A EURYARCHAEAL LYSYL-TRANSFER-RNA SYNTHETASE - RESEMBLANCE TO CLASS-ISYNTHETASES

The sequencing of euryarchaeal genomes has suggested that the essentia l 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 g ene that encodes this protein was cloned, The predicted amino acid seq uence of M. maripaludis LysRS is similar to open reading frames of una ssigned function in both Methanobacterium thermoautotrophicum and Meth anococcus jannaschii but is unrelated to canonical LysRS proteins repo rted in eubacteria, eukaryotes, and the crenarchaeote Sulfolobus solfa taricus. The presence of amino acid motifs characteristic of the Rossm ann dinucleotide-binding domain identifies M. maripaludis LysRS as a c lass 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.