title: Multiplicity of leucine transport systems in Escherichia coli K-12.
abstract: The major component of leucine uptake in Escherichia coli K-12 is a common system for l-leucine, l-isoleucine, and l-valine (LIV-I) with a Michaelis constant (K(m)) value of 0.2 muM (LIV-I system). The LIV-binding protein appears to be associated with this system. It now appears that the LIV-I transport system and LIV-binding protein also serve for the entry of l-alanine, l-threonine, and possibly l-serine. A minor component of l-leucine entry occurs by a leucine-specific system (L-system) for which a specific leucine-binding protein has been isolated. A mutant has been obtained that shows increased levels of the LIV-I transport activity and increased levels of both of the binding proteins. Another mutant has been isolated that shows only a major increase in the levels of the leucine-specific transport system and the leucine-specific binding protein. A third binding protein that binds all three branched-chain amino acids but binds isoleucine preferentially has been identified. The relationship of the binding proteins to each other and to transport activity is discussed. A second general transport system (LIV-II system) with a K(m) value of 2 muM and a relatively low V(max) can be observed in E. coli. The LIV-II system is not sensitive to osmotic shock treatment nor to growth of cells in the presence of leucine. This high K(m) system, which is specific for the branched-chain amino acids, can be observed in membrane vesicle preparations.
mesh_terms: D000409:Alanine / Q000378:metabolism; D001426:Bacterial Proteins / Q000378:metabolism; D001693:Biological Transport, Active; D002848:Chromatography, DEAE-Cellulose; D004926:Escherichia coli / Q000378:metabolism*; D007532:Isoleucine / Q000378:metabolism; D007930:Leucine / Q000378:metabolism*; D009154:Mutation; D009604:Nitrosoguanidines; D009995:Osmosis; D011485:Protein Binding; D012694:Serine / Q000378:metabolism; D013237:Stereoisomerism; D013912:Threonine / Q000378:metabolism; D014161:Transduction, Genetic; D014633:Valine / Q000378:metabolism
chemical_list: D001426:Bacterial Proteins; D009604:Nitrosoguanidines; D007532:Isoleucine; D013912:Threonine; D012694:Serine; D007930:Leucine; D014633:Valine; D000409:Alanine