A zinc(II)/lead(II)/cadmium(II)-inducible operon from the Cyanobacterium anabaena is regulated by AztR, an alpha3N ArsR/SmtB metalloregulator.
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).
Liu
Tong
T
Department of Biochemistry and Biophysics, 2128 TAMU, Texas A&M University, College Station, Texas 77843-2128, USA.
Golden
James W
JW
Giedroc
David P
DP
D000595:Amino Acid Sequence; D017033:Anabaena / Q000235:genetics*; D001426:Bacterial Proteins / Q000737:chemistry / Q000378:metabolism*; D001483:Base Sequence; D001665:Binding Sites; D002104:Cadmium / Q000378:metabolism*; D002413:Cations, Divalent; D000458:Cyanobacteria / Q000235:genetics* / Q000378:metabolism*; D004268:DNA-Binding Proteins / Q000737:chemistry / Q000378:metabolism*; D006706:Homeostasis; D007854:Lead / Q000378:metabolism*; D008667:Metalloproteins / Q000737:chemistry / Q000378:metabolism; D008969:Molecular Sequence Data; D009876:Operon / Q000187:drug effects / Q000235:genetics*; D012091:Repetitive Sequences, Nucleic Acid; D012097:Repressor Proteins / Q000737:chemistry / Q000378:metabolism*; D016415:Sequence Alignment; D017386:Sequence Homology, Amino Acid; D015534:Trans-Activators / Q000737:chemistry / Q000378:metabolism*; D015032:Zinc / Q000378:metabolism*
D001426:Bacterial Proteins; D002413:Cations, Divalent; D004268:DNA-Binding Proteins; D008667:Metalloproteins; D012097:Repressor Proteins; D015534:Trans-Activators; D002104:Cadmium; D007854:Lead; D015032:Zinc