Transfer RNA Aminoacylation and Modified Nucleosides
Richard Giegé and Jacques Lapointe
Among RNAs, the transfer RNAs are those showing the highest level of posttranscriptional modifications. After an overview on early data, the chapter discusses the present knowledge on the role modified nucleosides have on tRNA structure and function with emphasis on tRNA aminoacylation. The concept of tRNA aminoacylation identity will be outlined and the cases discussed where individual modified nucleosides act either as positive determinants (for recognition by the cognate synthetases) or negative antideterminants (preventing recognition by a noncognate synthetase). Furthermore, the collective participation of the ensemble of modified nucleosides in a given tRNA will also be analyzed. Evolutionary aspects will be illustrated by the unprecedented property of a paralog of bacterial glutamyl‑tRNA synthetase restricted to the catalytic module of the synthetase, that aminoacylates the Q‑base of bacterial tRNAAsp. This has evolutionary implications suggesting that modern tRNA originated by duplication of an ancestral minihelix and finds support with the existence of sequence similarities between the anticodon stem‑loop of tRNAAsp and the accepting end of tRNAGlu. Altogether and contrarily to a common belief, posttranscriptional modifications in tRNA play an active role in a majority of aminoacylation systems, although in many cases by indirect structure‑dependent effects.