ASPARTATE IDENTITY OF TRANSFER-RNAS

Structure/function relationships accounting for specific tRNA charging by class II aspartyl-tRNA synthetases from Saccharomyces cerevisiae, Escherichia coli and Thermus thermophilus are reviewed. Effects direct ly linked to tRNA features are emphasized and aspects about synthetase contribution in expression of tRNA(Asp) identity are also covered. Ma jor identity nucleotides conferring aspartate specificity to yeast, E coli and T thermophilus tRNAs comprise G34, U35, C36, C38 and G73, a s et of nucleotides conserved in tRNA(Asp) molecules of other biological origin. Aspartate specificity can be enhanced by negative discriminat ion preventing, eg mischarging of native yeast tRNA(Asp) by yeast argi nyl-tRNA synthetase. In the yeast system crystallography shows that id entity nucleotides are in contact with identity amino acids located in the catalytic and anticodon binding domains of the synthetase. Specif icity of RNA/protein interaction involves a conformational change of t he tRNA that optimizes the H-bonding potential of the identity signals on both partners of the complex. Mutation of identity nucleotides lea ds to decreased aspartylation efficiencies accompanied by a loss of sp ecific H-bonds and an altered adaptation of tRNA on the synthetase. Sp ecies-specific characteristics of aspartate systems are the number, lo cation and nature of minor identity signals. These features and the st ructural variations in aspartate tRNAs and synthetases are correlated with mechanistic differences in the aminoacylation reactions catalyzed by the various aspartyl-tRNA synthetases. The reality of the aspartat e identity set is verified by its functional expression in a variety o f RNA frameworks. Inversely a number of identities can be expressed wi thin a tRNA(Asp) framework. From this emerged the concept of the RNA s tructural frameworks underlying expression of identities which is illu strated with data obtained with engineered tRNAs. Efficient aspartylat ion of minihelices is explained by the primordial role of G73. From th is and other considerations it is suggested that aspartate identity ap peared early in the history of tRNA aminoacylation systems.