability, like that of the maternal Eg2 transcript, was abolished by treatment of the embryos with cycloheximide. Analysis of the polysome distribution of the maternal Eg2 mRNA in cycloheximide-treated and untreated embryos showed that Eg2 mRNA was released from polysomes after fertilization and that the stabilization caused by cycloheximide treatment was not due to a reloading of ribosomes onto the mRNA. Insertion of a stable hairpin loop (delta G = -50 kcal/mol) 5' to the reporter gene in the injected plasmid caused a 10- to 20-fold decrease in translation from the transcribed mRNAs. This decrease in translation did not abolish the instability conferred by the 3' Eg2 region. Therefore, the degradation of these chimeric mRNAs in Xenopus embryos requires the translation of another maternal mRNA coding for a trans-acting factor involved in mRNA degradation. Further restriction of the 3' Eg2 region, placed 3' to the reporter gene, showed that a cis-acting instability-conferring sequence is contained in a 497-nucleotide fragment.