Incorrect Aminoacylations Catalysed by the Phenylalanyl‐ and Valyl‐tRNA Synthetases from Yeast

Abstract

It has been found that purified phenylalanyl‐ and valyl‐tRNA synthetases from yeast catalyse the mischarging of numerous heterologous Escherichia coli and even of homologous yeast tRNAs when special aminoacylation conditions are used. For instance E. coli tRNAAla, tRNAVal, tRNALys and yeast tRNAAla, tRNAVal can be incorrectly aminoacylated by yeast phenylalanyl‐tRNA synthetase, whereas E. coli tRNAAla, tRNAMet, tRNAIle, tRNAThr and yeast tRNAAla, tRNAPhe can be mischarged by yeast valyl‐tRNA synthetase. The production of errors is highly dependant on the experimental conditions used, as their number and their level is increased when the Mg2+/ATP ratio or the enzyme concentration in the medium is increased or when the reactions are performed in the presence of dimethylsulfoxide. In the case of phenylalanyl‐tRNA synthetase, conditions can be found where practically all E. coli tRNAs are wrongly aminoacylated, although at different levels. On the other hand, in the case of homologous systems some errors can only be detected when purified tRNAs are used, thus suggesting, when total tRNA is used, a competition between the cognate and non‐cognate tRNAs which minimises the mischarging. The comparison of the sequences of the cognate and non cognate tRNAs which are aminoacylated either by the yeast phenylalanyl‐ or valyl‐tRNA synthetase led us to ascribe some importance to several regions inside of these tRNAs, for instance (a) the dihydrouracil arm, (b) the terminal part of the amino‐acid acceptor stem and (c) the extra‐loop. These regions should be essentially necessary for the establisment of the correct tri‐dimensional conformation necessary for the recognition by the aminoacyl‐tRNA synthetases. Copyright © 1972, Wiley Blackwell. All rights reserved