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Journal of Bacteriology, January 2004, p. 343-350, Vol. 186, No. 2
0021-9193/04/$08.00+0     DOI: 10.1128/JB.186.2.343-350.2004
Copyright © 2004, American Society for Microbiology. All Rights Reserved.

Identification of the LIV-I/LS System as the Third Phenylalanine Transporter in Escherichia coli K-12

Takashi Koyanagi, Takane Katayama, Hideyuki Suzuki, and Hidehiko Kumagai*

Division of Integrated Life Science, Graduate School of Biostudies, Kyoto University, Kyoto, Japan

Received 7 October 2003/ Accepted 21 October 2003

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 ({Delta}aroP {Delta}pheP {Delta}mtr {Delta}tna {Delta}tyrP). 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 Km values for phenylalanine in the LIV-I and LS systems were determined to be 19 and 30 µM, 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.

* Corresponding author. Mailing address: Division of Integrated Life Science, Graduate School of Biostudies, Kyoto University, Kitashirakawa, Sakyo-ku, Kyoto 606-8502, Japan. Phone: 81-75-753-6276. Fax: 81-75-753-6275. E-mail: hidekuma{at}kais.kyoto-u.ac.jp.

Journal of Bacteriology, January 2004, p. 343-350, Vol. 186, No. 2
0021-9193/04/$08.00+0     DOI: 10.1128/JB.186.2.343-350.2004
Copyright © 2004, American Society for Microbiology. All Rights Reserved.



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