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Transition metals, including manganese, are required for the proper virulence and persistence of many pathogenic bacteria. In Streptococcus pneumoniae (Spn), manganese homeostasis is controlled by a high-affinity Mn(II) uptake complex, PsaBCA, and a constitutively expressed efflux transporter, MntE. psaBCA expression is transcriptionally regulated by the DtxR/MntR family metalloregulatory protein pneumococcal surface antigen repressor (PsaR) in Spn. Here, we present a comprehensive analysis of the metal and DNA binding properties of PsaR. PsaR is a homodimer in the absence and presence of metals and binds two manganese or zinc atoms per protomer (four per dimer) in two pairs of structurally distinct sites, termed site 1 and site 2. Site 1 is likely filled with Zn(II) in vivo (K(Zn1) ≥ 10¹³ M⁻¹; K(Mn1) ≈ 10⁸ M⁻¹). The Zn(II)-site 1 complex adopts a pentacoordinate geometry as determined by X-ray absorption spectroscopy containing a single cysteine and appears to be analogous to the Cd(II) site observed in Streptococcus gordonii ScaR. Site 1 is necessary but not sufficient for full positive allosteric activation of DNA operator binding by metals as measured by ΔGc, the allosteric coupling free energy, because site 1 mutants show an intermediate ΔGc. Site 2 is the primary regulatory site and governs specificity for Mn(II) over Zn(II) in PsaR, where ΔGc(Zn,Mn) &gt;&gt; ΔGc(Zn,Zn) despite the fact that Zn(II) binds site 2 with an affinity 40-fold higher than that of Mn(II); i.e., K(Zn2) &gt; K(Mn2). Mutational studies reveal that Asp7 in site 2 is a critical ligand for Mn(II)-dependent allosteric activation of DNA binding. These findings are discussed in the context of other well-studied DtxR/MntR Mn(II)/Fe(II) metallorepressors.

Physical Characterization of the Manganese-Sensing Pneumococcal Surface Antigen Repressor from Streptococcus pneumoniae