Poly(A) binds to initiation factors and increases cap-dependent translation in vitro.

Both the 5' cap (m7GpppN) and the 3' poly(A) tail of eukaryotic mRNAs are important regulators of translation efficiency in vivo. Their function, however, is markedly reduced in in vitro translation systems derived from either rabbit reticulocytes or wheat germ. The impact of exogenous poly(A) on cap-dependent translation was examined in vitro. The translation of uncapped mRNA was preferentially repressed in the presence of exogenous poly(A). As a result, translation became increasingly cap-dependent with the increase in exogenous poly(A). The translation in wheat germ lysate was stimulated by the addition of purified eukaryotic initiation factor (eIF)-4B or eIF-4F; however, addition of poly(A) prevented this eIF-4B- or eIF-4F-mediated stimulation. Addition of eIF-4F or eIF-4B, alone, was not sufficient to restore translation in lysate to which poly(A) had been added. Restoration, however, was observed when eIF-4F, eIF-4B, and eIF-4A were added in combination. These data suggest that 1) exogenous poly(A) may bind to and sequester factors required for translation and 2) that capped messages compete with poly(A) more efficiently for these factors than do uncapped mRNAs. Gel shift analysis of purified initiation factors isolated from wheat germ confirmed that eIF-4B and eIF-4F do in fact form complexes with poly(A) in vitro.