The Tryptophanyl- and Tyrosyl-tRNA Synthetases from Bacillus stearothermophilus

Abstract

Aminoacyl-tRNA synthetases are ancient enzymes, as evidenced by their central role in metabolism (Granick 1950). They may well have evolved their specificities for amino acid and tRNA as the early cell developed pathways for amino acid synthesis and as the first codons were assigned to each amino acid (Wong 1975). Similarities between synthetases of different amino acid specificity might therefore reflect the mechanisms of earliest evolution. At first sight, the aminoacyl-tRNA synthetases seem to be a diverse group of enzymes with a range of quaternary structures (α₁, α₂, and α₂β₂) and subunit sizes (33,000–110,000 daltons) (see Table 1 of Hartley, this volume). Closer inspection reveals a loose class of large polypeptide chains centered on 100,000 daltons (and often monomeric) and a loose class of smaller chains centered on 50,000 daltons (and often dimeric). Peptide mapping, isolation of several tryptic peptides in greater than molar yield, and more detailed sequence studies have revealed that the large polypeptide chains contain areas of repeated sequence. (Kula 1973; Waterson and Konigsberg 1974; Koch et al. 1974). These larger chains could therefore have arisen by duplication and fusion of a common ancestral gene originally coding for the smaller chains. On the basis of this model, most synthetases would be composed of either a dimer of two identical chains or a monomer of fused domains. This arrangement would allow the binding of tRNA by contact of the pseudosymmetric regions on both tRNA and synthetase, each arm of the tRNA binding to corresponding portions of...