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°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 tRNAPhe preserved its native structure. In the Th. thermophilus system, the enzyme showed extreme thermostability (up to 90°C), and the reduction in the tRNA aminoacylation rate after the 78°C treatment was ascribed to denaturation of the tRNAPhe. Since the enzyme did not lose the [32P]PPi-ATP exchange activity up to 80°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.
