Synthesis of cysteinyl-tRNA(Cys) by a genome that lacks the normal cysteine-tRNA synthetase

Synthesis of cysteinyl-tRNA(Cys) by cysteine-tRNA synthetase is required fo r decoding cysteine codons in all known organisms. The genome of the archae on Methanococcus jannaschii lacks the gene for a normal cysteine-tRNA synth etase. The activity of the enzyme, however, was identified recently, and it allowed the purification of the enzyme and cloning of its gene. Sequence a nalysis of the gene showed that it encodes proline-tRNA synthetase and, thu s, raised the possibility of dual activities in a single aminoacyl-tRNA syn thetase. Assays of aminoacyl-adenylate synthesis confirmed the ability of t he enzyme to activate proline and cysteine and showed that both activities were independent of tRNA. Assays of tRNA aminoacylation established the spe cific attachment of proline to tRNA(Pro) and cysteine to tRNA(Cys). However , in contrast to a recent report of comparable activities with cysteine and proline, results here indicate that the adenylate synthesis and aminoacyla tion activities with cysteine are significantly lower than the respective a ctivity with proline. In addition, there is evidence of overlapping amino a cid-binding sites and tRNA-binding sites. These considerations, among other s, raised the distinct possibility that the M. jannaschii proline-tRNA synt hetase may recruit additional protein or RNA factors to facilitate the synt hesis of cysteinyl-tRNA(Cys).