The structural features of the G.U wobble pair in Escherichia coli alanine transfer RNA (tRNA(Ala)) that are associated with aminoacylation by alanyl-tRNA synthetase (AlaRS) were investigated in vivo for wild-type tRNA(Ala) and mutant tRNAs with G.U substitutions. tRNA(Ala) with G.U, C.A, or G.A gave similar amounts of charged tRNA(Ala) and supported viability of E. coli lacking chromosomal tRNA(Ala) genes. tRNA(Ala) with G.C was inactive. Recognition of G.U by AlaRS thus requires more than the functional groups on G.U in a regular helix and may involve detection of a helical distortion.

The 2.6 A resolution crystal structure of an inactive complex between yeast tRNA(Asp) and Escherichia coli aspartyl-tRNA synthetase reveals the molecular details of a tRNA-induced mechanism that controls the specificity of the reaction. The dimer is asymmetric, with only one of the two bound tRNAs entering the active site cleft of its subunit. However, the flipping loop, which controls the proper positioning of the amino acid substrate, acts as a lid and prevents the correct positioning of the terminal adenosine. The structure suggests that the acceptor stem regulates the loop movement through sugar phosphate backbone- protein interactions. Solution and cellular studies on mutant tRNAs confirm the crucial role of the tRNA three-dimensional structure versus a specific recognition of bases in the control mechanism.

The previously uncharacterized determinants of the specificity of tRNAPro for aminoacylation (tRNAPro identity) were defined by a computer comparison of all Escherichia coli tRNA sequences and tested by a functional analysis of amber suppressor tRNAs in vivo. We determined the amino acid specificity of tRNA by sequencing a suppressed protein and the aminoacylation efficiency of tRNA by examining the steady-state level of aminoacyl-tRNA. On substituting nucleotides derived from the acceptor end and variable pocket of tRNAPro for the corresponding nucleotides in a tRNAPhe gene, the identity of the resulting tRNA changed substantially but incompletely to that of tRNAPro. The redesigned tRNAPhe was weakly active and aminoacyl-tRNA was not detected. Ethyl methanesulfonate mutagenesis of the redesigned tRNAPhe gene produced a mutant with a wobble pair in place of a base pair in the end of the acceptor-stem helix of the transcribed tRNA. This mutant exhibited both a tRNAPro identity and substantial aminoacyl-tRNA. The results speak for the importance of a distinctive conformation in the acceptor-stem helix of tRNAPro for aminoacylation by the prolyl-tRNA synthetase. The anticodon also contributes to tRNAPro identity but is not necessary in vivo.

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We have identified six new aminoacylation determinants of Escherichia coli tRNA Gln in a genetic and biochemical analysis of suppressor tRNA. The new determinants occupy the interior of the acceptor stem, the inside corner of the L shape, and the anticodon loop of the molecule. They supplement the primary determinants located in the anticodon and acceptor end of tRNA Gln described previously. Remarkably, the three-dimensional structure of the complex between tRNA Gln and glutaminyl-tRNA synthetase shows that the enzyme interacts with the phosphate-sugar backbone but not the base of every new determinant. Moreover, a small protein motif interacts with five of these determinants, and it binds proximal to the sixth. The motif also interacts with the middle base of the anticodon and with the backbones of six other nucleotides. Our results emphasize that synthetase recognition of tRNA is more elaborate than amino acid side chains of the enzyme interacting with nucleotide bases of the tRNA. Recognition also includes synthetase interaction with tRNA backbone functionalities whose distinctive locations in three-dimensional space are exquisitely determined by the tRNA sequence.

The G x U pair at the third position in the acceptor helix of Escherichia coli tRNA(Ala) is critical for aminoacylation. The features that allow G x U recognition are likely to include direct interaction of alanyl-tRNA synthetase with distinctive atomic groups and indirect recognition of the structural and stability information encoded in the sequence of G x U and its immediate context. The present work investigates the thermodynamic stability and acceptor activity for a comprehensive set of variant RNAs with substitutions of the G x U pair of E. coli tRNA(Ala). The four RNAs with Watson-Crick substitutions had a lower acceptor activity and a higher stability relative to the G x U RNA. On the other hand, the RNAs with mispair substitutions had a lower stability, but either a higher or a lower acceptor activity. Thus, the entire set of variant RNAs does not exhibit a correlation between thermodynamic stability of the free, unbound tRNA and its acceptor activity. The substantial acceptor activity of tRNAs with particular mispair substitutions may be explained by their ability to assume the conformational preferences of alanyl-tRNA synthetase. Moreover, the G x U pair may provide a point of deformability for the substrate tRNA to adapt to the enzyme's active site.

Protein-RNA recognition between aminoacyl-tRNA synthetases and tRNA is highly specific and essential for cell viability. We investigated the structure-function relationships involved in the interaction of the Escherichia coli tRNA Asp acceptor stem with aspartyl-tRNA synthetase. The goal was to isolate functionally active mutants and interpret them in terms of the crystal structure of the synthetase-tRNA Asp complex. Mutants were derived from Saccharomyces cerevisiae tRNA Asp , which is inactive with E. coli aspartyl-tRNA synthetase, allowing a genetic selection of active tRNAs in a tRNA Asp knockout strain of E. coli. The mutants were obtained by directed mutagenesis or library selections that targeted the acceptor stem of the yeast tRNA Asp gene. The mutants provide a rich source of tRNA Asp sequences, which show that the sequence of the acceptor stem can be extensively altered while allowing the tRNA to retain substantial aminoacylation and cell-growth functions. The predominance of tRNA backbone-mediated interactions observed between the synthetase and the acceptor stem of the tRNA in the crystal and the mutability of the acceptor stem suggest that many of the corresponding wild-type bases are replaceable by alternative sequences, so long as they preserve the initial backbone structure of the tRNA. Backbone interactions emerge as an important functional component of the tRNA-synthetase interaction.

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