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div>PCC 6803, a manganese ABC transporter, MntCAB, operates with a high affinity (Km, 1�C3?��M), which is induced under manganese-starvation conditions (Bartsevich & Pakrasi, 1996). The crystal structure of its SBP, MntC, was resolved with bound Mn2+ and revealed the presence of an unusual, but functionally crucial disulfide (Rukhman et al., 2005). Manganese ABC transporters related to an oxidative stress response were found in numerous bacteria (Claverys, 2001; Horsburgh et al., 2002). The tuclazepam putative iron/manganese transporter of the root�Cnodule symbiont S. meliloti was shown to be important for the organism's response to oxidative stress. The SitABCD system for which a homolog exists in S. Typhimurium (Kehres et al., 2002) (Uptake of nutrients from the host) favors manganese over iron as a substrate. A mutant lacking the solute-binding subunit, SitA, is symbiotically defective and displays elevated sensitivity to superoxide due to decreased levels of superoxide dismutase B. The S. meliloti SodB (also called SodA in older references) can operate with iron or manganese, therefore termed ��cambialistic��, but exhibits a higher activity with the latter. The mutant was rescued by exogenous addition of 10?��M MnSO4, but not FeSO4, although the latter was not applied under reducing conditions to maintain the Fe(II) redox state. The authors concluded that the SitABCD transporter plays a crucial role in manganese uptake (Davies & Walker, 2007), which is see more consistent with data obtained for the Salmonella Sit system. Escherichia coli and others have a ZnuABC system, consisting of the extracellular binding protein, ZnuA, a membrane-integral subunit, ZnuB, and an ATPase, ZnuC. Expression of the znuABC gene cluster is regulated by zinc and a specific repressor, Zur (Hantke, 2005). Crystal structures of Ec-ZnuA are available and show that both ��-/��-domains are connected by an ��-helix as it is characteristic of family 3 receptors (Wilkinson & Verschueren, XAV-939 research buy 2003). Three conserved histidine residues and a glutamate were found to coordinate the zinc ion (Chandra et al., 2007; Li & Jogl, 2007; Yatsunyk et al., 2008). The glutamate was replaced by water in a structure of the homologous ZnuA protein from Synechocystis (Banerjee et al., 2003). Similar to MntC, the Ec-ZnuA structures show an unusual disulfide bond in the C-terminal domain, which might be important for structural integrity or regulation of Zn2+ binding. In one structure, a second metal-binding site with an unclear function was observed (Yatsunyk et al., 2008). Ec-ZnuA binds Zn2+ with an estimated Kd of <20?nM and other divalent cations, but not manganese. Only Zn2+, Cd2+ and Cu2+ caused conformational changes in the protein thought to be required for metal delivery to the cognate transport complex (Yatsunyk et al., 2008). A highly charged and mobile loop observed in the vicinity of the binding cleft is proposed to act as a zinc chaperone to facilitate acquisition (Banerjee et al., 2003).