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.