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. 1996 Jan 12;271(2):623-6.
doi: 10.1074/jbc.271.2.623.

Internal Initiation of Translation Directed by the 5'-untranslated Region of the mRNA for eIF4G, a Factor Involved in the Picornavirus-Induced Switch From Cap-Dependent to Internal Initiation

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Internal Initiation of Translation Directed by the 5'-untranslated Region of the mRNA for eIF4G, a Factor Involved in the Picornavirus-Induced Switch From Cap-Dependent to Internal Initiation

W Gan et al. J Biol Chem. .
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Abstract

The eIF4 group initiation factors carry out recognition of the mRNA cap, unwinding of mRNA secondary structure, and binding of mRNA to the 43 S preinitiation complex. Infection by picornaviruses results in proteolytic cleavage of one of these factors, eIF4G, an event that severely restricts cap-dependent translation but permits cap-independent initiation to proceed from internal ribosome entry sequences in picornaviral RNAs. The 5'-untranslated region (5'-UTR) of eIF4G mRNA resembles such picornaviral sequences in being unusually long and containing multiple open reading frames and a polypyrimidine tract. When inserted upstream of a luciferase reporter gene, this 5'-UTR served as a translational enhancer in four different cell lines. Mutation of all four upstream ATG codons to AAG did not alter the translational enhancement. The presence of the eIF4G 5'-UTR between an RNA hairpin and the luciferase cistron stimulated expression 119-fold. Similarly, the presence of the 5'-UTR between the two cistrons of a bicistronic mRNA stimulated expression of the downstream cistron 42-fold. These results indicate that the eIF4G 5'-UTR directs internal initiation. The ability to continue synthesis of eIF4G when the cell is unable to carry out normal cap-dependent translation may represent an autoregulatory mechanism or be part of the cellular response to stresses that interrupt cap-dependent translation.

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