The trace element selenium is present in the form of selenocysteine in selected proteins of organisms from all three domains of life. Most of the selenoproteins (i.e. proteins containing selenocysteine) investigated thus far are oxidoreductases with selenocysteine present in the active site and playing a crucial role in – or creating the required environment for – catalysis. Selenocysteine incorporation is DNA encoded, determined by the UGA codon, and occurs via a unique mechanism representing an expansion of the genetic code. Despite the conceptual conservation of the principal mechanism of selenocysteine synthesis and incorporation in all lines of descent, the bacterial system differs from the archaeal/eukaryal system. Detailed biochemical and structural knowledge of selenoprotein synthesis has allowed rewiring bacterial translation for targeted selenocysteine insertion.

Key Concepts

  • Selenium is an essential trace element for many organisms including humans.
  • The most important form of biologically active selenium is the amino acid selenocysteine.
  • Selenocysteine is co‐translationally inserted into growing polypeptides.
  • Selenocysteine insertion proceeds via a unique mechanism recoding UGA.
  • The mechanism of selenocysteine insertion differs in Bacteria, Archaea, and Eukarya.

Keywords: selenoproteins; SelB; selenocysteyl‐tRNA; SECIS; recoding

Figure 1. Cloverleaf structures of selenocysteine tRNAs from E. coli, M. jannaschii and humans. Bold arrows indicate novel tertiary interactions. Positions conserved in all elongator tRNAs are circled; positions deviating from the consensus are boxed (data from Baron and Böck, ; Sturchler et al., ).
Figure 2. Interactions of SelB with ligands. Domains I, II and III are homologous to those from EF‐Tu and interact with guanine nucleotides and selenocysteyl‐tRNA; domain IVb binds the SECIS element. Binding of the charged tRNA confers on SelB a conformation with a higher affinity for the SECIS structure. This form of SelB is able to interact with the ribosome, induce GTP hydrolysis and release the charged tRNA. The isolated domain IVb has the same affinity for the SelB protein as the whole‐length protein carrying the tRNA. Numbers give the equilibrium constants as determined by stopped‐flow analysis (data from Thanbichler and Böck, ).
Figure 3. Model of the function of the SECIS element in selenocysteine insertion by Bacteria (Baron and Böck, ), Eukarya (Squires and Berry, ) and Archaea (Rother and Krzycki, ); the black two‐headed arrow indicates potential competitive binding; the red two‐headed arrow indicates an unknown recoding mechanism; factors are not drawn to scale; see text for details.


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Further Reading

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Lobanov AV, Turanov AA, Hatfield DL and Gladyshev VN (2010) Dual functions of codons in the genetic code. Critical Reviews in Biochemistry and Molecular Biology 45: 257–265.

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Rother, Michael(Apr 2015) Selenocysteine. In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1002/9780470015902.a0000688.pub3]