Genetic and biochemical studies of transport systems for branched-chain amino acids in Escherichia coli.

Mutants of Escherichia coli K-12 requiring high concentrations of branched-chain amino acids for growth were isolated. One of the mutants was shown to be defective in transport activity for branched-chain amino acids. The locus of the mutation (hrbA) was mapped at 8.9 min on the E. coli genetic map by conjugational and transductional crosses. The gene order of this region is proC-hrbA-tsx. The hrbA system was responsible for the uptake activity of cytoplasmic membrane vesicles. It was not repressed by leucine. The substrate specificities and kinetics of the uptake activities were studied using cytoplasmic membrane vesicles and intact cells of the mutants grown in the presence or absence of leucine. Results showed that there are three transport systems for branched-chain amino acids, LIV-1, -2, and -3. The LIV-2 and -3 transport systems are low-affinity systems, the activities of which are detectable in cytoplasmic membrane vesicles. The systems are inhibited by norleucine but not by threonine. The LIV-2 system is also repressed by leucine. The LIV-1 transport system is a high-affinity system that is sensitive to osmotic shock. When the leucine-isoleucine-valine-threonine-binding protein is derepressed, the high-affinity system can be inhibited by threonine.