“…Previous studies on the regulation of amino acid biosynthesis in Escherichia coli and Salmonella typhimurium have indicated that there are several different mechanisms by which amino acids regulate the synthesis of the enzymes in their respective biosynthetic pathways (7,25). In the histidine and branchedchain amino acid pathways, the aminoacyltransfer ribonucleic acids (tRNA's) have been implicated as corepressors (5,13,26), whereas in the aromatic amino acid pathway, the evidence has indicated that the free amino acids function as corepressors (12,14,19,20,23). hisT mutants of S. typhimurium are of particular interest because they contain normal levels of the aminoacyl-tRNA's, and yet the enzymes of the histidine and isoleucine-valine pathways are derepressed (6,8,10,21).…”