A comparative study of the relationship between thermostability and function of phenylalanyl‐tRNA synthetases from Escherichia coli and Thermus thermophilus

The relationship between thermostability and functional activities of phenylalanyl-tRNA synthetases (EC 6.1.1.20) from E. coli and Thermus thermophilus has been studied. In the case of the E. coli enzyme, the activity decreased after the 43 degrees C treatment, both in the [32P]PPi-ATP exchange reaction and the overall aminoacylation reaction, due to thermo-inactivation of the phenylalanyl-tRNA synthetase, whereas tRNA(Phe) preserved its native structure. In the Th. thermophilus system, the enzyme showed extreme thermostability (up to 90 degrees C), and the reduction in the tRNA aminoacylation rate after the 78 degrees C treatment was ascribed to denaturation of the tRNA(Phe). Since the enzyme did not lose the [32P]PPi-ATP exchange activity up to 85 degrees C, the observed lower thermo-resistance of the tRNA is evidence that the native structure of ribonucleic acids should be one of the most difficult to stabilize at high temperatures.