| Abstrakt: |
To test the hypothesis that tRNATyrrecognition differs between bacterial and human tyrosyl-tRNA synthetases, we sequenced several clones identified as human tyrosyl-tRNA synthetase cDNAs by the Human Genome Project. We found that human tyrosyl-tRNA synthetase is composed of three domains: 1) an amino-terminal Rossmann fold domain that is responsible for formation of the activated E·Tyr-AMP intermediate and is conserved among bacteria, archeae, and eukaryotes; 2) a tRNA anticodon recognition domain that has not been conserved between bacteria and eukaryotes; and 3) a carboxyl-terminal domain that is unique to the human tyrosyl-tRNA synthetase and whose primary structure is 49% identical to the putative human cytokine endothelial monocyte-activating protein II, 50% identical to the carboxyl-terminal domain of methionyl-tRNA synthetase from Caenorhabditis elegans, and 43% identical to the carboxyl-terminal domain of Arc1p from Saccharomyces cerevisiae. The first two domains of the human tyrosyl-tRNA synthetase are 52, 36, and 16% identical to tyrosyl-tRNA synthetases from S. cerevisiae,Methanococcus jannaschii,and Bacillus stearothermophilus, respectively. Nine of fifteen amino acids known to be involved in the formation of the tyrosyl-adenylate complex in B. stearothermophilusare conserved across all of the organisms, whereas amino acids involved in the recognition of tRNATyrare not conserved. Kinetic analyses of recombinant human and B. stearothermophilustyrosyl-tRNA synthetases expressed in Escherichia coliindicate that human tyrosyl-tRNA synthetase aminoacylates human but not B. stearothermophilustRNATyr, and vice versa, supporting the original hypothesis. It is proposed that like endothelial monocyte-activating protein II and the carboxyl-terminal domain of Arc1p, the carboxyl-terminal domain of human tyrosyl-tRNA synthetase evolved from gene duplication of the carboxyl-terminal domain of methionyl-tRNA synthetase and may direct tRNA to the active site of the enzyme. |
