Selenocysteine incorporation directed from the 3'UTR: characterization of eukaryotic EFsec and mechanistic implications.

The mechanism of selenocysteine incorporation in eukaryotes has been assumed for almost a decade to be inherently different from that in prokaryotes, due to differences in the architecture of selenoprotein mRNAs in the two kingdoms. After extensive efforts in a number of laboratories spanning the same time frame, some of the essential differences between these mechanisms are finally being revealed, through identification of the factors catalyzing cotranslational selenocysteine insertion in eukaryotes. A single factor in prokaryotes recognizes both the selenoprotein mRNA, via sequences in the coding region, and the unique selenocysteyl-tRNA, via both its secondary structure and amino acid. The corresponding functions in eukaryotes are conferred by two distinct but interacting factors, one recognizing the mRNA, via structures in the 3' untranslated region, and the second recognizing the tRNA. Now, with these factors in hand, crucial questions about the mechanistic details and efficiency of this intriguing process can begin to be addressed.