Abstract: High-resolution X-ray structures for the tRNA/aminoacyl-tRNA synthetase complexes between Escherichia coli tRNAGln/GlnRS and yeast tRNAAsp/AspRS have been determined. Positive identity nucleotides that direct aminoacylation specificity have been defined in both cases; E. coli tRNAGln identity is governed by 10 elements scattered in the tRNA structure, while specific aminoacylation of yeast tRNAAsp is dependent on 5 positions. Both identity sets are partially overlapping and share 3 nucleotides. Interestingly, the two enzymes belong to two different classes described for aminoacyl-tRNA synthetases. The class I glutaminyl-tRNA synthetase and the class II aspartyl-tRNA synthetase recognize their cognate tRNA from opposite sides. Mutants derived from glutamine and aspartate tRNAs have been created by progressively introducing identity elements from one tRNA into the other one. Glutaminylation and aspartylation assays of the transplanted tRNAs show that identity nucleotides from a tRNA originally aminoacylated by a synthetase from one class are still recognized if they are presented to the enzyme in a structural framework corresponding to a tRNA aminoacylated by a synthetase belonging to the other class. The simple transplantation of the glutamine identity set into tRNAAsp is sufficient to obtain glutaminylatable tRNA, but additional subtle features seem to be important for the complete conversion of tRNAGln in an aspartylatable substrate. This study defines C38 in yeast tRNAAsp as a new identity nucleotide for aspartylation. We show also in this paper that, during the complex formation, aminoacyl-tRNA synthetases are at least partially responsible for conformational changes which involve structural constraints in tRNA molecules.