Translation of the human c-myc P0 tricistronic mRNA involves two independent internal ribosome entry sites

The human c-myc proto-oncogene is transcribed from four alternative promote rs (P0, P1, P2, and P3) giving rise to mRNAs having 5 ' leader sequences of various length. The c-myc P0 mRNA contains three open reading frames (ORFs ), the last one encoding c-Myc1 and c-Myc2 proteins generated by alternativ e translation initiated at CUG and AUG codons, The middle ORF (MYCHEX1) and the 5 ' ORF (ORF1) code for proteins 188 and 114 amino acids in length, re spectively. We and others previously identified an internal ribosome entry site (IRES) in P0 and P2 c-myc mRNAs, promoting the cap-independent transla tion of c-Myc1 and c-Myc2, Here, we report the presence of a second IRES (n amed IRES1) promoting the cap-independent translation of MYCHEX1 in c-myc P 0 mRNA, Using deletion analysis, we mapped an 80-nt region essential for IR ES1 activity. c-myc P0 mRNA is thus the first eukaryotic polycistronic mRNA described for which translation initiation of two different open reading f rames (MYCHEX1 and c-Myc1/c-Myc2) involves internal ribosome entry.