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Journal of Bacteriology, October 2001, p. 5862-5869, Vol. 183, No. 20
0021-9193/01/$04.00+0   DOI: 10.1128/JB.183.20.5862-5869.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.

Bacillus subtilis YxkJ Is a Secondary Transporter of the 2-Hydroxycarboxylate Transporter Family That Transports L-Malate and Citrate

Bastiaan P. Krom, Ronald Aardema, and Juke S. Lolkema*

Department of Microbiology, Groningen Biomolecular Sciences and Biotechnology Institute, University of Groningen, Haren, The Netherlands

Received 18 May 2001/Accepted 20 July 2001

The genome of Bacillus subtilis contains two genes that code for membrane proteins that belong to the 2-hydroxycarboxylate transporter family. Here we report the functional characterization of one of the two, yxkJ, which codes for a transporter protein named CimHbs. The gene was cloned and expressed in Escherichia coli and complemented the citrate-negative phenotype of wild-type E. coli and the malate-negative phenotype of the E. coli strain JRG4008, which is defective in malate uptake. Subsequent uptake studies in whole cells expressing CimHbs clearly demonstrated the citrate and malate transport activity of the protein. Immunoblot analysis showed that CimHbs is a 48-kDa protein that is well expressed in E. coli. Studies with right-side-out membrane vesicles demonstrated that CimHbs is an electroneutral proton-solute symporter. No indications were found for the involvement of Na+ ions in the transport process. Inhibition of the uptake catalyzed by CimHbs by divalent metal ions, together with the lack of effect on transport by the chelator EDTA, showed that CimHbs translocates the free citrate and malate anions. Among a large set of substrates tested, only malate, citramalate, and citrate competitively inhibited citrate transport catalyzed by CimHbs. The transporter is strictly stereoselective, recognizing only the S enantiomers of malate and citramalate. Remarkably, though citramalate binds to the transporter, it is not translocated.

* Corresponding author. Mailing address: Department of Microbiology, Biological Center, University of Groningen, Kerklaan 30, 9751 NN Haren, The Netherlands. Phone: (31) 50-3632150. Fax: (31) 50-3632154. E-mail: j.s.lolkema{at}biol.rug.nl.

Journal of Bacteriology, October 2001, p. 5862-5869, Vol. 183, No. 20
0021-9193/01/$04.00+0   DOI: 10.1128/JB.183.20.5862-5869.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.


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