Membrane topology of the p1258 CadA Cd(II)/Pb(II)/Zn(II)-translocating P-type ATPase

Enhancement effects of dissolved organic matter leached from sewage sludge on microbial reduction and immobilization of Cr(VI) by Geobacter sulfurreducens

Sewage sludge has a high concentration of dissolved organic matter (DOM) which contains compounds that can serve as electron donors or shuttles for metal reduction by dissimilatory metal reducing bacteria (DMRB). In this study, Cr(VI) removal by G. sulfurreducens, a common DMRB present in anaerobic soils, was examined in the presence or absence of sludge DOM. Two different types of sludge DOM were tested; composted sludge DOM (C-DOM) and anaerobically digested sludge DOM (A-DOM). Both sludge DOMs enhanced Cr(VI) reduction by G. sulfurreducens, but C-DOM was more effective likely because it had higher concentrations of humic substances that served as electron shuttles. Transcriptomic studies indicated that G. sulfurreducens utilizes several different mechanisms to tolerate chromium including extracellular Cr(VI) reduction and immobilization by outer membrane c-type cytochromes and electrically conductive pili, intracellular Cr(VI) reduction by triheme cytochromes and NAD(P)H FMN reductase proteins, and chromium efflux by several P-type ATPase and RND transporter proteins. Microscopy experiments also showed that Cr(III) crystals formed on the surface of the cells, indicating that extracellular Cr(VI) reduction and adsorption was involved in the chromium removal process. These results help provide insight into the potential use of sewage sludge as an additive to enhance the bioremediation of chromium contaminated soils.

Genome sequencing of biocontrol strain Bacillus amyloliquefaciens Bam1 and further analysis of its heavy metal resistance mechanism

Plant growth-promoting rhizobacteria (PGPR) or Biocontrol strains inevitably encounter heavy metal excess stress during the product's processing and application. Bacillus amyloliquefaciens Bam1 was a potential biocontrol strain with strong heavy metal resistant ability. To understand its heavy metal resistance mechanism, the complete genome of Bam1 had been sequenced, and the comparative genomic analysis of Bam1 and FZB42, an industrialized PGPR and biocontrol strain with relatively lower heavy metal tolerance, was conducted. The comparative genomic analysis of Bam1 and the other nine B. amyloliquefaciens strains as well as one Bacillus velezensis (genetically and physiologically very close to B. amyloliquefaciens) was also performed. Our results showed that the complete genome size of Bam1 was 3.95 Mb, 4219 coding sequences were predicted, and it possessed the highest number of unique genes among the eleven analyzed strains. Nine genes related to heavy metal resistance were detected within the twelve DNA islands of Bam1, while only two of them were detected within the seventeen DNA islands of FZB42. When compared with B. amyloliquefaciens type strain DSM7, Bam1 lacked contig L, whereas FZB42 lacked contig D and I, as well as just possessed contig B with a very small size. Our results could also deduce that Bam1 promoted its essential heavy metal resistance mainly by decreasing the import and increasing the export of heavy metals with the corresponding homeostasis systems, which are regulated by different metalloregulators. While Bam1 promoted its non-essential heavy metal resistance mainly by the activation of some specific or non-specific exporters responding to different heavy metals. The variation of the genes related to heavy metal resistance and the other differences of the genomes, including the different number and arrangement of contigs, as well as the number of the heavy metal resistant genes in Prophages and Genomic islands, led to the significant different resistance of Bam1 and FZB42 to heavy metals.

Zinc Resistance Mechanisms of P1B-type ATPases in Sinorhizobium meliloti CCNWSX0020

The Sinorhizobium meliloti (S. meliloti) strain CCNWSX0020 displayed tolerance to high levels exposures of multiple metals and growth promotion of legume plants grown in metal-contaminated soil. However, the mechanism of metal-resistant strain remains unknown. We used five P_1B-ATPases deletions by designating as ∆copA1b, ∆fixI1, ∆copA3, ∆zntA and ∆nia, respectively to investigate the role of P_1B-ATPases in heavy metal resistance of S. meliloti. The ∆copA1b and ∆zntA mutants were sensitive to zinc (Zn), cadmium (Cd) and lead (Pb) in different degree, whereas the other mutants had no significant influence on the metal resistance. Moreover, the expression of zntA was induced by Zn, Cd and Pb whereas copA1b was induced by copper (Cu) and silver (Ag). This two deletions could led to the increased intracellular concentrations of Zn, Pb and Cd, but not of Cu. Complementation of ∆copA1b and ∆zntA mutants showed a restoration of tolerance to Zn, Cd and Pb to a certain extent. Taken together, the results suggest an important role of copA1b and zntA in Zn homeostasis and Cd and Pb detoxification in S. meliloti CCNWSX0020.

Cd(2+) extrusion by P-type Cd(2+)-ATPase of Staphylococcus aureus 17810R via energy-dependent Cd(2+)/H(+) exchange mechanism

Cd²⁺ is highly toxic to Staphylococcus aureus since it blocks dithiols in cytoplasmic 2-oxoglutarate dehydrogenase complex (ODHC) participating in energy conservation process. However, S. aureus 17810R is Cd²⁺-resistant due to possession of cadA-coded Cd²⁺ efflux system, recognized here as P-type Cd²⁺-ATPase. This Cd²⁺ pump utilizing cellular energy—ATP, ∆μ_H⁺ (electrochemical proton potential) and respiratory protons, extrudes Cd²⁺ from cytoplasm to protect dithiols in ODHC, but the mechanism of Cd²⁺ extrusion remains unknown. Here we propose that two Cd²⁺ taken up by strain 17810R via Mn²⁺ uniporter down membrane potential (∆ψ) generated during glutamate oxidation in 100 mM phosphate buffer (high P_iB) are trapped probably by high affinity sites in cytoplasmic domain of Cd²⁺-ATPase, forming SCdS. This stops Cd²⁺ transport towards dithiols in ODHC, allowing undisturbed NADH production, its oxidation and energy conservation, while ATP could change orientation of SCdS towards facing transmembrane channel. Now, increased number of P_i-dependent protons pumped electrogenically via respiratory chain and countertransported through the channel down ∆ψ, extrude two trapped cytoplasmic Cd²⁺, which move to low affinity sites, being then extruded into extracellular space via ∆ψ-dependent Cd²⁺/H⁺ exchange. In 1 mM phosphate buffer (low P_iB), external Cd²⁺ competing with decreased number of P_i-dependent protons, binds to ψ_s of Cd²⁺-ATPase channel, enters cytoplasm through the channel down ∆ψ via Cd²⁺/Cd²⁺ exchange and blocks dithiols in ODHC. However, Mg²⁺ pretreatment preventing external Cd²⁺ countertransport through the channel down ∆ψ, allowed undisturbed NADH production, its oxidation and extrusion of two cytoplasmic Cd²⁺ via Cd²⁺/H⁺ exchange, despite low P_iB.

Recent developments in plant zinc homeostasis and the path toward improved biofortification and phytoremediation programs

Zinc (Zn) is an essential micronutrient for all living organisms. Plants serve as a major entry point for this element into the food chain. Zn deficiency has become a widespread nutritional condition, which mirror the inadequate Zn reserves in significant proportion of the earth's arable land. A recent assessment by the World Health Organization revealed that one third of the world's population is at risk of Zn deficiency. To counter this alarming situation, substantial efforts have been made to increase Zn content and availability in staple crops and grains. Nevertheless, the absence of fundamental information has held back progress in this field. Developing a better understanding of how Zn homeostasis is regulated in plants, such as Zn transporters at loading bottlenecks, is of primary interest to biofortification and phytoremediation programs. Many reviews have been published on this subject, and here we briefly summarize the regulation of one limiting step in Zn distribution within plants - the loading of Zn into root xylem.

Damage control: regulating defenses against toxic metals and metalloids

Some elements are essential for life and others closely related to them are very toxic. In exploiting unique ecological niches many prokaryotes have evolved the means to defend themselves against and even to derive energy from deleterious elements. Toxic metal defense systems are related to those providing homeostasis of essential metals and metalloid elements. Expression of these multiprotein systems is costly but they must respond rapidly and, so, all are well controlled. Seven diverse families of metalloregulators are presently recognized for essential metal homeostasis in prokaryotes. Two of these, the ArsR and MerR families, figure more often than the others in controlling responses to toxic transition metals and metalloids. This review emphasizes recent advances in these two metalloregulator families and highlights emerging regulatory motifs of other types.

A Multicopper oxidase (Cj1516) and a CopA homologue (Cj1161) are major components of the copper homeostasis system of Campylobacter jejuni

Metal ion homeostasis mechanisms in the food-borne human pathogen Campylobacter jejuni are poorly understood. The Cj1516 gene product is homologous to the multicopper oxidase CueO, which is known to contribute to copper tolerance in Escherichia coli. Here we show, by optical absorbance and electron paramagnetic resonance spectroscopy, that purified recombinant Cj1516 contains both T1 and trinuclear copper centers, which are characteristic of multicopper oxidases. Inductively coupled plasma mass spectrometry revealed that the protein contained approximately six copper atoms per polypeptide. The presence of an N-terminal "twin arginine" signal sequence suggested a periplasmic location for Cj1516, which was confirmed by the presence of p-phenylenediamine (p-PD) oxidase activity in periplasmic fractions of wild-type but not Cj1516 mutant cells. Kinetic studies showed that the pure protein exhibited p-PD, ferroxidase, and cuprous oxidase activities and was able to oxidize an analogue of the bacterial siderophore anthrachelin (3,4-dihydroxybenzoate), although no iron uptake impairment was observed in a Cj1516 mutant. However, this mutant was very sensitive to increased copper levels in minimal media, suggesting a role in copper tolerance. This was supported by increased expression of the Cj1516 gene in copper-rich media. A mutation in a second gene, the Cj1161c gene, encoding a putative CopA homologue, was also found to result in copper hypersensitivity, and a Cj1516 Cj1161c double mutant was found to be more copper sensitive than either single mutant. These observations and the apparent lack of alternative copper tolerance systems suggest that Cj1516 (CueO) and Cj1161 (CopA) are major proteins involved in copper homeostasis in C. jejuni.

Expression of copA and cusA in Shewanella during copper stress

Copper homeostasis is tightly regulated in all living cells as a result of the necessity and toxicity of this metal in free cationic form. In Gram-negative bacteria CPx-type ATPases (e.g. CopA in Escherichia coli) and heavy-metal efflux RND proteins (e.g. CusA in E. coli) play an important role in transport of copper across the cytoplasmic and outer membrane. We investigated the expression of CusA- and CopA-like proteins in Shewanella oneidensis MR1 and Shewanella strain MB4, a Mn(IV)-reducing isolate from a metal-polluted harbour sediment. Q-PCR analysis of total mRNA extracted from cultures grown under aerobic conditions with 25 microM copper showed significantly increased expression of cusA (Student's t-test: MR1, P<0.0001; MB4, P=0.0006). This gene was also induced in the presence of 100 microM copper and 10 or 25 microM cadmium in both tested strains. In the absence of oxygen, with fumarate as final electron acceptor and 100 microM copper, a prolonged lag phase (5 h) was observed and general fitness decreased as evidenced by twofold lower copy numbers of 16S rRNA compared to aerobic conditions. cusA expression in cells grown under these conditions remained at comparable levels (MR1) or was slightly decreased (MB4), compared to aerobic copper challenges. A gene homologous to the copA gene of S. oneidensis was not detected in strain MB4. Although low copA copy numbers were observed in strain MR1 under conditions with 25 and 100 microM copper, copA was not detected in mRNA from cultures grown in the presence of 10 microM cadmium, or in the absence of added heavy metals. However, copA was highly induced under anaerobic conditions with 100 microM copper (P=0.0011). These results suggest essentially different roles for the two proteins CopA and CusA in the copper response in S. oneidensis MR1, similar to findings in more metal-resistant bacteria such as Escherichia coli and Cupriavidus metallidurans.

Activation of Archaeoglobus fulgidus Cu+-ATPase CopA by cysteine

CopA, a thermophilic ATPase from Archaeoglobus fulgidus, drives the outward movement of Cu(+) across the cell membrane. Millimolar concentration of Cys dramatically increases ( congruent with 800%) the activity of CopA and other P(IB)-type ATPases (Escherichia coli ZntA and Arabidopsis thaliana HMA2). The high affinity of CopA for metal ( congruent with 1 microM) together with the low Cu(+)-Cys K(D) (<10(-10)M) suggested a multifaceted interaction of Cys with CopA, perhaps acting as a substitute for the Cu(+) chaperone protein present in vivo. To explain the activation by the amino acid and further understand the mechanism of metal delivery to transport ATPases, Cys effects on the turnover and partial reactions of CopA were studied. 2-20 mM Cys accelerates enzyme turnover with little effect on CopA affinity for Cu(+), suggesting a metal independent activation. Furthermore, Cys activates the p-nitrophenyl phosphatase activity of CopA, even though this activity is metal independent. Cys accelerates enzyme phosphorylation and the forward dephosphorylation rates yielding higher steady state phosphoenzyme levels. The faster dephosphorylation would explain the higher enzyme turnover in the presence of Cys. The amino acid has no significant effect on low affinity ATP K(m) suggesting no changes in the E(1)<-->E(2) equilibrium. Characterization of Cu(+) transport into sealed vesicles indicates that Cys acts on the cytoplasmic side of the enzyme. However, the Cys activation of truncated CopA lacking the N-terminal metal binding domain (N-MBD) indicates that activation by Cys is independent of the regulatory N-MBD. These results suggest that Cys is a non-essential activator of CopA, interacting with the cytoplasmic side of the enzyme while this is in an E1 form. Interestingly, these effects also point out that Cu(+) can reach the cytoplasmic opening of the access path into the transmembrane transport sites either as a free metal or a Cu(+)-Cys complex.

The structure and function of heavy metal transport P1B-ATPases

P(1B)-type ATPases transport heavy metals (Cu+, Cu2+, Zn2+, Co2+, Cd2+, Pb2+) across membranes. Present in most organisms, they are key elements for metal homeostasis. P(1B)-type ATPases contain 6-8 transmembrane fragments carrying signature sequences in segments flanking the large ATP binding cytoplasmic loop. These sequences made possible the differentiation of at least four P(1B)-ATPase subgroups with distinct metal selectivity: P(1B-1): Cu+, P(1B-2): Zn2+, P(1B-3): Cu2+, P(1B-4): Co2+. Mutagenesis of the invariant transmembrane Cys in H6, Asn and Tyr in H7 and Met and Ser in H8 of the Archaeoglobus fulgidus Cu+-ATPase has revealed that their side chains likely coordinate the metals during transport and constitute a central unique component of these enzymes. The structure of various cytoplasmic domains has been solved. The overall structure of those involved in enzyme phosphorylation (P-domain), nucleotide binding (N-domain) and energy transduction (A-domain), appears similar to those described for the SERCA Ca2+-ATPase. However, they show different features likely associated with singular functions of these proteins. Many P(1B)-type ATPases, but not all of them, also contain a diverse arrangement of cytoplasmic metal binding domains (MBDs). In spite of their structural differences, all N- and C-terminal MBDs appear to control the enzyme turnover rate without affecting metal binding to transmembrane transport sites. In addition, eukaryotic Cu+-ATPases have multiple N-MBD regions that participate in the metal dependent targeting and localization of these proteins. The current knowledge of structure-function relationships among the different P(1B)-ATPases allows for a description of selectivity, regulation and transport mechanisms. Moreover, it provides a framework to understand mutations in human Cu+-ATPases (ATP7A and ATP7B) that lead to Menkes and Wilson diseases.

Genes involved in cadmium resistance inCaulobacter crescentus

A transposon library of 5700 mutants was constructed in Caulobacter crescentus strain NA1000. Thirteen mutant strains were isolated that presented growth deficiency in the presence of 20 microM cadmium chloride, and the disrupted genes were identified by DNA sequencing. Among the predicted products were found two putative transcriptional regulators, one histidine kinase/response regulator hybrid protein, two 2Fe-2S family proteins, and also members of a putative metal efflux system. Certain of these mutants also showed an increased sensitivity to zinc and copper, and the genes encoding the efflux system were induced by cadmium and zinc, indicating that this system may be important for homeostasis of both metal ions.

The plant P1B-type ATPase AtHMA4 transports Zn and Cd and plays a role in detoxification of transition metals supplied at elevated levels

The transition metal Zn is essential for many physiological processes in plants, yet at elevated concentrations this, and the related non-essential metal Cd, can be toxic. Arabidopsis thaliana HMA4, belonging to the Type P1B subfamily of P-type ATPases, has recently been implicated in Zn nutrition, having a role in root to shoot Zn translocation. Using Arabidopsis insertional mutants, it is shown here that disruption of AtHMA4 function also results in increased sensitivity to elevated levels of Cd and Zn, suggesting that AtHMA4 serves an important role in metal detoxification at higher metal concentrations. AtHMA4 and a truncated form lacking the last 457 amino acids both confer Cd and Zn resistance to yeast but a mutant version of the full-length AtHMA4 (AtHMA4-C357G) does not; this demonstrates that the C-terminal region is not essential for this function. Evidence is presented that AtHMA4 functions as an efflux pump.

Overexpression of AtHMA4 enhances root-to-shoot translocation of zinc and cadmium and plant metal tolerance

AtHMA4 is an Arabidopsis thaliana P1B-ATPase which transports Zn and Cd. Here, we demonstrate that AtHMA4 is localized at the plasma membrane and expressed in tissues surrounding the root vascular vessels. The ectopic overexpression of AtHMA4 improved the root growth in the presence of toxic concentrations of Zn, Cd and Co. A null mutant exhibited a lower translocation of Zn and Cd from the roots to shoot. In contrast, the AtHMA4 overexpressing lines displayed an increase in the zinc and cadmium shoot content. Altogether, these results strongly indicate that AtHMA4 plays a role in metal loading in the xylem.

Arabidopsis HMA2, a divalent heavy metal-transporting P(IB)-type ATPase, is involved in cytoplasmic Zn2+ homeostasis

PIB-type ATPases transport heavy metal ions (Cu+, Cu2+, Zn2+, Cd2+, Co2+, etc.) across biological membranes. Several members of this subfamily are present in plants. Higher plants are the only eukaryotes where putative Zn(2+)-ATPases have been identified. We have cloned HMA2, a PIB-ATPase present in Arabidopsis (Arabidopsis thaliana), and functionally characterized this enzyme after heterologous expression in yeast (Saccharomyces cerevisiae). HMA2 is a Zn(2+)-dependent ATPase that is also activated by Cd2+ and, to a lesser extent, by other divalent heavy metals (Pb2+, Ni2+, Cu2+, and Co2+). The enzyme forms an acid-stable phosphorylated intermediate and is inhibited by vanadate. HMA2 interacts with Zn2+ and Cd2+ with high affinity (Zn2+ K(1/2) = 0.11 +/- 0.03 microm and Cd2+ K(1/2) = 0.031 +/- 0.007 microm). However, its activity is dependent on millimolar concentrations of Cys in the assay media. Zn2+ transport determinations indicate that the enzyme drives the outward transport of metals from the cell cytoplasm. Analysis of HMA2 mRNA suggests that the enzyme is present in all plant organs and transcript levels do not change in plants exposed to various metals. Removal of HMA2 full-length transcript results in Zn2+ accumulation in plant tissues. hma2 mutant plants also accumulate Cd2+ when exposed to this metal. These results suggest that HMA2 is responsible for Zn2+ efflux from the cells and therefore is required for maintaining low cytoplasmic Zn2+ levels and normal Zn2+ homeostasis.

Identification of the transmembrane metal binding site in Cu+-transporting PIB-type ATPases

P(IB)-type ATPases have an essential role maintaining copper homeostasis. Metal transport by these membrane proteins requires the presence of a transmembrane metal occlusion/binding site. Previous studies showed that Cys residues in the H6 transmembrane segment are required for metal transport. In this study, the participation in metal binding of conserved residues located in transmembrane segments H7 and H8 was tested using CopA, a model Cu(+)-ATPase from Archaeoglobus fulgidus. Four invariant amino acids in the central portion of H7 (Tyr(682) and Asn(683)) and H8 (Met(711) and Ser(715)) were identified as required for Cu(+) binding. Replacement of these residues abolished enzyme activity. These proteins did not undergo Cu(+)-dependent phosphorylation by ATP but were phosphorylated by P(i) in the absence of Cu(+). Moreover, the presence of Cu(+) could not prevent the enzyme phosphorylation by P(i). Other conserved residues in the H7-H8 region were not required for metal binding. Mutation of two invariant Pro residues had little effect on enzyme function. Replacement of residues located close to the cytoplasmic end of H7-H8 led to inactive enzymes. However, these were able to interact with Cu(+) and undergo phosphorylation. This suggests that the integrity of this region is necessary for conformational transitions but not for ligand binding. These data support the presence of a unique transmembrane Cu(+) binding/translocation site constituted by Tyr-Asn in H7, Met and Ser in H8, and two Cys in H6 of Cu(+)-ATPases. The likely Cu(+) coordination during transport appears distinct from that observed in Cu(+) chaperone proteins or catalytic/redox metal binding sites.

Molecular evidence for the evolution of metal homeostasis genes by lateral gene transfer in bacteria from the deep terrestrial subsurface

Lateral gene transfer (LGT) plays a vital role in increasing the genetic diversity of microorganisms and promoting the spread of fitness-enhancing phenotypes throughout microbial communities. To date, LGT has been investigated in surface soils, natural waters, and biofilm communities but not in the deep terrestrial subsurface. Here we used a combination of molecular analyses to investigate the role of LGT in the evolution of metal homeostasis in lead-resistant subsurface bacteria. A nested PCR approach was employed to obtain DNA sequences encoding P(IB)-type ATPases, which are proteins that transport toxic or essential soft metals such as Zn(II), Cd(II), and Pb(II) through the cell wall. Phylogenetic incongruencies between a 16S rRNA gene tree and a tree based on 48 P(IB)-type ATPase amplicons and sequences available for complete bacterial genomes revealed an ancient transfer from a member of the beta subclass of the Proteobacteria (beta-proteobacterium) that may have predated the diversification of the genus Pseudomonas. Four additional phylogenetic incongruencies indicate that LGT has occurred among groups of beta- and gamma-proteobacteria. Two of these transfers appeared to be recent, as indicated by an unusual G+C content of the P(IB)-type ATPase amplicons. This finding provides evidence that LGT plays a distinct role in the evolution of metal homeostasis in deep subsurface bacteria, and it shows that molecular evolutionary approaches may be used for investigation of this process in microbial communities in specific environments.

Archaeoglobus fulgidus CopB is a thermophilic Cu2+-ATPase: functional role of its histidine-rich-N-terminal metal binding domain

P1B-type ATPases transport heavy metal ions across cellular membranes. Archaeoglobus fulgidus CopB is a member of this subfamily. We have cloned, expressed in Escherichia coli, and functionally characterized this enzyme. CopB and its homologs are distinguished by a metal binding sequence Cys-Pro-His in their sixth transmembrane segment (H6) and a His-rich N-terminal metal binding domain (His-N-MBD). CopB is a thermophilic protein active at 75 degrees C and high ionic strength. It is activated by Cu2+ with high apparent affinity (K1/2 = 0.28 microm) and partially by Cu+ and Ag+ (22 and 55%, respectively). The higher turnover was associated with a faster phosphorylation rate in the presence of Cu2+. A truncated CopB lacking the first 54 amino acids was constructed to characterize the His-N-MBD. This enzyme showed reduced ATPase activity (50% of wild type) but no changes in metal selectivity, ATP dependence, or phosphorylation levels. However, a slower rate of dephosphorylation of the E2P(Cu2+) form was observed for truncated CopB. The data suggest that the presence of the His residue in the putative transmembrane metal binding site of CopB determines a selectivity for this enzyme that is different for that observed in Cu+/Ag+-ATPases carrying a Cys-Pro-Cys sequence. The His-NMBD appears to have a regulatory role affecting the metal transport rate by controlling the metal release/dephosphorylation rates.

Molecular analysis of the copper-transporting efflux system CusCFBA of Escherichia coli

The cus determinant of Escherichia coli encodes the CusCFBA proteins that mediate resistance to copper and silver by cation efflux. CusA and CusB were essential for copper resistance, and CusC and CusF were required for full resistance. Replacements of methionine residues 573, 623, and 672 with isoleucine in CusA resulted in loss of copper resistance, demonstrating their functional importance. Substitutions for several other methionine residues of this protein did not have any effect. The small 10-kDa protein CusF (previously YlcC) was shown to be a periplasmic protein. CusF bound one copper per polypeptide. The pink CusF copper protein complex exhibited an absorption maximum at around 510 nm. Methionine residues of CusF were involved in copper binding as shown by site-directed mutagenesis. CusF interacted with CusB and CusC polypeptides in a yeast two-hybrid assay. In contrast to other well-studied CBA-type heavy metal efflux systems, Cus was shown to be a tetrapartite resistance system that involves the novel periplasmic copper-binding protein CusF. These data provide additional evidence for the hypothesis that Cu(I) is directly transported from the periplasm across the outer membrane by the Cus complex.

Functional expression of AtHMA4, a P1B-type ATPase of the Zn/Co/Cd/Pb subclass

Mechanisms are required by all organisms to maintain the concentration of essential heavy metals (e.g. Zn and Cu) within physiological limits and to minimise the detrimental effects of non-essential heavy metals (e.g. Cd). Heavy-metal P-type ATPases (HMAs) are a subgroup of the P-type ATPase superfamily that may contribute to metal homeostasis in plants. We cloned and characterised a member of this family, AtHMA4, from Arabidopsis thaliana that clusters with the Zn/Co/Cd/Pb subclass of HMAs on phylogenetic analysis. Sequencing of the AtHMA4 cDNA showed that it contained the conserved motifs found in all P-type ATPases and also motifs that are characteristic of heavy-metal ATPases. Escherichia coli mutants defective in the HMAs, CopA and ZntA, were used in functional complementation studies. AtHMA4 was able to restore growth at high [Zn] in the zntA mutant but not at high [Cu] in the copA mutant, suggesting a role in zinc transport. Heterologous expression of AtHMA4 in Saccharomyces cerevisiae made the yeast more resistant to Cd but did not affect sensitivity to other metals compared with vector-transformed controls. The organ specificity of AtHMA4 was analysed in Arabidopsis and showed that AtHMA4 was expressed in a range of tissues with highest expression in roots. AtHMA4 was upregulated in roots exposed to elevated levels of Zn and Mn but downregulated by Cd. Possible physiological roles of this transporter in Arabidopsis are discussed.

Zn(II) metabolism in prokaryotes

It is difficult to over-state the importance of Zn(II) in biology. It is a ubiquitous essential metal ion and plays a role in catalysis, protein structure and perhaps as a signal molecule, in organisms from all three kingdoms. Of necessity, organisms have evolved to optimise the intracellular availability of Zn(II) despite the extracellular milieu. To this end, prokaryotes contain a range of Zn(II) import, Zn(II) export and/or binding proteins, some of which utilise either ATP or the chemiosmotic potential to drive the movement of Zn(II) across the cytosolic membrane, together with proteins that facilitate the diffusion of this ion across either the outer or inner membranes of prokaryotes. This review seeks to give an overview of the systems currently classified as altering Zn(II) availability in prokaryotes.

Efflux-mediated heavy metal resistance in prokaryotes

What makes a heavy metal resistant bacterium heavy metal resistant? The mechanisms of action, physiological functions, and distribution of metal-exporting proteins are outlined, namely: CBA efflux pumps driven by proteins of the resistance-nodulation-cell division superfamily, P-type ATPases, cation diffusion facilitator and chromate proteins, NreB- and CnrT-like resistance factors. The complement of efflux systems of 63 sequenced prokaryotes was compared with that of the heavy metal resistant bacterium Ralstonia metallidurans. This comparison shows that heavy metal resistance is the result of multiple layers of resistance systems with overlapping substrate specificities, but unique functions. Some of these systems are widespread and serve in the basic defense of the cell against superfluous heavy metals, but some are highly specialized and occur only in a few bacteria. Possession of the latter systems makes a bacterium heavy metal resistant.

Biochemical characterization of CopA, the Escherichia coli Cu(I)-translocating P-type ATPase

Escherichia coli CopA is a copper ion-translocating P-type ATPase that confers copper resistance. CopA formed a phosphorylated intermediate with [gamma-(32)P]ATP. Phosphorylation was inhibited by vanadate and sensitive to KOH and hydroxylamine, consistent with acylphosphate formation on conserved Asp-523. Phosphorylation required a monovalent cation, either Cu(I) or Ag(I). Divalent cations Cu(II), Zn(II), or Co(II) could not substitute, signifying that the substrate of this copper-translocating P-type ATPase is Cu(I) and not Cu(II). CopA purified from dodecylmaltoside-solubilized membranes similarly exhibited Cu(I)/Ag(I)-stimulated ATPase activity, with a K(m) for ATP of 0.5 mm. CopA has two N-terminal Cys(X)(2)Cys sequences, Gly-Leu-Ser-Cys(14)-Gly-His-Cys(17), and Gly-Met-Ser-Cys(110)-Ala-Ser-Cys(113), and a Cys(479)-Pro-Cys(481) motif in membrane-spanning segment six. The requirement of these cysteine residues was investigated by the effect of mutations and deletions. Mutants with substitutions of the N-terminal cysteines or deletion of the first Cys-(X)(2)-Cys motif formed acylphosphate intermediates. From the copper dependence of phosphoenzyme formation, the mutants appear to have 2-3 fold higher affinity for Cu(I) than wild type CopA. In contrast, substitutions in Cys(479) or Cys(481) resulted in loss of copper resistance, transport and phosphoenzyme formation. These results imply that the cysteine residues of the Cys-Pro-Cys motif (but not the N-terminal cysteine residues) are required for CopA function.