title: Genetic separation of high- and low-affinity transport systems for branched-chain amino acids in Escherichia coli K-12.
abstract: The Escherichia coli K-12 mutant strain AE4107 (livH::Mu) is defective in the high-affinity binding protein-mediated uptake system for L-leucine, L-valine, and L-isoleucine (LIV-I). We have used this strain to produce mutations in the residual LIV-II membrane-bound branched-chain amino acid uptake system. Mutants selected for their inability to utilize exogenous L-leucine were found to be defective in the LIV-II system and fell into two classes. One class, represented by strain AE410709 (livP9), showed a complete loss of saturable uptake for L-leucine, L-valine, and L-isoleucine up to 50 muM, and a second class, represented by strain AE4017012 (liv-12), showed a residual component of saturable leucine uptake with increased Km. These mutations, livP9 and liv-12, were closely linked and mapped in the 74 to 78 min region of the E. coli genetic map. Strains constructed so that they lacked both LIV-I and LIV-II transport systems excreted leucine. Strains of the genotype livH+ livP were found to have normal high-affinity binding protein-mediated transport (LIV-I and leucine specific), whereas the low-affinity (LIV-II) transport was completely missing. We concluded from these studies that the high-affinity binding protein-mediated transport systems (LIV-I and leucine specific) can operate independently of the membrane-bound LIV-II system.
mesh_terms: D001693:Biological Transport, Active; D002874:Chromosome Mapping; D002876:Chromosomes, Bacterial; D004926:Escherichia coli / Q000235:genetics / Q000378:metabolism*; D005796:Genes; D008040:Genetic Linkage; D007532:Isoleucine / Q000378:metabolism*; D007930:Leucine / Q000378:metabolism*; D009154:Mutation; D014633:Valine / Q000378:metabolism*
chemical_list: D007532:Isoleucine; D007930:Leucine; D014633:Valine