Cys-tRNACys formation and cysteine biosynthesis in methanogenic archaea:: two faces of the same problem?

Aminoacyl-tRNA ( transfer RNA) synthetases are essential components of the cellular translation machinery as they provide the ribosome with aminoacyl-tRNAs. Aminoacyl-tRNA synthesis is generally well understood. However, the mechanism of Cys-tRNACys formation in three methanogenic archaea (Methanocaldococcus jannaschii, Methanothermobacter thermautotrophicus and Methanopyrus kandleri) is still unknown, since no recognizable gene for a canonical cysteinyl-tRNA synthetase could be identified in the genome sequences of these organisms. Here we review the different routes recently proposed for Cys-tRNA(Cys) formation and discuss its possible link with cysteine biosynthesis in these methanogenic archaea.