Effects Of C-Myc First Exons And 5' Synthetic Hairpins On Rna Translation In Oocytes And Early Embryos Of Xenopus Laevis

Mammalian c-myc transcripts have long G/C-rich 5' untranslated regions (UTRs) that may fold into secondary structural elements that may impede translation. We have examined the effects of different c-myc first exons, which produce most of the 5' UTR of c-myc transcripts, on translation in Xenopus oocytes and embryos, by placing these structures upstream of a chloramphenicol acetyltransferase (CAT) reporter. Our results demonstrate that the human c-myc first exon inhibits reporter translation in both oocytes and embryos. Unlike their mammalian counterparts, Xenopus c-mycI first exons initiated at either promoter 1 or promoter 2 do not impede translation. We conclude that translation inhibition reported in a previous investigation (Lazarus, 1992. Oncogene, 7:1037) utilizing Xenopus c-mycI 5' non-coding elements was due to the inclusion of non-relevant non-transcribed sequences. Previous investigators have reported that inhibition of translation in Xenopus oocytes by 5' secondary structure is alleviated after fertilization (Lazarus et al., 1988. Oncogene 3:517; Fu et al., 1991. Science 251:807). We repeated the experiments of Fu et al., examining the effects on translation by a highly stable synthetic hairpin. The hairpin severly restricted translation in both oocytes and embryos, indicating that highly stable 5' secondary structure is equally inhibitory in oocytes and embryos.