Branched-Chain Amino Acid Transport in Escherichia coli

The transport of the branched-chain amino acids in Escherichia coli is mediated by at least three kinetically and genetically distinct active transport systems (6. 16, 18). There are two high-affinity, regulated, osmotic shock-sensitive systems (LIV -I and LS) and one low-affinity, constitutive, osmotic shock-resistant system (LJV-U) (8, 13). The LIV-I system is a general transport system for the transport of L-Ieucine, L-isoleucine, L-vaJine, L-threonine, L-a1anine, and L-serine, whereas the LS system is specific for Land o-leucine. The LlV-II is a general transport system with a lower affinity for all three branched-chain amino acids (2, 13, 16). Genetic analyses have identified four genes, IivJ, livK, /ivH, and livG, for the two highaffinity transport systems (I, 12) and only one, livP, for the LlV-II transport system (2). Mutations in IivJ and IivK define the genes coding for the periplasmic binding proteins of the LIV-I and LS transport systems, respectively. Mutations in IivH and !ivG define the genes for the membrane-associated proteins which are common components of both high-affinity transport systems (1. 12; manuscripts in preparation). All four genes are clustered at min 74.5 on the E. coli chromosome (4). Mutations in livP map at min 76 and affect only the LIV-IJ lnmsport system. This mutation presllmably defines a membrane-bound component of this transport system. Two additional mutations, livR and ISIR, which lead to derepression of the high-affinity transport systems have also been identified and mapped at min 20 on the E. coli chromosome (3). By use of standard cloning techniques. the entire Iiv genetic locus for the high-affinity branchedchain amino acid transport systems contained in a 13-kilobase EcoRl fragment has been cloned into the pACYC184 plasmid vector, yielding the pOXI plasmid (12).