Abstract
The aspartate identity of tRNA for AspRS from Thermus thermophilus has been investigated by kinetic analysis of the aspartylation reaction of different tRNA molecules and their variants as well as of tRNA super(Phe) variants with transplanted aspartate identity elements. It is shown that G sub(10), G sub(34), U sub(35), C sub(36), C sub(38), and G sub(73) determine recognition and aspartylation of yeast and T. thermophilus tRNA super(Asp) by the thermophilic AspRS. This set of nucleotides specifies also tRNA aspartylation in the homologous yeast and Escherichia coli systems. Structural considerations indicate that the major aspartate identity elements interact with amino acids conserved in all AspRSs. It follows that the structural features of tRNA and synthetase specifying aspartylation are mainly conserved in various structural contexts and in organisms adapted to different life conditions. Mutations of tRNA identity elements provoke drastic losses of charging in the heterologous system involving yeast tRNA super(Asp) and T. thermophilus AspRS. In the homologous systems, the mutational effects are less pronounced. However, effects in E. coli and T. thermophilus exceed those in yeast which are particularly moderate, indicating variations in the individual contributions of identity elements for aspartylation in prokaryotes and eukaryotes. Analysis of multiple tRNA mutants reveals cooperativity between the cluster of determinants of the anticodon loop and the additional determinants G sub(10) and G sub(73) for efficient aspartylation in the thermophilic system, suggesting that conformational changes trigger formation of the functional tRNA/synthetase complex.