Complementary metal ion specificity of the metal-citrate transporters CitMand CitH of Bacillus subtilis

Citrate uptake in Bacillus subtilis is stimulated by a wide range of divale nt metal ions. The metal ions were separated into two groups based on the e xpression pattern of the uptake system. The two groups correlated with the metal ion specificity of two homologous B, subtilis secondary citrate trans porters, CitM and CitH, upon expression in Escherichia coli, CitM transport ed citrate in complex with Mg2+, Ni2+, Mn2+, Co2+, and Zn2+ but not in comp lex with Ca2+, Ba2+, and Sr2+. CitH transported citrate in complex with Ca2 +, Ba2+, and Sr2+ but not in complex with Mg2+, Ni2+, Mn2+, Co-,(2+) and Zn 2+. Both transporter. did not transport free citrate. Nevertheless, free ci trate uptake could be demonstrated in B. subtilis, indicating the expressio n 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 t he metal ion, indicating that the complex is the transported species. The r esults indicate that CitM and CitM are secondary transporters that transpor t complexes of divalent metal ions and citrate but with a complementary met al ion specificity. The potential physiological function of the two transpo rters is discussed.