A Zinc(II)/lead(II)/cadmium(II)-inducible operon from the cyanobacterium Anabaena is regulated by AztR, an α3N ArsR/SmtB metalloregulator

Department of Biochemistry/Biophysics, Texas A&M University, College Station, Texas, United States
Biochemistry (Impact Factor: 3.02). 07/2005; 44(24):8673-83. DOI: 10.1021/bi050450+
Source: PubMed


A novel Zn(II)/Pb(II)/Cd(II)-responsive operon that consists of genes encoding a Zn(II)/Pb(II) CPx-ATPase efflux pump (aztA) and a Zn(II)/Cd(II)/Pb(II)-specific SmtB/ArsR family repressor (aztR) has been identified and characterized from the cyanobacterium Anabaena PCC 7120. In vivo real time quantitative RT-PCR assays reveal that both aztR and aztA expression are induced by divalent metal ions Zn(II), Cd(II), and Pb(II) but not by other divalent [Co(II), Ni(II)] or monovalent metal ions [Cu(I) and Ag(I)]. The introduction of a plasmid containing the azt operon into a Zn(II)/Cd(II)-hypersensitive Escherichia coli strain GG48 functionally restores Zn(II) and Pb(II) resistance with a limited effect on Cd(II) resistance. Gel mobility shift assays and aztR O/P-lacZ induction experiments confirm that AztR is the metal-regulated repressor of this operon. In vitro biochemical and mutagenesis studies indicate that AztR contains a sole metal-binding site, designated the alpha3N site, that binds Zn(II), Cd(II), and Pb(II) with a high affinity. Optical absorption spectra of Co(II)- and Cd(II)-substituted AztR and (113)Cd NMR spectroscopy of (113)Cd(II)-substituted AztR reveal that the sole alpha3N site in AztR is a CadC-like distorted tetrahedral S(3)(N,O) metal site. The first metal-coordination shell in the AztR alpha3N site differs from other alpha3N family members that sense Cd(II)/Pb(II) and those alpha5 repressors that sense Zn(II)/Co(II). Our results reveal that the alpha3N site in AztR mediates derepression of the azt operon in the presence of Zn(II), as well as Cd(II) and Pb(II); this might have provided Anabaena with an evolutionary advantage to adapt to heavy-metal-rich environments, while maintaining homeostasis of an essential metal ion, Zn(II).

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    • "For some proteins, negative cooperativity between the two ligand binding sites suggests that binding of a single ligand is sufficient to effect regulation. Examples include the cAMP-receptor protein (CRP) (23,24), the mercury-sensing activator MerR (25), and ArsR family metalloregulators M. tuberculosis CmtR and Anabaena AztR (26,27) and S. aureus CzrA (18). In other cases, binding of effectors to both monomers is required to elicit a regulatory response. "
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    • "CPx – ATPase efflux pump (aztA) was identified and characterized by Liu et al. (2005). "
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