Abstract
Isoleucyl-tRNA synthetase has been purified to homogeneity from Mycobacterium smegmatis. The influence of spermine on the kinetics of valyl-tRNA and isoleucyl-tRNA formation has been investigated by Cleland's method (Cleland, W.W. (1963) Biochim. Biophys. Acta 67, 104–137, 173–187, 188–196). The results suggest that in the presence of spermine and suboptimal concentration of Mg 2+, the formation of valyl-tRNA and isoleucyl-tRNA follows a sequential ∗ ∗ In Cleland's nomenclature ‘sequential’ mechanism is equivalent to the ‘concerted’ mechanism used by others for aminoacylation. The ‘ping-pong’ mechanism of Cleland is equivalent to ‘step-wise’ or ‘sequential’ mechanism as used for aminoacylation. In the summary and results, Cleland's nomenclature is used; in discussion, a general one is used. mechanism. In the presence of an optimal concentration of Mg 2+, both valyl-tRNA and isoleucyl-tRNA formation proceeds by a ping-pong mechanism. However, in the presence of spermine and optimal concentrations of Mg 2+, valyl-tRNA formation follows the ping-pong mechanism while isoleucyl-tRNA formation follows the sequential mechanism.
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