IDENTITY SWITCHES BETWEEN TRANSFER-RNAS AMINOACYLATED BY CLASS-I GLUTAMINYL-TRANSFER-RNA AND CLASS-II ASPARTYL-TRANSFER-RNA SYNTHETASES

High-resolution X-ray structures for the tRNA/aminoacyl-tRNA synthetas e complexes between Escherichia coli tRNA(Gln)/GlnRS and yeast tRNA(As p)/AspRS have been determined. Positive identity nucleotides that dire ct aminoacylation specificity have been defined in both cases; E. coil tRNA(Gln) identity is governed by 10 elements scattered in the tRNA s tructure, while specific aminoacylation of yeast tRNA(Asp) is dependen t on 5 positions. Both identity sets are partially overlapping and sha re 3 nucleotides. Interestingly, the two enzymes belong to two differe nt classes described for aminoacyl-tRNA synthetases. The class I gluta minyl-tRNA synthetase and the class II aspartyl-tRNA synthetase recogn ize their cognate tRNA from opposite sides. Mutants derived from gluta mine and aspartate tRNAs have been created by progressively introducin g 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 synt hetase belonging to the other class. The simple transplantation of the glutamine identity set into tRNA(Asp) is sufficient to obtain glutami nylatable tRNA, but additional subtle features seem to be important fo r the complete conversion of tRNA(Gln) in an aspartylatable substrate. This study defines C38 in yeast tRNA(Asp) as a new identity nucleotid e for aspartylation. We show also in this paper that, during the compl ex formation, aminoacyl-tRNA synthetases are at least partially respon sible for conformational changes which involve structural constraints in tRNA molecules.