ATP-Induced Specificity of the Binding of Serine tRNAs from Rat Liver to Seryl-tRNA Synthetase from Yeast

The binding of yeast tRNAserII, rat liver tRNAserI and tRNAserIII to yeast seryl-tRNA synthetase has been investigated by use of the tryptophan fluorescence quenching of the enzyme. Whereas one tRNAser binding site was detected per subunit of the enzyme in the homologous system, about 5 tRNAser molecules per subunit can be bound in the heterologous system in absence of ATP. This unspecific binding of both rat liver tRNAser to the yeast enzyme occurs with a 2–10-fold lower binding constant compared to yeast tRNAserII. If the enzyme binding sites for ATP are saturated, a subsequent binding of yeast tRNAser results in only a slightly increased intrinsic association constant but does not influence the number of binding sites. Under the same conditions, a strict 1:1 stochiometry for both rat liver tRNAs per subunit of the yeast enzyme is observed paralledled by a strong increase in the binding constant. The same specificity is also induced by ATP in the course of the aminoacylation reaction since Michaelis-Menten kinetics of the three tRNAs are very similar. CTP and UTP do not induce any change in the number of binding sites or the association constant in a subsequent binding of rat liver tRNAser to the yeast enzyme. Thus, ATP is not only crucial in the amino-acid activation but plays, in addition, an important role in the recognition process.