Poster Sessions

To maintain high fidelity in translation, twenty kinds of aminoacyltRNA synthetases (ARSs) exist in general for twenty kinds of amino acids, each ARS being specialized to recognize only the cognate amino acid (A) and the cognate tRNA. Some organisms, however, have two genes for ThrRS, and considered that their proteins (ThrRS-1 and ThrRS-2) are complementary to each other in functions, one for catalysis and the other for editing. In order to clarify their three-dimensional structures, we started X-ray analyses of putatively assigned ThrRS-1 (APE0809) and ThrRS-2 (APE0117) from Aeropyrum pernix (Ap), and those (ST0966 and ST2187) from Sulfolobus tokodaii (St). These proteins were overexpressed in E. coli, purified, and crystallized. The crystal structure of ApThrRS-1 has been successfully determined at 2.3 Å resolution, as the first example. Ap-ThrRS-1 is a dimeric enzyme composed of the two identical subunits, each containing two domains for the catalytic reaction and for the anticodon-binding. The essential editing domain is, however, completely missing as expected. These structural features are consistent with that ThrRS-1 catalyze only the aminoacylation of the cognate tRNA, and suggest the necessity of the second enzyme ThrRS-2 for editing. Since the N-terminal sequence of Ap-ThrRS-2 is similar to the sequence of the editing domain of ThrRS from Pyrococcus abyssi, Ap-ThrRS-2 is expected to catalyze de-aminoacylation of the misacylated serine moiety at the CCA terminus.