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Journal of Bacteriology, November 2000, p. 6374-6381, Vol. 182, No. 22
0021-9193/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.

Complementary Metal Ion Specificity of the Metal-Citrate Transporters CitM and CitH of Bacillus subtilis

Bastiaan P. Krom, Jessica B. Warner, Wil N. Konings, and Juke S. Lolkema*

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

Received 28 June 2000/Accepted 23 August 2000

Citrate uptake in Bacillus subtilis is stimulated by a wide range of divalent metal ions. The metal ions were separated into two groups based on the expression pattern of the uptake system. The two groups correlated with the metal ion specificity of two homologous B. subtilis secondary citrate transporters, CitM and CitH, upon expression in Escherichia coli. CitM transported citrate in complex with Mg2+, Ni2+, Mn2+, Co2+, and Zn2+ but not in complex with Ca2+, Ba2+, and Sr2+. CitH transported citrate in complex with Ca2+, Ba2+, and Sr2+ but not in complex with Mg2+, Ni2+, Mn2+, Co2+, and Zn2+. Both transporters did not transport free citrate. Nevertheless, free citrate uptake could be demonstrated in B. subtilis, indicating the expression of at least a third citrate transporter, whose identity is not known. For both the CitM and CitH transporters it was demonstrated that the metal ion promoted citrate uptake and, vice versa, that citrate promoted uptake of the metal ion, indicating that the complex is the transported species. The results indicate that CitM and CitH are secondary transporters that transport complexes of divalent metal ions and citrate but with a complementary metal ion specificity. The potential physiological function of the two transporters is discussed.

* Corresponding author. Mailing address: Department of Microbiology, University of Groningen, Biological Center, 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, November 2000, p. 6374-6381, Vol. 182, No. 22
0021-9193/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.


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