Tong Liu

Texas A&M University, College Station, TX, United States

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Publications (8)34.46 Total impact

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    ABSTRACT: ArsR (or ArsR/SmtB) family metalloregulatory homodimeric repressors collectively respond to a wide range of metal ion inducers in regulating homeostasis and resistance of essential and nonessential metal ions in bacteria. BxmR from the cyanobacterium Osciliatoria brevis is the first characterized ArsR protein that senses both Cu (I)/Ag (I) and divalent metals Zn (II)/Cd (II) in cells by regulating the expression of a P-type ATPase efflux pump (Bxa1) and an intracellular metallothionein (BmtA). We show here that both pairs of predicted alpha3N and alpha5 sites bind metal ions, but with distinct physicochemical and functional metal specificities. Inactivation of the thiophilic alpha3N site via mutation (C77S) abolishes regulation by both Cd (II) and Cu (I), while Zn (II) remains a potent allosteric negative effector of operator/promoter binding (Delta G c >or= +3.2 kcal mol (-1)). In contrast, alpha5 site mutant retains regulation by all four metal ions, albeit with a smaller coupling free energy (Delta G c approximately +1.7 (+/-0.1) kcal mol (-1)). Unlike the other metals ions, the BxmR dimer binds 4 mol equiv of Cu (I) to form an alpha3N binuclear Cu (I) 2S 4 cluster by X-ray absorption spectroscopy. BxmR is thus distinguishable from other closely related ArsR family sensors, in having evolved a metalloregulatory alpha3N site that can adopt an expanded range of coordination chemistries while maintaining redundancy in the response to Zn (II). The evolutionary implications of these findings for the ArsR metal sensor family are discussed.
    Biochemistry 10/2008; 47(40):10564-75. · 3.38 Impact Factor
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    ABSTRACT: Transition metal-transporting P1B-type CPx ATPases play crucial roles in mediating metal homeostasis and resistance in all cells. The degree to which N-terminal metal binding domains (MBDs) confer metal specificity to the transporter is unclear. We show that the two MBDs of the Zn/Cd/Pb effluxing pump Anabaena AztA are functionally nonequivalent, but only with respect to zinc resistance. Inactivation of the a-MBD largely abrogates resistance to high intracellular Zn(II) levels, whereas inactivation of the b-MBD is not as deleterious. In contrast, inactivation of either the a- or b-MBD has little measurable impact on Cd(II) and Pb(II) resistance. The membrane proximal b-MBD binds Zn(II) with a higher affinity than the distal N-terminal a-MBD. Facile Zn(II)-specific intermolecular transfer from the a-MBD to the higher-affinity b-MBD is readily observed by 1H-15N HSQC spectroscopy. Unlike Zn(II), Cd(II) and Pb(II) form saturated 1:1 S4 or S3(O/N) complexes with AztAaHbH, where a single metal ion bridges the two MBDs. We propose that the tandem MBDs enhance Zn(II)-specific transport, while stabilizing a non-native inter-MBD Cd/Pb cross-linked structure that is a poor substrate and/or regulator for the transporter.
    Biochemistry 11/2007; 46(39):11057-68. · 3.38 Impact Factor
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    ABSTRACT: Copper is an essential element that becomes highly cytotoxic when concentrations exceed the capacity of cells to sequester the ion. Here, we identify a new copper-specific repressor (CsoR) of a copper-sensitive operon (cso) in Mycobacterium tuberculosis (Mtb) that is representative of a large, previously uncharacterized family of proteins (DUF156). Electronic and X-ray absorption spectroscopies reveal that CsoR binds a single-monomer mole equivalent of Cu(I) to form a trigonally coordinated (S(2)N) Cu(I) complex. The 2.6-A crystal structure of copper-loaded CsoR shows a homodimeric antiparallel four-helix bundle architecture that represents a novel DNA-binding fold. The Cu(I) is coordinated by Cys36, Cys65' and His61' in a subunit bridging site. Cu(I) binding negatively regulates the binding of CsoR to a DNA fragment encompassing the operator-promoter region of the Mtb cso operon; this results in derepression of the operon in Mtb and the heterologous host Mycobacterium smegmatis. Substitution of Cys36 or His61 with alanine abolishes Cu(I)- and CsoR-dependent regulation in vivo and in vitro. Potential roles of CsoR in Mtb pathogenesis are discussed.
    Nature Chemical Biology 02/2007; 3(1):60-8. · 12.95 Impact Factor
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    ABSTRACT: A cysteine-rich metal binding protein MT (metallothionein) (named BmtA) is induced upon exposure to multiple heavy metal ions in the freshwater cyanobacterium Oscillatoria brevis. The SmtB/ArsR family repressor BxmR from O. brevis represses the expression of an operon encoding bmtA and bxmR. In the present study, the expression of bmtA was induced in vivo by diamide, a specific thiol oxidant, in O. brevis cells. In vitro electrophoretic gel mobility shift experiments revealed that the incubation with diamide induces disassembly of the BxmR-bxmR/bmtA operator (O)/promoter (P) complex [multiple resolvable complexes of BxmR with oligonucleotide (named P5) containing a single 12-2-12 inverted repeat derived from the O/P region of bxmR/bmtA]. Thus, the exposure to diamide induces MT mRNA in O. brevis, and this induction is associated with diamide-mediated inhibition of BxmR-P5 complex. BxmR is more sensitive to diamide than to H(2)O(2). Furthermore, pretreatment of O. brevis with Zn decreased intracellular peroxidation products caused by diamide. Thus, these results imply that MT induced by Zn-pretreatment functions to protect O. brevis cells against diamide stress.
    Toxicology Letters 07/2006; 163(3):250-6. · 3.15 Impact Factor
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    ABSTRACT: 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).
    Biochemistry 07/2005; 44(24):8673-83. · 3.38 Impact Factor
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    ABSTRACT: A novel SmtB/ArsR family metalloregulator, denoted BxmR, has been identified and characterized from the cyanobacterium Oscillatoria brevis. Genetic and biochemical evidence reveals that BxmR represses the expression of both bxa1, encoding a CPx-ATPase metal transporter, as well as a divergently transcribed operon encoding bxmR and bmtA, a heavy metal sequestering metallothionein. Derepression of the expression of all three genes is mediated by both monovalent (Ag(I) and Cu(I)) and divalent (Zn(II) and Cd(II)) heavy metal ions, a novel property among SmtB/ArsR metal sensors. Electrophoretic gel mobility shift experiments reveal that apoBxmR forms multiple resolvable complexes with oligonucleotides containing a single 12-2-12 inverted repeat derived from one of the two operator/promoter regions with similar apparent affinities. Preincubation with either monovalent or divalent metal ions induces disassembly of both the BxmR-bxa1 and BxmR-bxmR/bmtA operator/promoter complexes. Interestingly, the temporal regulation of expression of bxa1 and bmtA mRNAs is different in O. brevis with bxa1 induced first upon heavy metal treatment, followed by bmtA/bxmR. A dynamic interplay among Bxa1, BmtA, and BxmR is proposed that maintains metal homeostasis in O. brevis by balancing the relative rates of metal storage and efflux of multiple heavy metal ions.
    Journal of Biological Chemistry 05/2004; 279(17):17810-8. · 4.65 Impact Factor
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    ABSTRACT: A metallothionein (BmtA) and a CPx-ATPase (Bxa1) have been identified and characterized from the cyanobacterium Oscillatoria brevis. Both bmtA and bxa1 expression can be markedly induced in vivo by Zn(2+) or Cd(2+). Over-expression of bmtA or bxa1 in Escherichia coli enhances Zn(2+) and Cd(2+) tolerance in the transformant. Dynamic studies on the expression of two genes showed that the maximum expression of bxa1 induced by Zn(2+) and Cd(2+) was much quicker than that of bmtA, suggesting distinct physiological roles of metallothionein and CPx-ATPase in the handling of surplus metal.
    FEBS Letters 06/2003; 542(1-3):159-63. · 3.58 Impact Factor
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    ABSTRACT: A metallothionein (BmtA) and a CPx-ATPase (Bxa1) have been identified and characterized from the cyanobacterium Oscillatoria brevis. Both bmtA and bxa1 expression can be markedly induced in vivo by Zn2+ or Cd2+. Over-expression of bmtA or bxa1 in Escherichia coli enhances Zn2+ and Cd2+ tolerance in the transformant. Dynamic studies on the expression of two genes showed that the maximum expression of bxa1 induced by Zn2+ and Cd2+ was much quicker than that of bmtA, suggesting distinct physiological roles of metallothionein and CPx-ATPase in the handling of surplus metal.
    Febs Letters - FEBS LETT. 01/2003; 542(1):159-163.