ALTERNATIVE TRANSLATION INITIATION OF THE MOLONEY MURINE LEUKEMIA-VIRUS MESSENGER-RNA CONTROLLED BY INTERNAL RIBOSOME ENTRY INVOLVING THE P57 PTB SPLICING FACTOR/

Moloney murine leukemia virus (Mo-MuLV) genomic mRNA codes for two gag precursors by alternative initiations of translation. An AUG codon go verns the synthesis of the retroviral capsid proteins precursor, where as a CUG codon directs the synthesis of a glycosylated cell surface an tigen, the gross cell surface antigen. Control of the relative synthes is of the two precursors is crucial for MuLV infectivity and pathology . Furthermore, the MuLV mRNA leader sequence is very long and should i nhibit translation according to the classical scanning model. This sug gests a different translation initiation mechanism allowing gag effici ent expression. We demonstrate, by using bicistronic vectors expressed in COS-7 cells, that the Mo-MuLV mRNA leader drives translation initi ation by internal ribosome entry. We have localized the internal ribos ome entry site (IRES) between the two initiation codons. This 126 nucl eotide long IRES implies an oligopyrimidine tract located 45 nucleotid es upstream of AUG codon. UV crosslinking and affinity chromatography experiments show that the PTB/p57 splicing factor specifically interac ts with this oligopyrimidine tract. The MuLV IRES controls alternative translation initiation by activating the capsid protein precursor exp ression. This gag translational enhancer could exist in other retrovir uses.