The levels of glutamate synthase and of glutamine synthetase are both derepressed 10-fold in strain JP1449 of Escherichia coli carrying a thermosensitive mutation in the glutamyl-transfer ribonucleic acid (tRNA) synthetase and growing exponentially but at a reduced rate at a partially restrictive temperature, compared with the levels in strain AB347 isogenic with strain JP1449 except for this thermosensitive mutation and the marker aro. These two enzymes catalyze one of the two pathways for glutamate biosynthesis in E. coli, the other being defined by the glutamate dehydrogenase. We observed a correlation between the percentage of charged tRNAGlu and the level of glutamate synthase in various mutants reported to have an altered glutamyl-tRNA synthetase activity. These results suggest that a glutamyl-tRNA might be involved in the repression of the biosynthesis of the glutamate synthase and of the glutamine synthetase and would couple the regulation of the biosynthesis of these two enzymes, which can work in tandem to synthesize glutamate when the ammonia concentration is low in E. coli but whose structural genes are quite distant from each other. No derepression of the level of the glutamate dehydrogenase was observed in mutant strain JP1449 under the conditions where the levels of the glutamine synthetase and of the glutamate synthase were derepressed. This result indicates that the two pathways for glutamate biosynthesis in E. coli are under different regulatory controls. The glutamate has been reported to be probably the key regulatory element of the biosynthesis of the glutamate dehydrogenase. Our results indicate that the cell has chosen the level of glutamyl-tRNA as a more sensitive probe to regulate the biosynthesis of the enzymes of the other pathway, which must be energized at a low ammonia concentration.