Binding of eukaryotic translation initiation factor 4E (eIF4E) to eIF4G represses translation of uncapped mRNA.

mRNA translation in crude extracts from the yeast Saccharomyces cerevisiae is stimulated by the cap structure and the poly(A) tail through the binding of the cap-binding protein eukaryotic translation initiation factor 4E (eIF4E) and the poly(A) tail-binding protein Pab1p. These proteins also bind to the translation initiation factor eIF4G and thereby link the mRNA to the general translational apparatus. In contrast, uncapped, poly(A)-deficient mRNA is translated poorly in yeast extracts, in part because of the absence of eIF4E and Pab1p binding sites on the mRNA. Here, we report that uncapped-mRNA translation is also repressed in yeast extracts due to the binding of eIF4E to eIF4G. Specifically, we find that mutations which weaken the eIF4E binding site on the yeast eIF4G proteins Tif4631p and Tif4632p lead to temperature-sensitive growth in vivo and the stimulation of uncapped-mRNA translation in vitro. A mutation in eIF4E which disturbs its ability to interact with eIF4G also leads to a stimulation of uncapped-mRNA translation in vitro. Finally, overexpression of eIF4E in vivo or the addition of excess eIF4E in vitro reverses these effects of the mutations. These data support the hypothesis that the eIF4G protein can efficiently stimulate translation of exogenous uncapped mRNA in extracts but is prevented from doing so as a result of its association with eIF4E. They also suggest that some mRNAs may be translationally regulated in vivo in response to the amount of free eIF4G in the cell.     

Feedback regulation of ribosomal protein synthesis in E. coli: Localization of the mRNA target sites for repressor action of ribosomal protein L1

E. coli ribosomal protein L1 is a translational repressor of the synthesis in vitro of both proteins encoded in the L11 operon (L11 and L1). L1 is shown to act at a single target site within the first 160 bases of the bicistronic mRNA, near (or at) the translation initiation site of the L11 cistron. Synthesis of L1 apparently requires translation of the preceding L11 cistron, allowing regulation of the synthesis of both proteins from a single mRNA target site. This observation suggests a sequential translation mechanism that results in the equimolar synthesis rates of the two proteins observed in vivo. It was found that the presence of 23S rRNA, but not 16S rRNA, relieves translational inhibition by L1. L1 presumably recognizes structural features of the mRNA target site that are homologous to the L1-binding site of 23S rRNA. Although previous work indicated that translationally inhibited ribosomal protein mRNA is degraded in vivo, L1 repressor action in the present in vitro system was found not to involve mRNA degradation.     

Structure and function of poly(A) binding proteins

Poly (A) tails are found at the 3' ends of almost all eukaryotic mRNAs. They are bound by two different poly (A) binding proteins, PABPC in the cytoplasm and PABPN1 in the nucleus. PABPC functions in the initiation of translation and in the regulation of mRNA decay. In both functions, an interaction with the m7G cap at the 5' end of the message plays an important role. PABPN1 is involved in the synthesis of poly (A) tails, increasing the processivity of poly (A) polymerase and contributing to defining the length of a newly synthesized poly (A) tail.