Abstract
We have investigated the consequences of two mutations, leu X and leu Y, which affect the level of leucyl-tRNA synthetase on the amount of tRNA and other aminoacyl-tRNA synthetases and on the expression of the ilv and leu operons in Escherichia coli . Neither mutation appears to alter the cellular concentrations of other aminoacyl-tRNA synthetases or tRNA isoacceptor families. It was shown that leucyl-tRNA rather than leucyl-tRNA synthetase is the defective control element responsible for derepression of the leu and ilv operon in leu S31 strains. Steady-state levels of leucine, isoleucine, valine, ppGpp, pppGpp and leucyl-tRNA, and the rate of protein synthesis have been measured in leu S + , leu S − , leu S − leu X − , and leu S − leu Y − strains. The levels of magic spot compounds appear to correlate with the extent that protein synthesis is arrested rather than the concentration of limiting leucyl-tRNA.
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