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.
Highlights
The c-myc proto-oncogene has a fundamental role in various cellular events, including proliferation, differentiation, and apoptosis [1]
We demonstrate the presence of an internal ribosome entry site (IRES) in P0, P1, and P2 c-myc mRNAs, implying an element located between nucleotides Ϫ363 and Ϫ95 upstream from the CUG start codon
An IRES Controls the Synthesis of c-Myc1 and c-Myc2 Proteins—To determine whether the cap-independent translation initiation, observed in the rabbit reticulocyte lysate (RRL) system for the P0 leader and for the P1 and P2 leaders, occurred by a process of ribosome entry, we looked for the presence of an IRES in the c-myc mRNA
Summary
The c-myc proto-oncogene has a fundamental role in various cellular events, including proliferation, differentiation, and apoptosis [1]. P1 and P2 mRNAs encode two proteins of 64 and 67 kDa, designated as c-Myc and c-Myc, respectively, by a process of alternative initiation of translation starting at two inframe codons, CUG and AUG [5]. The transcription starting points of the P0, P1, and P2 c-myc mRNAs are located 1172, 524, and 363 nt upstream from the CUG initiation codon, respectively. The best characterized of these factors is the polypyrimidine tract-binding protein, known as a splicing factor [22,23,24] Due to their positive regulation by cellular factors, IRESs can be considered as translation enhancers, allowing activation of tightly controlled genes in response to specific stimuli [25]. We demonstrate the presence of an IRES in P0, P1, and P2 c-myc mRNAs, implying an element located between nucleotides Ϫ363 and Ϫ95 upstream from the CUG start codon
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
