Leucine binding protein and regulation of transport in E. coli

Abstract

Leucine is transported into E. coli cells by high-affinity transport systems (LIV-I and leucine-specific systems) which are sensitive to osmotic shock and require periplasmic binding proteins. In addition leucine is transported by a low-affinity system (LIV-II) which is membrane bound and retained in membrane vesicle preparations. The LIV-I system serves for threonine and alanine in addition to the 3 branched-chain amino acids. The LIV-II system is more specific for leucine, isoleucine, and valine while the high-affinity leucine-specific system has the greatest specificity. A regulatory locus, livR at minute 22 on the E. coli chromosome produces negatively regulated leucine transport and synthesis of the binding proteins. Valine-resistant strains have been selected to screen for transport mutants. High-affinity leucine transport mutants that have been identified include a LIV-binding protein mutant, livJ , a leucine-specific binding protein mutant livK and a nonbinding protein component of the LIV-I system, livH. A fourth mutant, livP , appears to be required only for the low-affinity LIV-II system. The existence of this latter mutant indicates that LIV-I and LIV-II are parallel transport systems. The 4 mutations concerned with high-affinity leucine transport form a closely linked cluster of genes on the E. coli chromosome at minute 74. The results of recent studies on the regulation of the high-affinity transport systems suggests that an attenuator site may be operative in its regulation. This complex regulation appears to require a modified leucyl-tRNA along with the transcription termination factor rho. Regulation of leucine transport is also defective in relaxed strains. Among the branched-chain amino acids only leucine produces regulatory changes in LIV-I activity suggesting a special role of this amino acid in the physiology of E. coli. It was shown that the rapid exchange of external leucine for intracellular isoleucine via the LIV-I system could create an isolucine pseudoauxotrophy and account for the leucine sensitivity of E. coli.