EXON 2-MEDIATED C-MYC MESSENGER-RNA DECAY IN-VIVO IS INDEPENDENT OF ITS TRANSLATION

We have previously shown that the steady-state level of c-myc mRNA in vivo is primarily controlled by posttranscriptional regulatory mechani sms. To identify the sequences involved in this process, we constructe d a series of H-2/myc transgenic lines in which various regions of the human c-MYC gene were placed under the control of the quasi-ubiquitou s H-2K class I regulatory sequences. We demonstrated that the presence of one of the two coding exons, exon 2 or exon 3, is sufficient to co nfer a level of expression of transgene mRNA similar to that of endoge nous c-myc in various adult tissues as well as after partial hepatecto my or after protein synthesis inhibition. We now focus on the molecula r mechanisms involved in modulation of expression of mRNAs containing c-myc exon 2 sequences, with special emphasis on the coupling between translation and c-myc mRNA turnover. We have undertaken an analysis of expression, both at the mRNA level and at the protein level, of new t ransgenic constructs in which the translation is impaired either by di sruption of the initiation codon or by addition of stop codons upstrea m of exon 2. Our results show that the translation of c-myc exon 2 is not required for regulated expression of the transgene in the differen t situations analyzed, and therefore they indicate that the mRNA desta bilizing function of exon 2 is independent of translation by ribosomes . Our investigations also reveal that, in the thymus, some H-2/myc tra nsgenes express high levels of mRNA but low levels of protein. Besides the fact that these results suggest the existence of tissue-specific mechanisms that control c-myc translatability in vivo, they also bring another indication of the uncoupling of c-myc mRNA translation and de gradation.