Identification of livG, a membrane-associated component of the branched-chain amino acid transport in Escherichia coli

Abstract

Branched-chain amino acids are transported into Escherichia coli by two osmotic shock-sensitive systems (leucine-isoleucine-valine and leucine-specific transport systems). These high-affinity systems consist of separate periplasmic binding protein components and at least three common membrane-bound components. In this study, one of the membrane-bound components, livG, was identified. A toxic analog of leucine, azaleucine, was used to isolate a large number of azaleucine-resistant mutants which were defective in branched-chain amino acid transport. Genetic complementation studies established that two classes of transport mutants with similar phenotypes, livH and livG, were obtained which were defective in one of the membrane-associated transport components. Since the previously cloned plasmid, pOX1, genetically complemented both livH and livG mutants, we were able to verify the physical location of the livG gene on this plasmid. Recombinant plasmids which carried different portions of the pOXl plasmid were constructed and subjected to complementation analysis. These results established that livG was located downstream from livH with about 1 kilobase of DNA in between. The expression of these plasmids was studied in minicells; these studies indicate that livG appears to be membrane bound and to have a molecular weight of 22,000. These results establish that livG is a membrane-associated component of the branched-chain amino acid transport system in E. coli.

Plasticity in amino acid sensing of the chimeric receptor Taz

“…If their effect were exerted through changes in membrane structure then it would not be stereo-specific and other hydrophobic amino acids, like Ile, Phe and Trp would have a similar effect. The same reasoning partially applies for changes in membrane potential or any other possible effect caused by the active transport of the amino acids; although stereo-specificity could arise from the transport system(s), other amino acids would be expected to have an inhibitory effect, especially Ile, which shares the same pathway of active transport with Val (Nazos et al, 1985).…”

Multiple determinants influence root colonization and induction of induced systemic resistance by Pseudomonas chlororaphis O6

“…Mutations in the livF and livH genes did not show a reduced colonization potential, but displayed an impaired induction of systemic resistance. In E. coli, LivF and LivH are associated with the ABC transport system for branched-chain amino acids (Nazos et al, 1985). Based on sequence similarities, LivH is the membrane component and LivF is the ATP-binding component of the ABC transport complexes (Adams et al, 1990).…”

The LIV-I/LS system as a determinant of azaserine sensitivity ofEscherichia coliK-12

“…Quite recently, we found that the LIV‐I/LS system [22–27], initially assumed to be a branched‐chain amino acid‐specific transporter, is the third phenylalanine transporter and has a broader substrate specificity than previously reported [28]. This finding led us to envisage the possible linkage between the alleviating effects of phenylalanine and branched‐chain amino acids on azaserine toxicity, and prompted us to examine whether or not the LIV‐I/LS system is involved in the azaserine sensitivity of E. coli .…”