Linking the 3 ' poly(A) tail to the subunit joining step of translation initiation: Relations of Pab1p, eukaryotic translation initiation factor 5B (Fun12p), and Ski2p-Slh1p

The 3 ' poly(A) structure improves translation of a eukaryotic mRNA by 50-f old in vivo. This enhancement has been suggested to be due to an interactio n of the poly(A) binding protein, Pab1p, with eukaryotic translation initia tion factor 4G (eIF4G), However, we find that mutation of eIF4G eliminating its interaction with Pab1p does not diminish the preference for poly(A)(+) mRNA in vivo, indicating another role for poly(A). We show that either the absence of Fun12p (eIF5B), or a defect in eIF5, proteins involved in 60S r ibosomal subunit joining, specifically reduces the translation of poly(A)() mRNA, suggesting that poly(A) may have a role in promoting the joining st ep. Deletion of two nonessential putative RNA helicases (genes SKI2 and SLH 1) makes poly(A) dispensable for translation. However, in the absence of Fu n12p, eliminating Ski2p and Slh1p shows little enhancement of expression of non-poly(A) mRNA. This suggests that Ski2p and Slh1p block translation of non-poly(A) mRNA by an effect on Fun12p, possibly by affecting 60S subunit joining.