REGULATION OF TRANSLATION BY SPECIFIC PROTEIN MESSENGER-RNA INTERACTIONS

This review will focus on cases of specific translational control by p rotein/RNA interactions in the 5'- or 3'-UTR of eukaryote mRNA where e ither the cis-acting RNA determinant or the trans-acting protein (or p referably both) have been identified with fair certainty. Examples of messages that are regulated by 5' motifs, which are proposed to occlud e ribosome binding when bound by their specific factors, include ferri tin and ribosomal protein mRNAs and the autoregulated thymidylate synt hase and poly(A)binding mRNAs. However, it has become increasingly evi dent recently that 3' UTR determinants and their specific binding prot eins also regulate translation efficiency either directly, or indirect ly via an influence on the polyadenylation status of the mRNA. It is s till unclear how events at the 3' end of mRNA influence ribosome bindi ng. Most, if not all, of the mRNAs known to be regulated by 3' UTR mot ifs are subject to regulation during early development or during diffe rentiation such as several spermatocyte and oocyte mRNAs and erythroid lipoxygenase mRNA. To date, in all cases where translation is control led directly by specific protein/mRNA interactions, the protein seems to act as a negative regulator, a translational repressor whose bindin g to the specific site on the mRNA results in inhibition of initiation . The only cases of translational activation known so far concern inte rnal initiation of translation of picornaviral RNAs, but this topic is beyond the scope of this review.