The canonical pathway for selenocysteine insertion is dispensable in Trypanosomes

Proc Natl Acad Sci U S A. 2009 Mar 31;106(13):5088-92. doi: 10.1073/pnas.0901575106. Epub 2009 Mar 11.


The micronutrient selenium is found in proteins as selenocysteine (Sec), the 21st amino acid cotranslationally inserted in response to a UGA codon. In vitro studies in archaea and mouse showed that Sec-tRNA(Sec) formation is a 3-step process starting with serylation of tRNA(Sec) by seryl-tRNA synthetase (SerRS), phosphorylation of serine to form phosphoserine (Sep)-tRNA(Sec) by phosphoseryl-tRNA(Sec) kinase (PSTK), and conversion to Sec-tRNA(Sec) by Sep-tRNA:Sec-tRNA synthase (SepSecS). However, a complete study of eukaryotic selenoprotein synthesis has been lacking. Here, we present an analysis of Sec-tRNA(Sec) formation in the parasitic protozoon Trypanosoma brucei in vivo. Null mutants of either PSTK or SepSecS abolished selenoprotein synthesis, demonstrating the essentiality of both enzymes for Sec-tRNA(Sec) formation. Growth of the 2 knockout strains was not impaired; thus, unlike mammals, trypanosomes do not require selenoproteins for viability. Analysis of conditional RNAi strains showed that SerRS, selenophosphate synthase, and the Sec-specific elongation factor, EFSec, are also essential for selenoprotein synthesis. These results with T. brucei imply that eukaryotes have a single pathway of Sec-tRNA(Sec) synthesis that requires Sep-tRNA(Sec) as an intermediate.

Publication types

  • Research Support, N.I.H., Extramural
  • Research Support, Non-U.S. Gov't

MeSH terms

  • Animals
  • Archaea
  • Metabolic Networks and Pathways*
  • Mice
  • Phosphotransferases / metabolism
  • RNA, Transfer, Amino Acid-Specific / metabolism
  • RNA, Transfer, Amino Acyl / metabolism
  • Selenocysteine / metabolism*
  • Selenoproteins / biosynthesis*
  • Trypanosoma brucei brucei / enzymology
  • Trypanosoma brucei brucei / metabolism*


  • RNA, Transfer, Amino Acid-Specific
  • RNA, Transfer, Amino Acyl
  • Selenoproteins
  • selenocysteinyl-tRNA
  • tRNA, selenocysteine-
  • Selenocysteine
  • Phosphotransferases