RECOGNITION OF TRANSFER RNATYR BY TYROSYL-TRANSFER RNA-SYNTHETASE

In this review, I have brought together and compared the available data on the interaction between tRNATyr and tyrosyl-tRNA synthetases (TyrTS) of prokaryotic origins. The amino acid sequences of the heterologous TyrTS that can charge Escherichia coli tRNATyr, show that the residues involved in the binding and recognition of tyrosine are strictly conserved whereas those involved in the interaction with tRNATyr are only weakly similar. The results of in vivo genetic complementation experiments indicate that the identity elements of tRNAs and the recognition mechanisms of such elements by the synthetases have been conserved during evolution. Heterologous or mutant tRNATyr are quantitatively charged by E coli TyrTS; the set of their common residues contains less than 10 elements if one excludes the invariant and semi-invariant residues of tRNAs. The residues of this set are candidates for a specific recognition by TyrTS. So far, adenosine-73 is the only residue for which a specific recognition of the base has been demonstrated. The residues that might serve as identify elements for E coli tRNATyr [McClain WH, Nicholas Jr HB (1987) J Mol Biol 194, 635-642] do not belong to the above set of conserved residues and therefore probably play negative roles, enabling tRNATyr to avoid non-cognate synthetases. Comparison of the charging and stability properties of mutant tRNATyrsu+3 shows that bases 1 and 72 must pair (either by Watson-Crick or non-canonical hydrogen bonds) and adopt a geometry which is compatible with the helical structure of the acceptor stem in order for the mutant tRNATyr to be charged with tyrosine. If bases 1 and 72 or bases 2 and 71 cannot form such pairings, the suppressor phenotype of the mutant tRNATyrsu+3 becomes thermosensitive. The weakening of base pair 1 / 72 by mutation or the change of adenosine-73 into guanosine results in the charging of tRNATyrsu+3 with glutamine. Comparison of the structural model of the TyrTS/tRNATyr complex with the crystallographic structure of the GlnTS/tRNAGln complex indicates that the mechanisms for the recognition of the acceptor arm are different in the 2 cases. Chemical attack and molecular modeling experiments have indicated that the acceptor end of tRNATyr ... CCA3′-OH, remains mobile after the initial binding of tRNATyr to TyrTS. © 1990.