title: Alternative translation of the proto-oncogene c-myc by an internal ribosome entry site.
abstract: The human proto-oncogene c-myc encodes two proteins, c-Myc1 and c-Myc2, from two initiation codons, CUG and AUG, respectively. It is also transcribed from four alternative promoters (P0, P1, P2, and P3), giving rise to different RNA 5'-leader sequences, the long sizes of which suggest that they must be inefficiently translated by the classical ribosome scanning mechanism. Here we have examined the influence of three c-myc mRNA 5'-leaders on the translation of chimeric myc-CAT mRNAs. We observed that in the reticulocyte rabbit lysate, these 5'-leaders lead to cap-independent translation initiation. To determine whether this kind of initiation resulted from the presence of an internal ribosome entry site (IRES), COS-7 cells were transfected with bicistronic vectors containing the different c-myc 5'-leaders in the intercistronic region. An IRES was identified, requiring elements located within the P2 leader, between nucleotides -363 and -94 upstream from the CUG start codon. This is the first demonstration of the existence of IRES-dependent translation for a proto-oncogene. This IRES could be a translation enhancer, allowing activation of c-myc expression under the control of trans-acting factors and in response to specific cell stimuli.
authors.affiliation: INSERM U397, Endocrinologie et Communication Cellulaire, Institut Louis Bugnard, Centre Hospitalier Universitaire Rangueil, Avenue Jean Poulhès, 31403 Toulouse Cedex 04, France.
authors.forename: C
authors.initials: C
authors.lastname: Nanbru
authors.forename: I
authors.initials: I
authors.lastname: Lafon
authors.forename: S
authors.initials: S
authors.lastname: Audigier
authors.forename: M C
authors.initials: MC
authors.lastname: Gensac
authors.forename: S
authors.initials: S
authors.lastname: Vagner
authors.forename: G
authors.initials: G
authors.lastname: Huez
authors.forename: A C
authors.initials: AC
authors.lastname: Prats
mesh_terms: D000818:Animals; D001483:Base Sequence; D019556:COS Cells; D015500:Chloramphenicol O-Acetyltransferase / Q000235:genetics; D003062:Codon; D016259:Genes, myc; D006801:Humans; D008969:Molecular Sequence Data; D009690:Nucleic Acid Conformation; D011401:Promoter Regions, Genetic; D014176:Protein Biosynthesis; D000090063:Proto-Oncogene Mas; D012315:RNA Caps; D012333:RNA, Messenger / Q000737:chemistry / Q000235:genetics; D011993:Recombinant Fusion Proteins / Q000235:genetics; D012270:Ribosomes / Q000378:metabolism*; D012689:Sequence Homology, Nucleic Acid; D014407:Tumor Cells, Cultured
chemical_list: D003062:Codon; C000715470:MAS1 protein, human; D000090063:Proto-Oncogene Mas; D012315:RNA Caps; D012333:RNA, Messenger; D011993:Recombinant Fusion Proteins; D015500:Chloramphenicol O-Acetyltransferase