The 5′-leader of tobacco mosaic virus promotes translation through enhanced recruitment of eIF4F

The 5′-leader sequence (called Ω) of tobacco mosaic virus (TMV) functions as a translational enhancer in plants. A poly(CAA) region within Ω is responsible for the translation enhancement and serves as a binding site for the heat shock protein, HSP101, which is required for the translational enhancement. Genetic analysis of the HSP101-mediated enhancement of translation from Ω-containing mRNA suggested that two eukaryotic initiation factors (eIFs), i.e. eIF4G and eIF3, were necessary. In this study, the functional interaction between Ω and other RNA elements known to participate in the recruitment of eIF4G, i.e. the 5′-cap and the poly(A) tail, was examined. Ω exhibited functional overlap with the 5′-cap and the poly(A) tail but not with the native TMV 3′-UTR which contains an independent translational enhancer. Consistent with the role of HSP101 in mediating the translational function of Ω, the enhancement afforded by Ω increased following a heat shock, which elevates expression of HSP101. The use of a fractionated translation lysate revealed that of the two eIF4F proteins present in plants, eIF4F was specifically required for the activity of Ω. The data suggest that Ω is functionally similar to a 5′-cap and a poly(A) tail in that it serves to recruit eIF4F in order to enhance translation from an mRNA.