Leucine binding protein and regulation of transport in E. coli.

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-11) 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-I1 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 2 2 on the E. coli chromosome produces negatively regulated leucine transport and synthesis of the binding proteins. Valineresistant 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 l ivK and a nonbinding protein component of the LIV-I system, l i v l f . A fourth mutant, / ivP, appears to be required only for the low-affinity LIV-I1 system. The existence of this latter mutant indicates that LIV-I and LIV-11 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.