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.

Regulation of peptide transport in Escherichia coli: induction of the trp-linked operon encoding the oligopeptide permease

“…2, band 3) was unaffected by inclusion of leucine in the growth medium (Table 5), whereas synthesis of the LIV-binding protein (Fig. 2, band 2), the product of the livJ gene (16), was repressed upon the addition of leucine to the E. coli cultures as has been previously described (17,19). The magnitude of the leucine-specific increase in OppA protein shown in Fig.…”

Cloning and characterization of livH, the structural gene encoding a component of the leucine transport system in Escherichia coli

“…These conditions provided a selection for random TnS insertions either in the chromosome or in plasmid pOX1 since Tetr selects for pOX1 and Kanr selects To confirm these results, we measured the high-affinity L-leucine transport activities of these transformants. Strain AE205(pOX1:: Tn5) transformants with a defective livJ gene would be expected to show a normal L-leucine transport activity due to the presence of the high-affinity leucinespecific transport system (4), while AE205(pOX1:: Tn5) transformants with a defective livH gene would be expected to have very low L-leucine transport activity since the livH gene is a common component for both of the high-affinity transport systems (4,29). By using these screening methods, we identified both livJ:: TnS and livH:: Tn5 insertions on pOX1.…”

“…The second system is a general system, designated LIV-I, and utilizes the leucineisoleucine-valine (LIV) binding protein, the livJ gene product. Genetic analysis has indicated that both high-affinity transport systems require at least two additional components, the products of the livH and livG genes (4,24,29). All four genes, livJ, livK, livH, and livG, are clustered at minute 76 on the E. coli linkage map.…”

A regulatory livR mutation affecting high- and low-affinity transport systems for branched-chain amino acids in Salmonella typhimurium.

“…Unequal recombination between these largely homologous regions can lead to rearrangements of the chromosome and causes translocation of xyl gene near to the ilvC locus. In Escherichia coli, clusters of structural genes concerned with the highand low-affinity uptake systems of branched-chain amino acids have been located at 74 min (Oxender et al 1977;Anderson and Oxender 1978), and in the region of 76 to 77 min (Yamato and Anraku 1980), respectively, on its genetic map. Although existence of gene clusters, analogous to those found in E. coli, has as yet been unknown in S. typhimurium, it is possible that the livR gene is a member of one of such gene clusters.…”