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Abstract:
Several methanogenic archaea lack cysS the gene encoding cysteinyl-tRNA synthetase (CysRS), the essential enzyme that provides Cys-tRNACys for translation in most organisms. Using anaerobic techniques to fractionate and analyze the cell-extract of Methanocaldococcus jannaschii revealed that tRNACys becomes acylated with O-phosphoserine (Sep) by a class II-type O-phosphoseryl-tRNA synthetase. Sep-tRNACys is then converted to Cys-tRNACys by Sep-tRNA:Cys-tRNA synthase (SepCysS) in the presence of an unknown sulfur donor. Genomic analyses revealed that this pathway is found in all organisms lacking CysRS as well as some other archaea that contain cysS. Genetic experiments demonstrated that this pathway is also the sole route for tRNA-dependent cysteine biosynthesis in some methanogens. The presence of Sep-tRNASec was established recently (Kaiser et al, Biochemistry 2005). As the conversions of Sep-tRNA to Cys-tRNA or Sec-tRNA are chemically analogous, the catalytic activity of SepCysS or of a similar enzyme may have provided the means by which selenocysteine evolved and was added to the genetic code. Experiments along these lines are currently in progress. (This work was supported by grants from NIGMS, National Institutes of Health, and the U. S. Department of Energy).
