Abstract
Threonyl-tRNA synthetase (ThrRS) has been isolated from an extreme thermophile Thermus thermophilus strain HB8. The enzyme was purified to electrophoretic homogeneity by combinations of column chromatographies on DEAE-Sepharose, S-Sepharose, ACA-44 Ultrogel and HA-Ultrogel. Seventeen mg of purified enzyme were obtained from 1 kg of biomass. In parallel, purified aspartyl- and phenylalanyl-tRNA synthetases were obtained. The purified ThrRS is composed of two identical subunits with a molecular mass of about 77 000 (virtually the same as E coli ThrRS). The N-terminal sequence has been determined. The homology between the first 45 amino acid residues of ThrRS from T thermophilus and E coli is about 29%. A comparative study of tRNA Thr charging by ThrRS from E coli and T thermophilus reveals a similar efficiency of the reaction in both homologous systems. This efficiency remains unchanged for aminoacylation of tRNA Thr from T thermophilus by the heterologous ThrRS from E coli, but decreases 700 times for aminoacylation of E coli tRNA Thr by ThrRS from T thermophilus.
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