[Phenylalanyl-tRNA synthetase of wheat embryos. Purification, molecular weight, structure, properties]

Recombinant expression and purification of phenylalanyl-tRNA synthetase from wheat: a long-lasting poly(U)-dependent poly(Phe) synthesis system

Synthetic genes for the two subunits of phenylalanyl-tRNA synthetase (PheRS) from wheat were expressed in Escherichia coli. When each gene was induced individually, the α subunit with a cleavable 6 × His tag at the amino terminus was largely soluble, while the β subunit was almost completely insoluble. When the two subunits were co-expressed, a soluble fraction containing the two subunits were obtained. This was purified by a standard method in which the tag was cleaved off with a specific protease after affinity purification. As the sample contained slightly more PheRSα than PheRSβ, we further resolved the sample by gel filtration to obtain the fraction that showed the size of the conventional α2β2 tetrameric complex and contains an almost equal amount of the two subunits. The final yield was 0.6 mg per 1 liter of the culture medium, and the specific activity was 28 nmol min-1 mg-1, which was higher than that of a fraction purified from wheat germ. This recombinant PheRS was used, along with purified samples of the elongation factors and the ribosomes from wheat germ, for a poly(U)-dependent poly(Phe) synthesis reaction. The reaction was dependent on the added components and lasted for more than several hours.

Phenylalanyl-tRNA synthetases as an example for comparative and evolutionary aspects of aminoacyl-tRNA synthetases

Aminoacyl-tRNA synthetases are indispensable components of protein synthesis in all three lines of evolutionary descent, eubacteria, archaebacteria and eukaryotes. Furthermore they are also present in the translational apparatus of the semi-autonomous organelles, mitochondria and chloroplasts, of the eukaryotic cell. Therefore aminoacyl-tRNA synthetases are appropriate objects for comparative molecular biology in order to obtain a comprehensive picture of the evolution of the translational process. The analysis of the phenylalanyl-tRNA synthetase in a large variety of organisms and organelles in this respect is the most advanced. In addition to comparison of quaternary structure, analysis includes functional aspects of accuracy mechanisms (proofreading) and comparison of structural features by means of substrate analogs. Evolutionary relationships are furthermore elucidated using the immunological approach and heterologous aminoacylation.

Purification and subunit structure studies of human placental threonyl-tRNA synthetase

Human threonyl-tRNA synthetase has been purified from full-term placenta over 5000-fold with a 13% overall yield by a combination of affinity chromatographic methods and conventional procedures. The product was apparently homogeneous by the criterion of sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The purified enzyme catalyzed the esterification of approximately 3500 nmol L-threonine to tRNA per min per mg of protein at 37 degrees, corresponding to a molecular activity of approximately 700/min. The apparent molecular weight of the native enzyme ranged from 210000 to 220000 by gel-filtration on Sephadex G-200, and was either approximately 110000 or 228000 by sucrose gradient centrifugation for different preparations. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis demonstrated a single band of molecular weight 85000 or 115000 for different preparations. These results suggest that human placental threonyl-tRNA synthetase has a subunit structure of the type alpha 2 with a subunit molecular weight of about 100000. However, some other molecular forms with lower specific activity were also found to exist under certain conditions.