7 Aminoacyl-tRNA Synthetases and Self-acylating Ribozymes

Abstract

The aminoacyl-tRNA synthetases (aaRS) are at the heart of modern translation, catalyzing the accurate biosynthesis of aminoacyl-tRNAs (aa-tRNAs), the immediate precursors for encoded peptides. However, the first catalysts that made aa-RNAs for coded protein synthesis probably appeared long before any protein aaRS, to serve a preexisting translation system (see below). It presently seems likely that this ancestral translation system relied on a molecule like RNA, and the proto-aaRS catalysts may themselves have been RNAs. As an exercise in the limits of RNA catalysis, and as data relevant to the existence of such an RNA World, we would like to know how closely RNA catalysis can approximate the essential capabilities of the modern aaRS. Below is, first, a brief introduction to these complex modern proteins, then similarly a description of the presently known self-acylating RNAs, and finally an assessment of the question of possible resemblance.

AMINOACYL-tRNA SYNTHETASE EVOLUTION
Current aaRS are uniformly large proteins (Schimmel and Söll 1979) and therefore could not have existed before translation itself. In fact, because aaRS share essential structures, like the nucleotide-binding Rossman fold, among themselves and with other proteins (Eriani et al. 1990), they presumably were derived from yet more ancient common ancestors. Thus, most aaRS cannot even have been among the first proteins.

Aminoacyl-tRNA synthetases from all species can be split into two equally sized protein families, termed type I and type II proteins, characterized by two different sets of conserved amino acid motifs in the active sites for amino acid activation and aa-tRNA...