Structural basis for the metal-selective activation of the manganese transport regulator of Bacillus subtilis

The manganese transport regulator (MntR) of Bacillus subtilis is activated by Mn2+ to repress transcription of genes encoding transporters involved in the uptake of manganese. N4ntR is also strongly activated by cadmium, both in vivo and in vitro. but it is poorly activated by other metal cations, including calcium and zinc. The previously published MntR center dot Mn2+ structure revealed a binuclear complex of manganese ions with a metal-metal separation of 3.3 angstrom (herein designated the AB conformer). Analysis of four additional crystal forms of MntR center dot Mn2+ reveals that the AB conformer is only observed in monoclinic crystals at 100 K, suggesting that this conformation may be stabilized by crystal packing forces. In contrast, monoclinic crystals analyzed at room temperature (at either pH 6.5 or pH 8.5), and a second hexagonal crystal form (analyzed at 100 K), all reveal the shift of one manganese ion by 2.5 angstrom thereby leading to a newly identified conformation (the AC conformer) with an internuclear distance of 4.4 angstrom. significantly, the cadmium and calcium complexes of MntR also contain binuclear complexes with a 4.4 angstrom internuclear separation. In contrast, the zinc complex of MntR contains only one metal ion per subunit, in the A site. Isothermal titration calorimetry confirms the stoichiometry of Mn2+, Cd2+, and Zn2+ binding to MntR. We propose that the specificity of MntR activation is tied to productive binding of metal ions at two sites; the A site appears to act as a selectivity filter, determining whether the B or C site will be Occupied and thereby fully activate MntR.