A euryarchaeal lysyl-tRNA synthetase: resemblance to class I synthetases

The sequencing of euryarchaeal genomes has suggested that the essential protein lysyl-transfer RNA (tRNA) synthetase (LysRS) is absent from such organisms. However, a single 62-kilodalton protein with canonical LysRS activity was purified from Methanococcus maripaludis, and the gene that encodes this protein was cloned. The predicted amino acid sequence of M. maripaludis LysRS is similar to open reading frames of unassigned function in both Methanobacterium thermoautotrophicum and Methanococcus jannaschii but is unrelated to canonical LysRS proteins reported in eubacteria, eukaryotes, and the crenarchaeote Sulfolobus solfataricus. The presence of amino acid motifs characteristic of the Rossmann dinucleotide-binding domain identifies M. maripaludis LysRS as a class I aminoacyl-tRNA synthetase, in contrast to the known examples of this enzyme, which are class II synthetases. These data question the concept that the classification of aminoacyl-tRNA synthetases does not vary throughout living systems.