ALTERNATIVE TRANSLATION OF THE PROTOONCOGENE C-MYC BY AN INTERNAL RIBOSOME ENTRY SITE

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 tr anscribed from four alternative promoters (P0, P1, P2, and P3), giving rise to different RNA 5'-leader sequences, the long sizes of which su ggest that they must be inefficiently translated by the classical ribo some 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 lea d 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 vect ors containing the different c-myc 5'-leaders in the intercistronic re gion. 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-depend ent translation for a proto-oncogene. This IRES could be a translation enhancer, allowing activation of c-myc expression under the control o f trans-acting factors and in response to specific cell stimuli.