INTERNAL INITIATION OF TRANSLATION DIRECTED BY THE 5'-UNTRANSLATED REGION OF THE MESSENGER-RNA FOR EIF4G, A FACTOR INVOLVED IN THE PICORNAVIRUS-INDUCED SWITCH FROM CAP-DEPENDENT TO INTERNAL INITIATION

The eIF4 group initiation factors carry out recognition of the mRNA ca p, unwinding of mRNA secondary structure, and binding of mRNA to the 4 3 S preinitiation complex, Infection by picornaviruses results in prot eolytic cleavage of one of these factors, eIF4G, an event that severel y restricts cap-dependent translation but permits cap independent init iation to proceed from internal ribosome entry sequences in picornavir al RNAs. The 5'-untranslated region (5'-UTR) of eIF4G mRNA resembles s uch picornaviral sequences in being unusually long and containing mult iple open reading frames and a polypyrimidine tract. When inserted ups tream of a luciferase reporter gene, this 5'-UTR served as a translati onal enhancer in four different cell lines. Mutation of all four upstr eam ATG codons to AAG did not alter the translational enhancement. The presence of the eIF4G 5'-UTR between an RNA hairpin and the luciferas e cistron stimulated expression 119-fold. Similarly, the presence of t he 5'-UTR between the two cistrons of a bicistronic mRNA stimulated ex pression of the downstream cistron 42-fold. These results indicate tha t the eIF4G 5'-UTR directs internal initiation. The ability to continu e synthesis of eIF4G when the cell is unable to carry out normal cap d ependent translation may represent an autoregulatory mechanism or be p art of the cellular response to stresses that interrupt cap-dependent translation.