Identification of the LIV-I/LS system as the third phenylalanine transporter in Escherichia coli K-12.
In Escherichia coli, the active transport of phenylalanine is considered to be performed by two different systems, AroP and PheP. However, a low level of accumulation of phenylalanine was observed in an aromatic amino acid transporter-deficient E. coli strain (DeltaaroP DeltapheP Deltamtr Deltatna DeltatyrP). The uptake of phenylalanine by this strain was significantly inhibited in the presence of branched-chain amino acids. Genetic analysis and transport studies revealed that the LIV-I/LS system, which is a branched-chain amino acid transporter consisting of two periplasmic binding proteins, the LIV-binding protein (LIV-I system) and LS-binding protein (LS system), and membrane components, LivHMGF, is involved in phenylalanine accumulation in E. coli cells. The K(m) values for phenylalanine in the LIV-I and LS systems were determined to be 19 and 30 micro M, respectively. Competitive inhibition of phenylalanine uptake by isoleucine, leucine, and valine was observed for the LIV-I system and, surprisingly, also for the LS system, which has been assumed to be leucine specific on the basis of the results of binding studies with the purified LS-binding protein. We found that the LS system is capable of transporting isoleucine and valine with affinity comparable to that for leucine and that the LIV-I system is able to transport tyrosine with affinity lower than that seen with other substrates. The physiological importance of the LIV-I/LS system for phenylalanine accumulation was revealed in the growth of phenylalanine-auxotrophic E. coli strains under various conditions.
Koyanagi
Takashi
T
Division of Integrated Life Science, Graduate School of Biostudies, Kyoto University, Kyoto, Japan.
Katayama
Takane
T
Suzuki
Hideyuki
H
Kumagai
Hidehiko
H
D026905:Amino Acid Transport Systems; D026921:Amino Acid Transport Systems, Neutral / Q000502:physiology; D001426:Bacterial Proteins / Q000502:physiology; D001483:Base Sequence; D001692:Biological Transport; D002352:Carrier Proteins / Q000032:analysis* / Q000502:physiology; D004926:Escherichia coli / Q000378:metabolism*; D029968:Escherichia coli Proteins / Q000502:physiology; D007700:Kinetics; D008969:Molecular Sequence Data; D010649:Phenylalanine / Q000378:metabolism*
D026905:Amino Acid Transport Systems; D026921:Amino Acid Transport Systems, Neutral; C063201:AroP protein, E coli; D001426:Bacterial Proteins; D002352:Carrier Proteins; D029968:Escherichia coli Proteins; C488923:LivJ protein, E coli; C028332:leucine-isoleucine-valine binding protein, bacteria; C069320:phenylalanine-specific permease, E coli; D010649:Phenylalanine