Molecule of the Month
August 2009
Selenocysteine Biosynthesis in Eukaryotes: A Three Step Pathway

The micronutrient selenium is found in proteins as selenocysteine (Sec), the 21st amino acid cotranslationally inserted in response to a UGA codon. The eukaryotic pathway for selenocysteinyl-tRNASec formation has been elusive for many years. Using the parasitic protozoa Trypanosoma brucei as a model system we were able to solve this problem.
We could show that eukaryotic selenocysteinyl-tRNASec formation is a three step process starting with serylation of tRNASec by seryl-tRNA synthetase, followed by phosphorylation of serine to form phosphoseryl (Sep)-tRNASec by phosphoseryl-tRNASec kinase and by the last step consisting in the conversion of phosphoserine-tRNASec to selenocysteinyl-tRNASec by Sep-tRNA:Sec-tRNA synthase.

This work was carried out in the group of Prof. André Schneider.


  • E. Aeby, S. Palioura, M. Pusnik, J. Marazzi, A. Lieberman, E. Ullu, D. Söll and A. Schneider;
    "The canonical pathway for selenocysteine insertion is dispensable in Trypanosomes"
    Proc. Natl. Acad. Sci. USA, 106, 5088-5092, (2009); doi:10.1073/pnas.0901575106.