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Journal of Bacteriology, October 2002, p. 5436-5448, Vol. 184, No. 19
0021-9193/02/$04.00+0     DOI: 10.1128/JB.184.19.5436-5448.2002
Copyright © 2002, American Society for Microbiology. All Rights Reserved.

A Monocarboxylate Permease of Rhizobium leguminosarum Is the First Member of a New Subfamily of Transporters

A. H. F. Hosie, D. Allaway, and P. S. Poole*

School of Animal and Microbial Sciences, University of Reading, Whiteknights, Reading RG6 6AJ, United Kingdom

Received 26 March 2002/ Accepted 21 June 2002

Amino acid transport by Rhizobium leguminosarum is dominated by two ABC transporters, the general amino acid permease (Aap) and the branched-chain amino acid permease (Bra). However, mutation of these transporters does not prevent this organism from utilizing alanine for growth. An R. leguminosarum permease (MctP) has been identified which is required for optimal growth on alanine as a sole carbon and nitrogen source. Characterization of MctP confirmed that it transports alanine (Km = 0.56 mM) and other monocarboxylates such as lactate and pyruvate (Km = 4.4 and 3.8 µM, respectively). Uptake inhibition studies indicate that propionate, butyrate, {alpha}-hydroxybutyrate, and acetate are also transported by MctP, with the apparent affinity for solutes demonstrating a preference for C3-monocarboxylates. MctP has significant sequence similarity to members of the sodium/solute symporter family. However, sequence comparisons suggest that it is the first characterized permease of a new subfamily of transporters. While transport via MctP was inhibited by CCCP, it was not apparently affected by the concentration of sodium. In contrast, glutamate uptake in R. leguminosarum by the Escherichia coli GltS system did require sodium, which suggests that MctP may be proton coupled. Uncharacterized members of this new subfamily have been identified in a broad taxonomic range of species, including proteobacteria of the ß-subdivision, gram-positive bacteria, and archaea. A two-component sensor-regulator (MctSR), encoded by genes adjacent to mctP, is required for activation of mctP expression.

* Corresponding author. Mailing address: School of Animal and Microbial Sciences, University of Reading, Whiteknights, P.O. Box 228, Reading RG6 6AJ, United Kingdom. Phone: (44) 118 931 8895. Fax: (44) 118 931 6671. E-mail: p.s.poole{at}reading.ac.uk.

Journal of Bacteriology, October 2002, p. 5436-5448, Vol. 184, No. 19
0021-9193/02/$04.00+0     DOI: 10.1128/JB.184.19.5436-5448.2002
Copyright © 2002, American Society for Microbiology. All Rights Reserved.



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