IDENTITY ELEMENTS FOR SPECIFIC AMINOACYLATION OF YEAST TRANSFER RNAASP BY COGNATE ASPARTYL-TRANSFER RNA-SYNTHETASE

The nucleotides crucial for the specific aminoacylation of yeast tRNA(Asp) by its cognate synthetase have been identified. Steady-state aminoacylation kinetics of unmodified tRNA transcripts indicate that G34, U35, C36, and G73 are important determinants of tRNA(Asp) identity. Mutations at these positions result in a large decrease (19- to 530-fold) of the kinetic specificity constant (ratio of the catalytic rate constant k(cat) and the Michaelis constant K(m)) for aspartylation relative to wild-type tRNA(Asp). Mutation to G10-C25 within the D-stem reduced k(cat)/K(m) eightfold. This fifth mutation probably indirectly affects the presentation of the highly conserved G10 nucleotide to the synthetase. A yeast tRNA(Phe) was converted into an efficient substrate for aspartyl-tRNA synthetase through introduction of the five identity elements. The identity nucleotides are located in regions of tight interaction between tRNA and synthetase as shown in the crystal structure of the complex and suggest sites of base-specific contacts.