Interpretation of tRNA-Mischarging Kinetics

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Abstract
Incorrect tRNA aminoacylation reactions are characterized by very slow reaction rates and by the fact that in most cases they are incomplete. In a previous study the incompleteness of the correct aminoacylation reactions of tRNA, which can be encountered under certain experimental conditions (for instance low enzyme concentration or high ionic strength) by an equilibrium between the aminoacylation and the deacylation reactions was explained. The present report presents evidence that the incorrect valylation of yeast .**GRAPHIC**. by yeast valyl-tRNA synthetase studied under standard experimental conditions, can also be described by a kinetic rate law including the rate equations of the aminoacylation and of the various deacylation reactions. It is shown that the incomplete mischarging plateaus reflect the existence of an equilibrium between the valylation reaction and the spontaneous and enzymic deacylation of .**GRAPHIC**. and the reverse of the valylation reaction. However, when the valyl-tRNA synthetase concentration is not very high the reverse reaction of the aminoacylation does not play a predominant part in the establishment of the plateau. These interpretations have been extended to other mischarging systems: valylation of yeast tRNAPhe by yeast valyl-tRNA synthetase and mischarging of .**GRAPHIC**. and .**GRAPHIC**. from yeast by yeast phenylalanyl-tRNA synthetase. Unusual mischarging kinetics were discussed. As in correct systems, during the mischarging of .**GRAPHIC**. 1 ATP is apparently hydrolyzed per tRNA charged with valine. The correct and the incorrect aminoacylation of tRNA apparently behave kinetically in a similar way.