Regulation of the human c-myc gene: 5' noncoding sequences do not affect translation.

Abstract

The influence of untranslated 5' sequences on c-myc expression was compared by measuring the translational efficiencies of mRNAs which contain leaders derived from exon 1 or intron 1 of the human c-myc gene. Expression plasmids were constructed and introduced into COS cells, and the levels of c-myc mRNA and protein were examined. Our results show that mRNAs transcribed from constructs containing exon 1 or intron 1, which have different folding potential, are translated with approximately equal efficiencies. This suggests that the translation of c-myc mRNA is not controlled by secondary structure alone. In addition, we observed that transcripts in which exon 1 was deleted are not translated more efficiently, but are present at a higher steady-state level. Thus, this example provides evidence for possible control at the transcriptional level. Finally, since the c-myc product was produced in each of our test systems, the results suggest that this protein does not regulate its own transcription or translation via a specific interaction with c-myc exon 1 alone.

cis-acting translational effects of the 5' noncoding region of c-myc mRNA.

“…Our findings that the 5' noncoding region of c-myc mRNA might have cis-acting inhibitory activity that is translation system specific has bearing on previous reports that the c-myc 5' noncoding region has no detrimental effect on translation in vivo in two different cell lines (6,38). An intriguing possibility is that translation in COS cells (6) has the same characteristics as translation in vitro in HeLa cell extracts and that in other cell lines (e.g., specialized cells) translation of c-myc mRNA is restricted by its 5' noncoding region. To explore this possibility, we began to analyze established cell lines, including COS-1, for translational effects of the c-myc mRNA 5' noncoding region by transient expression after transfection of c-myc or chimeric c-myc-CAT constructs into the cells.…”

“…Several groups have investigated this possibility (6,13,38,41) with various in vitro and in vivo systems. In the study by Butnick et al (6), exon 1 was found to have no effect on translational efficiency in COS-1 cells; however, the possibility exists that overexpression of c-myc mRNA in this type of system precludes the detection of translational effects of exon 1 which might be observed in other systems (see reference 6 and Discussion). Results from our in vitro translational studies showed that full-length c-myc mRNAs have lower translational efficiencies than those lacking exon 1, while the mRNAs had similar stabilities (13).…”

Features of the chicken c-myc gene that influence the structure of c-myc RNA in normal cells and bursal lymphomas.

“…Thus, the removal of exon 1 in most retrovirus-induced bursal lymphomas (11,37), as well as in the majority of c-myc-immunoglobulin locus translocations found in plasmacytomas (3,20,41,43) and some Burkitt's lymphomas (3,5), may enhance translation of c-myc (11,37). Experiments to address the issue of translation efficiency with truncated c-myc transcripts have yielded inconsistant data (7,11,30). Unfortunately, the amount of c-inyc protein in chicken bursal lymphomas with and without the first exons present in RNA has not yet been examined.…”

“…It has been proposed that secondary structures caused by self-annealing sequences derived from exons 1 and 2 of human c-mvc RNA can inhibit translation (7,30,37). Chicken c-mvc RNA can also potentially form a stem structure by annealing portions of exons 1 and 2 with a minimum free energy of AGO (250) of -45 kcal/mol (44) which is not as low as in the human RNA stem (-90 kcal/mol).…”

C-Myc 5′ untranslated region contains an internal ribosome entry segment

“…However, early studies in vivo examining translational eciencies of c-myc mRNA in Burkitt's lymphoma cell lines suggested that both truncated and full length transcripts were translated with equal eciencies (Nilsen and Maroney, 1984). Moreover, when expressed in numerous cell lines or translated in HeLa cell extracts the 5' UTR does not inhibit translation of either c-myc or reporter genes (Butnick et al, 1985;Parkin et al, 1988). This disparity suggests that non-canonical factors, which are lacking in rabbit reticulolysate and wheat germ extract, facilitate c-myc translation through the 5' UTR in vivo (Parkin et al, 1988).…”