The zinc metalloregulatory protein Synechococcus PCC7942 SmtB binds a single zinc ion per monomer with high affinity in a tetrahedral coordination geometry

The Synechococcus PCC7942 SmtB is a zinc-responsive transcriptional repress or and a member of the ArsR superfamily of prokaryotic metalloregulatory tr anscription factors. The mechanism of negative regulation by Zn(II) and oth er metals as well as the coordination chemistry (stoichiometry, affinity, a nd specificity) of SmtB is poorly understood, in contrast to previous resul ts [Kar, S. R., Adams, A. C., Lebowitz, J., Taylor, K. B., and Hall, L. M. (1997) Biochemistry 36, 15343-15348], we find that fully reduced SmtB binds 1 mol equiv of Zn(II) with a very high affinity, K-Zn in excess of 10(11) M-1 (pH 7.4, 0.15:M KCl, 22 degrees C). Optical spectroscopic experiments r eveal that SmtB binds I mol equiv of Co(II) in a tetrahedral or distorted t etrahedral environment with one or two cysteine thiolate ligands in the fir st coordination shell. Zn(II) and Co(II) EXAFS studies are consistent with the optical spectroscopic data, and further suggest the presence of a mixtu re of carboxylate and imidazole-containing ligands. K-Co was determined to be 1.7 (+/-0.1) x 10(9) M-1 in a chelator (EGTA) competition assay; 1 equiv of Zn(II) results in complete displacement of the bound Co(LI). SmtB also binds 1 mol equiv of Ni(II), which, when formed at low Ni(LI):SmtB molar ra tios, adopts a non-native, six-coordinate complex characterized by at least two histidine and no thiolate ligands. The hierarchy of metal binding affi nities is Zn(II) much greater than Co(II) much greater than Ni(II).