This proposed project will involve efforts to determine the mechanism for regulation of synthesis of aminoacyl-tRNA synthetases of Escherichia coli and Salmonella typhimurium. At present, we have provided evidence for regulation of synthesis for nine of the twenty aminoacyl-tRNA synthetases. The formation of these enzymes is regulated by a repression process in a parallel, but non-coordinate manner with the cognate amino acid biosynthetic enzymes, and aminoacylated-tRNA appears to be the functional co-repressor, at least, for the histidine, arginine and methionine synthetases. Furthermore, we obtained evidence which suggest that the regulation of histidyl-, valyl-, leucyl- and isoleucyl-tRNA synthetase formation is altered in regulatory mutants which exhibited altered repression control of each of the cognate amino acids.