Decoding UGA as selenocysteine requires a unique tRNA, a specialized elongation factor, and specific secondary structures in the mRNA, termed SECIS elements. Eukaryotic SECIS elements are found in the 3′ untranslated region of selenoprotein mRNAs while those in prokaryotes occur immediately downstream of UGA. Consequently, a single eukaryotic SECIS element can serve multiple UGA codons, whereas prokaryotic SECIS elements only function for the adjacent UGA, suggesting distinct mechanisms for recoding in the two kingdoms. We have identified and characterized the first eukaryotic selenocysteyl-tRNA-specific elongation factor. This factor forms a complex with mammalian SECIS binding protein 2, and these two components function together in selenocysteine incorporation in mammalian cells. Expression of the two functional domains of the bacterial elongation factor-SECIS binding protein as two separate proteins in eukaryotes suggests a mechanism for rapid exchange of charged for uncharged selenocysteyl-tRNA-elongation factor complex, allowing a single SECIS element to serve multiple UGA codons.

Selenocysteine is incorporated into proteins via "recoding" of UGA from a stop codon to a sense codon, a process that requires specific secondary structures in the 3 untranslated region, termed selenocysteine incorporation sequence (SECIS) elements, and the protein factors that they recruit. Whereas most selenoprotein mRNAs contain a single UGA codon and a single SECIS element, selenoprotein P genes encode multiple UGAs and two SECIS elements. We have identified evolutionary adaptations in selenoprotein P genes that contribute to the efficiency of incorporating multiple selenocysteine residues in this protein. The first is a conserved, inefficiently decoded UGA codon in the N-terminal region, which appears to serve both as a checkpoint for the presence of factors required for selenocysteine incorporation and as a "bottleneck," slowing down the progress of elongating ribosomes. The second adaptation involves the presence of introns downstream of this inefficiently decoded UGA which confer the potential for nonsense-mediated decay when factors required for selenocysteine incorporation are limiting. Third, the two SECIS elements in selenoprotein P mRNA function with differing efficiencies, affecting both the rate and the efficiency of decoding different UGAs. The implications for how these factors contribute to the decoding of multiple selenocysteine residues are discussed.Selenoprotein P is an enigma of genetic recoding. It is the only known protein in which multiple potential stop codons are recoded to function as sense codons. In selenoprotein mRNAs, UGA codons, which would normally signal the termination of protein synthesis, are decoded as the amino acid selenocysteine. This process involves a unique tRNA with an anticodon complementary to UGA and specialized secondary structures located in the 3Ј untranslated regions (UTRs) of selenoprotein mRNAs, termed selenocysteine incorporation sequence (SECIS) elements (3). For selenocysteine incorporation to occur, the SECIS element must recruit a SECIS binding protein, SBP2 (6). SBP2, in turn, recruits a dedicated elongation factor, EFsec (7, 26), complexed with selenocysteyl-tRNA (2, 31). Most selenoprotein mRNAs contain a single UGA codon and a single SECIS element. Selenoprotein P mRNAs contain 10 to 18 UGA codons, depending on the species, and two SECIS elements differing in secondary structure. The majority of studies to date on the mechanism of selenoprotein synthesis have focused on selenoprotein mRNAs containing a single UGA codon and a single SECIS element. Thus, little is known about the mechanism for incorporating multiple selenocysteines.The incorporation of selenium into selenoprotein P serves several important biological functions: it allows the accumulation of this essential but potentially toxic trace element in a biologically stable and nontoxic form, it plays a critical role in the transport of selenium from the liver via the circulation to target organs (13,25), it is thought to function in sequestering and thereby detoxifying heavy metals in plasma...

The aim of the present study was to investigate homoharringtonine alkaloid effect on: (i) the nonenzymatic and eEF‐1‐dependent Phe‐tRNAphe binding to poly(U)‐programmed human placenta 80 S ribosomes; (ii) diphenylalanine synthesis accompanying nonenzymatic Phe‐tRNAphe binding; and (iii) acetylphenylalanyl‐puromycin formation. Neither nonenzymatic nor eEF‐1‐dependent Phe‐tRNAphe binding were noticeably affected by the alkaloid, whereas diphenylalanine synthesis and puromycin reaction were strongly inhibited by homoharringtonine. It has been proposed that the site of homoharringtonine binding on 80 S ribosomes shouldoverlap or coincide with the acceptor site of the ribosome.