Secondary transporters of the bacterial CitMHS family transport citrate in complex with a metal ion. Different members of the family are specific for the metal ion in the complex and have been shown to transport Mg 2+ –cit-rate, Ca 2+ –citrate or Fe 3+ –citrate. The Fe 3+ –citrate transporter ofStrep-tococcus mutans clusters on the phylogenetic tree on a separate branch with a group of transporters found in the phylum Firmicutes which are believed to be involved in anaerobic citrate degradation. .
