Regulation of branched-chain amino acid transport in Escherichia coli

Abstract

The repression and derepression of leucine, isoleucine and valine transport in E. coli K-12 was examined by using strains auxotrophic for leucine, isoleucine, valine and methionine. In experiments designed to limit each amino acid separately, leucine limitation alone derepressed the leucine-binding protein, the high-affinity branched-chain amino acid transport system (LIV-I), and the membrane-bound, low-affinity system (LIV-II). This regulation did not involve inactivation of transport components, but represented an increase in the differential rate of synthesis of transport components relative to total cellular proteins. The apparent regulation of transport by isoleucine, valine and methionine reported elsewhere required an intact leucine biosynthetic operon and resulted from changes in the level of leucine biosynthetic enzymes. A functional leucyl-tRNA synthetase was also required for repression of transport. Transport regulation was essentially independent of ilvA or its gene product, threonine deaminase. The central role of leucine or its derivatives in cellular metabolism in general is discussed.