On the Interaction of Seryl-tRNA Synthetase with tRNASer. A Contribution to the Problem of Synthetase-tRNA Recognition

Abstract

By following the tryptophan fluorescence of yeast Saccharomyces cerevisiae seryl-tRNA synthetase on addition of tRNASer it was observed that the number of binding sites for tRNA decreases from 2 to 1 with increasing temperature, ATP or KCl concentration. Concomitantly a considerable decrease of the apparent binding constant was observed. The variation in the number of binding sites is explained by the presence of at least 1 temperature and ionic strength sensitive binding site and 1 temperature and ionic strength independent binding site. Relaxation kinetic experiments revealed 2 binding processes: a fast one depending on tRNA concentration and ionic strength and a slow one, which appeared to be independent of tRNA concentration and ionic strength. Enzyme kinetic studies showed that the activity of seryl-tRNA synthetase strongly depends on the KCl concentration and exhibits a maximum at 0.2 M KCl. Based on the data from relaxation and enzyme kinetic experiments a model is suggested for the recognition process involving a 1st unspecific step where all tRNAs, cognate and non-cognate, are bound to the synthetase (scanning step). The identification of the cognate tRNA is then performed at the recognition site by a conformational transition of the tRNA.cntdot.synthetase complex (identification step).