TroA was isolated from outer membrane preparations of Treponema pallidum (TpTroA) [18] . This protein belongs to a wellstudied family, formerly designated as cluster 9 substrate binding protein (SBP) [19] , but recently reclassified into the A-1 family of prokaryotic SBPs [20] . The SsTro operon contains five genes, an ATP-binding cassette metal transport system (troABCD) (encoding four proteins) and a transcriptional regulator (troR) (Fig. 1A) . The crystal structures of some A-1 family SBPs have been determined over the past decade. These structures include Mn-specific SBPs (e.g.,TpTroA [21] - [22] , Streptococcus pneumoniae PsaA [23] , and Synechocystis 6803 MntC [24] ), Zn-specific SBPs (e.g., S. pneumoniae AdcAII [25] , Synechocystis 6803 ZnuA [26, 27] , and S. pyogenes Lbp [28] ) and Fe-specific SBPs (Streptococcus pyogenes MtsA [29] ). They share a common structure of two (a/b) 4 domains linked by a long helix, with the metal bound at the interface between the domains. Hazlett et al. [30] and Desrosiers et al. [31] revealed that TpTroA has high binding affinity for both Zn 2+ and Mn 2+ , but Zn 2+ is considered to be the primary substrate of TpTroA [32] .