Cadmium resistance from Staphylococcus aureus plasmid pI258 cadA gene results from a cadmium-efflux ATPase

Stenotrophomonas maltophilia D457R contains a cluster of genes from gram-positive bacteria involved in antibiotic and heavy metal resistance

A cluster of genes involved in antibiotic and heavy metal resistance has been characterized from a clinical isolate of the gram-negative bacterium Stenotrophomonas maltophilia. These genes include a macrolide phosphotransferase (mphBM) and a cadmium efflux determinant (cadA), together with the gene cadC coding for its transcriptional regulator. The cadC cadA region is flanked by a truncated IS257 sequence and a region coding for a bin3 invertase. Despite their presence in a gram-negative bacterium, these genetic elements share a common gram-positive origin. The possible origin of these determinants as a remnant composite transposon as well as the role of gene transfer between gram-positive and gram-negative bacteria for the acquisition of antibiotic resistance determinants in chronic, mixed infections is discussed.

Rapid screening for putative exported proteins from Staphylococcus aureus using alkaline phosphatase as a reporter molecule

Staphylococcus aureus causes a wide range of infections in humans, ranging from superficial skin infections to the more serious toxin-mediated diseases such as toxic shock syndrome. Owing to the increasing resistance of this bacterium to a wide range of antibiotics, the need to determine the virulence factors involved in infection is becoming more important as these molecules are potential therapeutic targets. In this study, we have screened for putative exported proteins from S. aureus on the basis that these proteins are likely to be the first point of contact between the bacterium and host during infection. We have constructed gene fusions between S. aureus DNA and a truncated version of the Escherichia coli phoA gene, and we report on the characterization of the recombinants exhibiting alkaline phosphatase activity. As well as known S. aureus proteins, we have identified a number of putative open reading frames that encode proteins similar to those from nonstaphylococcal species and also unique proteins that do not have any homologues on the current databases.

A gene cluster involved in metal homeostasis in the cyanobacterium Synechocystis sp. strain PCC 6803

A gene cluster composed of nine open reading frames (ORFs) involved in Ni(2+), Co(2+), and Zn(2+) sensing and tolerance in the cyanobacterium Synechocystis sp. strain PCC 6803 has been identified. The cluster includes an Ni(2+) response operon and a Co(2+) response system, as well as a Zn(2+) response system previously described. Expression of the Ni(2+) response operon (nrs) was induced in the presence of Ni(2+) and Co(2+). Reduced Ni(2+) tolerance was observed following disruption of two ORFs of the operon (nrsA and nrsD). We also show that the nrsD gene encodes a putative Ni(2+) permease whose carboxy-terminal region is a metal binding domain. The Co(2+) response system is composed of two divergently transcribed genes, corR and corT, mutants of which showed decreased Co(2+) tolerance. Additionally, corR mutants showed an absence of Co(2+)-dependent induction of corT, indicating that CorR is a transcriptional activator of corT. To our knowledge, CorR is the first Co(2+)-sensing transcription factor described. Our data suggest that this region of the Synechocystis sp. strain PCC 6803 genome is involved in sensing and homeostasis of Ni(2+), Co(2+), and Zn(2+).

Cloning, expression, and characterization of cadmium and manganese uptake genes from Lactobacillus plantarum

An Mn(2+) and Cd(2+) uptake gene, mntA, was cloned from Lactobacillus plantarum ATCC 14917 into Escherichia coli. Its expression conferred on E. coli cells increased Cd(2+) sensitivity as well as energy-dependent Cd(2+) uptake activity. Both transcription and translation of mntA were induced by Mn(2+) starvation in L. plantarum, as indicated by reverse transcriptase PCR and immunoblotting. Two Cd(2+) uptake systems have been identified in L. plantarum: one is a high-affinity Mn(2+) and Cd(2+) uptake system that is expressed in Mn(2+)-starved cells, and the other is a nonsaturable Cd(2+) uptake system that is expressed in Cd(2+)-sufficient cells (Z. Hao, H. R. Reiske, and D. B. Wilson, Appl. Environ. Microbiol. 65:592-99, 1999). MntA was not detected in an Mn(2+)-dependent mutant of L. plantarum which had lost high-affinity Mn(2+) and Cd(2+) uptake activity. The results suggest that mntA is the gene encoding the high-affinity Mn(2+) and Cd(2+) transporter. On the basis of its predicted amino acid sequence, MntA belongs to the family of P-type cation-translocating ATPases. The topology and potential Mn(2+)- and Cd(2+)-binding sites of MntA are discussed. A second clone containing a low-affinity Cd(2+) transport system was also isolated.

Molecular mechanisms of copper homeostasis

Copper is an essential trace element which plays a pivotal role in cell physiology as it constitutes a core part of important cuproenzymes. Novel components of copper homeostasis in humans have been identified recently which have been characterised at the molecular level. These include copper-transporting P-type ATPases, Menkes and Wilson proteins, and copper chaperones. These findings have paved the way towards better understanding of the role of copper deficiency or copper toxicity in physiological and pathological conditions.

Helicobacter pylori cadA encodes an essential Cd(II)-Zn(II)-Co(II) resistance factor influencing urease activity

Inactivation of Helicobacter pylori cadA, encoding a putative transition metal ATPase, was only possible in one of four natural competent H. pylori strains, designated 69A. All tested cadA mutants showed increased growth sensitivity to Cd(II) and Zn(II). In addition, some of them showed both reduced 63Ni accumulation during growth and no or impaired urease activity, which was not due to lack of urease enzyme subunits. Gene complementation experiments with plasmid (pY178)-derived H. pylori cadA failed to correct the deficiencies, whereas resistance to Cd(II) and Zn(II) was restored. Moreover, pY178 conferred increased Co(II) resistance to both the cadA mutants and the wild-type strain 69A. Heterologous expression of H. pylori cadA in an Escherichia coli zntA mutant resulted in an elevated resistance to Cd(II) and Zn(II). Expression of cadA in E. coli SE5000 harbouring H. pylori nixA, which encodes a divalent cation importer along with the H. pylori urease gene cluster, led to about a threefold increase in urease activity compared with E. coli control cells lacking the H. pylori cadA gene. These results suggest that H. pylori CadA is an essential resistance pump with ion specificity towards Cd(II), Zn(II) and Co(II). They also point to a possible role of H. pylori CadA in high-level activity of H. pylori urease, an enzyme sensitive to a variety of metal ions.

Functional analysis of the N-terminal CXXC metal-binding motifs in the human Menkes copper-transporting P-type ATPase expressed in cultured mammalian cells

The Menkes protein (MNK) is a copper-transporting P-type ATPase, which has six highly conserved metal-binding sites, GMTCXXC, at the N terminus. The metal-binding sites may be involved in MNK trafficking and/or copper-translocating activity. In this study, we report the detailed functional analysis in mammalian cells of recombinant human MNK and its mutants with various metal-binding sites altered by site-directed mutagenesis. The results of the study, both in vitro and in vivo, provide evidence that the metal-binding sites of MNK are not essential for the ATP-dependent copper-translocating activity of MNK. Moreover, metal-binding site mutations, which resulted in a loss of ability of MNK to traffick to the plasma membrane, produced a copper hyperaccumulating phenotype. Using an in vitro vesicle assay, we demonstrated that the apparent K(m) and V(max) values for the wild type MNK and its mutants were not significantly different. The results of this study suggest that copper-translocating activity of MNK and its copper-induced relocalization to the plasma membrane represent a well coordinated copper homeostasis system. It is proposed that mutations in MNK which alter either its catalytic activity or/and ability to traffick can be the cause of Menkes disease.

ZntR is an autoregulatory protein and negatively regulates the chromosomal zinc resistance operon znt of Staphylococcus aureus

A chromosomally encoded znt operon of Staphylococcus aureus consists of two consecutive putative genes designated zntR and zntA. The zntA gene encodes a transmembrane protein that facilitates extrusion of Zn2+ and Co2+, whereas the zntR gene encodes a putative regulatory protein that controls the expression of the znt operon. The zntR gene was amplified using the polymerase chain reaction, cloned into Escherichia coli for overexpression as His-tagged ZntR and purified by Ni2+-affinity column. His-tag-free ZntR was purified to near homogeneity after digestion with enterokinase. Electrophoretic mobility shift assays (EMSAs) indicated that the ZntR bound to a fragment of DNA corresponding to the chromosomal znt promoter region with an affinity of about 8.0 x 10-12 M. The addition of 25 microM Zn2+ or Co2+ in the binding reaction completely or significantly inhibited association of ZntR with the znt promoter. DNase I footprinting assays identified a ZntR binding site encompassing 49 nucleotides in the znt promoter region that contained repeated TGAA sequences. These sequences have been proposed to be the binding sites for SmtB, a metallorepressor protein from the cyanobacterium Synechococcus, to its corresponding operator/promoter. In vitro transcription assays, using S. aureus RNA polymerase, revealed that ZntR represses transcription from the znt promoter in a concentration-dependent fashion. The EMSAs, DNase I footprinting and in vitro transcription assays indicate that ZntR is a trans-acting repressor protein that binds to the znt promoter region and regulates its own transcription together with that of zntA.

Cloning and expression of cadD, a new cadmium resistance gene of Staphylococcus aureus

A cadmium resistance gene, designated cadD, has been identified in and cloned from the Staphylococcus aureus plasmid pRW001. The gene is part of a two-component operon which contains the resistance gene cadD and an inactive regulatory gene, cadX*. A high degree of sequence similarity was observed between cadD and the cadB-like gene from S. lugdunensis, but no significant similarity was found with either cadA or cadB from the S. aureus plasmids pI258 and pII147. The positive regulatory gene cadX* is identical to cadX from pLUG10 over a stretch of 78 codons beginning at the N terminus, but it is truncated at this point and inactive. Sequence analysis showed that the cadmium resistance operon resides on a 3,972-bp element that is flanked by direct repeats of IS257. The expression of cadD in S. aureus and Bacillus subtilis resulted in low-level resistance to cadmium; in contrast, cadA and cadB from S. aureus induced higher level resistance. However, when the truncated version of cadX contained in pRW001 is complemented in trans with cadX from plasmid pLUG10, resistance increased approximately 10-fold suggesting that the cadmium resistance operons from pRW001 and pLUG10 are evolutionarily related. Moreover, the truncated version of cadX contained in pRW001 is nonfunctional and may have been generated by deletion during recombination to acquire the cadmium resistance element.

Chromosome-determined zinc-responsible operon czr in Staphylococcus aureus strain 912

A novel operon, czrAB (zinc-responsible genes), was identified in the chromosome of Staphylococcus aureus. The operon consists of two genes, czrA and czrB. The czrA gene, coding for an 11.5 kDa protein, was homologous to cadC, arsR of S. aureus plasmid pI258 and smtB of Synechococcus PCC7942. The czrB, coding for a 36 kDa membrane spanning protein, was homologous to the czcD gene, cobalt, zinc and the cadmium-resistant factor of Bacillus subtilis and Alcaligenes eutrophus. In the presence of zinc (0.1-10 mM), the transcription of czrAB was enhanced in a concentration-dependent manner. Other heavy metals, such as cobalt, copper, manganese and nickel showed no effect on czrAB expression. The disruptant of the czrB gene became sensitive to zinc ion (MIC, 2 mM; MBC, 10 mM), and the complementation with the plasmid recovered the resistance to zinc at the same concentration as a parental strain (MIC, 5 mM; MBC, 20 mM). The disruptant accumulated intracellular zinc up to 0.4 mg per g dry weight of the organism, while that of the parental strain was 0.25 mg per g dry weight. The findings indicated that the novel operon czrAB should play a role in the transportation of zinc across the cell membrane to maintain the proper intracellular concentration.

ZntR is a Zn(II)-responsive MerR-like transcriptional regulator of zntA in Escherichia coli

We have identified the promoter/operator region of the zntA gene of Escherichia coli and shown that Zn(II) is the primary inducer of expression of this Zn(II)/Cd(II) export gene. The promoter PzntA shows sequence similarities to the promoters of mercury resistance (mer) operons, including a long spacer region containing an inverted repeat sequence. The gene encoding the transcriptional regulator of PzntA, designated zntR, has been identified from genome sequence data, by expression of the gene product and by insertional inactivation/complementation. The ZntR product is a member of the MerR family of transcriptional regulators and appears to act as a hypersensitive transcriptional switch. A hybrid MerR/ZntR protein has been constructed and indicates that the C-terminal region of ZntR recognizes Zn(II).

Pb(II)-translocating P-type ATPases

The cad operon of Staphylococcus aureus plasmid pI258, which confers cadmium resistance, encodes a transcriptional regulator, CadC, and CadA, an ATP-coupled Cd(II) pump that is a member of the superfamily of cation-translocating P-type ATPases. The Escherichia coli homologue of CadA, termed ZntA, is a Zn(II)/Cd(II) pump. The results described in this paper support the hypothesis that ZntA and CadA are Pb(II) pumps. First, CadC is a metal-responsive repressor that responds to soft metals in the order Pb>Cd>Zn. Second, both CadA and ZntA confer resistance to Pb(II). Third, transport of 65Zn(II) in everted membrane vesicles of E. coli catalyzed by either of these two P-type ATPase superfamily members is inhibited by Pb(II).

Luminescent bacterial sensor for cadmium and lead

A sensor plasmid was constructed by inserting the regulation unit from the cadA determinant of plasmid pI258 to control the expression of firefly luciferase. The resulting sensor plasmid pTOO24 is capable of replicating in Gram-positive and Gram-negative bacteria. The expression of the reporter gene as a function of added extracellular heavy metals was studied in Staphylococcus aureus strain RN4220 and Bacillus subtilis strain BR151. Strain RN4220(pTOO24) mainly responded to cadmium, lead and antimony, the lowest detectable concentrations being 10 nM, 33 nM and 1 nM respectively. Strain BR151(pTOO24) responded to cadmium, antimony, zinc and tin at concentrations starting from 3.3 nM, 33 nM, 1 microM, and 100 microM, respectively. The luminescence ratios between induced and uninduced cells, the induction coefficients, of strains RN4220(pTOO24) and BR151(pTOO24) were 23-50 and about 5, respectively. These results were obtained with only 2-3 h incubation times. Freeze-drying of the sensor strains had only moderate effects on the performance with respect to sensitivity or induction coefficients.

An SmtB-like repressor from Synechocystis PCC 6803 regulates a zinc exporter

ORF slr0798, now designated ziaA, from Synechocystis PCC 6803 encodes a polypeptide with sequence features of heavy metal transporting P-type ATPases. Increased Zn2+ tolerance and reduced 65Zn accumulation was observed in Synechococcus PCC 7942, strain R2-PIM8(smt), containing ziaA and upstream regulatory sequences, compared with control cells. Conversely, reduced Zn2+ tolerance was observed following disruption of ziaA in Synechocystis PCC 6803, and ziaA-mediated restoration of Zn2+ tolerance has subsequently been used as a selectable marker for transformation. Nucleotide sequences upstream of ziaA, fused to a promoterless lacZ gene, conferred Zn2+-dependent beta-galactosidase activity when introduced into R2-PIM8(smt). The product of ORF sll0792, designated ZiaR, is a Zn2+-responsive repressor of ziaA transcription. Reporter gene constructs lacking ziaR conferred elevated Zn2+-independent expression from the ziaA operator-promoter in R2-PIM8(smt). Gel retardation assays detected ZiaR-dependent complexes forming with the zia operator-promoter and ZiaR-DNA binding was enhanced by treatment with a metal-chelator in vitro. Two mutants of ZiaR (C71S/C73S and H116R) bound to, and repressed expression from, the ziaA operator-promoter but were unable to sense Zn2+. Metal coordination to His-imidazole and Cys-thiolate ligands at these residues of ZiaR is thus implicated in Zn2+-perception by Synechocystis PCC 6803.

Molecular characterization of a chromosomal determinant conferring resistance to zinc and cobalt ions in Staphylococcus aureus

A DNA fragment conferring resistance to zinc and cobalt ions was isolated from a genomic DNA library of Staphylococcus aureus RN450. The DNA sequence analysis revealed two consecutive open reading frames, designated zntR and zntA. The predicted ZntR and ZntA showed significant homology to members of ArsR and cation diffusion families, respectively. A mutant strain containing the null allele of zntA was more sensitive to zinc and cobalt ions than was the parent strain. The metal-sensitive phenotype of the mutant was complemented by a 2.9-kb DNA fragment containing zntR and zntA. An S. aureus strain harboring multiple copies of zntR and zntA showed an increased resistance to zinc. The resistance to zinc in the wild-type strain was inducible. Transcriptional analysis indicated that zntR and zntA genes were cotranscribed. The zinc uptake studies suggested that the zntA product was involved in the export of zinc ions out of cells.

A swarming-defective mutant of Proteus mirabilis lacking a putative cation-transporting membrane P-type ATPase

The motile TnphoA mutant IC24 of Proteus mirabilis U6450 generates an aberrant swarming colony, and was shown to be impaired in swarm cell differentiation, i.e. cell elongation and hyperflagellation, causing delayed and slower population migration across a solid growth medium. Levels of transcript from the flagellin filament gene fliC, the flagellar master operon flhDC, and the leucine-responsive regulatory protein gene lrp, a regulator of swarming differentiation, were reduced in IC24 mutant swarm cells. The transposon had inserted into a gene encoding a putative P-type ATPase closely related to those transporting cations across bacterial membranes. This ppa gene (Proteus P-type ATPase) was maximally expressed in differentiated swarm cells. The data suggest an effect of ion homeostasis on swarm cell differentiation, possibly mediated via the lrp-flhDC pathway.

Metal ion homeostasis and intracellular parasitism

Bacteria possess multiple mechanisms for the transport of metal ions. While many of these systems may have evolved in the first instance to resist the detrimental effects of toxic environmental heavy metals, they have since become adapted to a variety of important homeostatic functions. The 'P'-type ATPases play a key role in metal ion transport in bacteria. A Cu+-ATPase from the intracellular bacterium Listeria monocytogenes is implicated in pathogenesis, and similar pumps in Mycobacterium tuberculosis and M. leprae may play a comparable role. Intracellular bacteria require transition metal cations for the synthesis of superoxide dismutases and catalases, which constitute an important line of defence against macrophage-killing mechanisms. The macrophage protein Nramp1, which confers resistance to a variety of intracellular pathogens, has also been shown recently to be a divalent amphoteric cation transporter. Mycobacterial homologues have recently been identified by genomic analysis. These findings suggest a model in which competition for divalent cations plays a pivotal role in the interaction between host and parasite.

Properties of the P-type ATPases encoded by the copAP operons of Helicobacter pylori and Helicobacter felis

The cop operons of Helicobacter pylori and Helicobacter felis were cloned by gene library screening. Both operons contain open reading frames for a P-type ion pump (CopA) with homology to Cd2+ and Cu2+ ATPases and a putative ion binding protein (CopP), the latter representing a CopZ homolog of the copYZAB operon of Enterococcus hirae. The predicted CopA ATPases contained an N-terminal GMXCXXC ion binding motif and a membrane-associated CPC sequence. A synthetic N-terminal peptide of the H. pylori CopA ATPase bound to Cu2+ specifically, and gene disruption mutagenesis of CopA resulted in an enhanced growth sensitivity of H. pylori to Cu2+ but not to other divalent cations. As determined experimentally, H. pylori CopA contains four pairs of transmembrane segments (H1 to H8), with the ATP binding and phosphorylation domains lying between H6 and H7, as found for another putative transition metal pump of H. pylori (K. Melchers, T. Weitzenegger, A. Buhmann, W. Steinhilber, G. Sachs, and K. P. Schäfer, J. Biol. Chem. 271:446-457, 1996). The corresponding transmembrane segments of the H. felis CopA pump were identified by hydrophobicity analysis and via sequence similarity. To define functional domains, similarly oriented regions of the two enzymes were examined for sequence identity. Regions with high degrees of identity included the N-terminal Cu2+ binding domain, the regions of ATP binding and phosphorylation in the energy transduction domain, and a transport domain consisting of the last six transmembrane segments with conserved cysteines in H4, H6, and H7. The data suggest that H. pylori and H. felis employ conserved mechanisms of ATPase-dependent copper resistance.

The zntA gene of Escherichia coli encodes a Zn(II)-translocating P-type ATPase

The first Zn(II)-translocating P-type ATPase has been identified as the product of o732, a potential gene identified in the sequencing of the Escherichia coli genome. This gene, termed zntA, was disrupted by insertion of a kanamycin gene through homologous recombination. The mutant strain exhibited hypersensitivity to zinc and cadmium salts but not salts of other metals, suggesting a role in zinc homeostasis in E. coli. Everted membrane vesicles from a wild-type strain accumulated 65Zn(II) and 109Cd(II) by using ATP as an energy source. Transport was sensitive to vanadate, an inhibitor of P-type ATPases. Membrane vesicles from the zntA::kan strain did not accumulate those metal ions. Both the sensitive phenotype and transport defect of the mutant were complemented by expression of zntA on a plasmid.

Genetic analysis of regions involved in replication and cadmium resistance of the plasmid pND302 from Lactococcus lactis

The 8.8-kb Lactococcus lactis plasmid pND302 encodes resistance to cadmium (CdR). Regions of pND302 involved in replication and CdR were subcloned and sequenced. The replication region is localized on a 1.5-kb region and consists of an open reading frame (repB) preceded by a noncoding AT-rich sequence (ori) which is highly homologous to lactococcal theta-type replicons. The CdR determinant is localized on a 2.9-kb region and encodes putative proteins similar to the Cd(2+)-specific P-type efflux ATPase (CadA) and the transcriptional regulatory repressor (CadC) identified in Staphylococcus aureus, Bacillus firmus, and Listeria monocytogenes. Similar CdR determinants were also detected by PCR in other CdR plasmids isolated from different L. lactis strains.

A promoter for the first nine genes of the Escherichia coli mra cluster of cell division and cell envelope biosynthesis genes, including ftsI and ftsW

We constructed a null allele of the ftsI gene encoding penicillin-binding protein 3 of Escherichia coli. It caused blockage of septation and loss of viability when expression of an extrachromosomal copy of ftsI was repressed, providing a final proof that ftsI is an essential cell division gene. In order to complement this null allele, the ftsI gene cloned on a single-copy mini-F plasmid required a region 1.9 kb upstream, which was found to contain a promoter sequence that could direct expression of a promoterless lacZ gene on a mini-F plasmid. This promoter sequence lies at the beginning of the mra cluster in the 2 min region of the E. coli chromosome, a cluster of 16 genes which, except for the first 2, are known to be involved in cell division and cell envelope biosynthesis. Disruption of this promoter, named the mra promoter, on the chromosome by inserting the lac promoter led to cell lysis in the absence of a lac inducer. The defect was complemented by a plasmid carrying a chromosomal fragment ranging from the mra promoter to ftsW, the fifth gene downstream of ftsI, but not by a plasmid lacking ftsW. Although several potential promoter sequences in this region of the mra cluster have been reported, we conclude that the promoter identified in this study is required for the first nine genes of the cluster to be fully expressed.

N-terminal domains of human copper-transporting adenosine triphosphatases (the Wilson's and Menkes disease proteins) bind copper selectively in vivo and in vitro with stoichiometry of one copper per metal-binding repeat

N-terminal domains of the Wilson's and Menkes disease proteins (N-WND and N-MNK) were overexpressed in a soluble form in Escherichia coli as fusions with maltose-binding protein, purified, and their metal-binding properties were characterized. Both N-MNK and N-WND bind copper specifically as indicated by the results of metal-chelate chromatography, direct copper-binding measurements, and chemical modification of Cys residues in the presence of different heavy metals. When E. coli cells are grown in the presence of copper, N-MNK and N-WND bind copper in vivo with stoichiometry of 5-6 nmol of copper/nmol of protein. Copper released from the copper-N-MNK and copper-N-WND complexes reacts with the Cu(I)-selective chelator bicinchoninic acid in the absence of reducing agents. This suggests that in proteins, it is bound in reduced Cu(I) form, in agreement with the spectroscopic properties of the copper-bound domains. Copper bound to the domains in vivo or in vitro specifically protects the N-MNK and N-WND against labeling with the cysteine-directed probe; this indicates that Cys residues in the repetitive motifs GMTCXXCXXXIE are involved in coordination of copper. Direct involvement of the N-terminal domains in the binding of copper suggests their important role in copper-dependent functions of human copper-transporting adenosine triphosphatases (Wilson's and Menkes disease proteins).

Cloning of a cDNA encoding a putative metal-transporting P-type ATPase from Arabidopsis thaliana

Metal-transporting P-type ATPases were recently proposed to constitute a newly emerged sub-family of cation-transporting P-type ATPases, and are known to occur widely in prokaryotes and eukaryotes. However, no instance has been reported for higher plants. A cDNA clone encoding a metal-transporting P-type ATPase was thus searched for, if present, and was identified in Arabidopsis thaliana. The amino acid sequence, predicted from the determined nucleotide sequence for the cloned cDNA, shows all the critical features common to known metal-transporting P-type ATPases. This plant P-type ATPase has a typical metal-binding motif at its N-terminal portion. The newly isolated Arabidopsis gene, named PAA1, provides us with the first instance of putative metal-transporting P-type ATPases in higher plants. Some results of genomic analyses for this gene are also presented.

Biotechnological aspects of membrane function

An overview is given of the biotechnological utilizability of various features of cell membranes. Techniques are given that describe how to make use of the barrier and transport functions of membranes for biotechnological purposes, ranging from cell permeabilization and construction of immobilized biocatalysts to manipulating excretion and uptake properties of the membranes by various methods. Glucose transporters, iron-transporting membrane systems, and pumps engaged in pleiotropic drug resistance are treated in more detail as particularly biotechnologically important membrane proteins.

Copper incorporation into superoxide dismutase in Menkes lymphoblasts

The incorporation of copper into Cu,Zn-superoxide dismutase (SOD) was examined in Menkes lymphoblasts that express a genetic defect of copper metabolism. SOD activity was approximately 40% higher in Menkes than normal lymphoblasts. Since Menkes lymphoblasts contain elevated copper levels, the higher SOD activity is most likely due to near copper saturation of an apoSOD pool that is in normal lymphoblasts. Cycloheximide markedly inhibited 64Cu(II) incorporation into SOD in Menkes lymphoblasts under conditions in which no significant, de novo synthesis of SOD protein was detected with normal lymphoblasts. The maximal amount of 64Cu incorporation into newly synthesized SOD in Menkes lymphoblasts was approximately equal to the maximal amount of 64Cu that could be incorporated into the apoSOD pool in normal lymphoblasts. The increased synthesis of SOD in Menkes lymphoblasts may play a protective role against copper toxicity in Menkes lymphoblasts. The protonophore, CCCP markedly inhibited 64Cu incorporation into SOD in both normal and Menkes lymphoblasts, which is consistent with 64Cu incorporation into SOD within a membrane-bounded compartment in both cell types. When 64Cu-incorporation into SOD was blocked with CCCP, copper accumulated in a Superose column fraction that contains S-adenosylhomocysteine hydrolase (SAHH), which has a high affinity for copper. SAHH may play a role in delivering copper to SOD.

A novel gene cluster including the Rhodococcus rhodochrous J1 nhlBA genes encoding a low molecular mass nitrile hydratase (L-NHase) induced by its reaction product

The 3.5 kilobases (kb) of the 5'-upstream region from nhlBA encoding a cobalt-containing low molecular mass nitrile hydratase (L-NHase) from Rhodococcus rhodochrous J1 was found to be required for the amide-dependent expression of nhlBA in experiments using a Rhodococcus transformation system. Sequence analysis of the 3.5-kb fragment revealed the presence of two open reading frames (nhlD and nhlC) in this fragment. NhlD has similarity to regulators MerR, CadC, and ArsR. NhlC has similarity to the regulators AmiC, for the expression of an aliphatic amidase from Pseudomonas aeruginosa, and NhhC, for the expression of a high molecular mass nitrile hydratase from R. rhodochrous J1. Assays of NHase activity of transformants carrying nhlD deletion or nhlC deletion mutations suggest a negative regulatory role for nhlD and a positive regulatory role for nhlC in the process of the L-NHase formation. Assays of NHase and amidase activities and Western blot analyses of each Rhodococcus transformant carrying various deletion plasmids, have shown that nhlBA and amdA encoding an amidase, which is located 1.9 kb downstream of nhlBA, were regulated in the same manner. These findings present the genetic evidence for a novel gene cluster controlling the expression of L-NHase, which is induced by the reaction product (amide) in the "practical microorganism" R. rhodochrous J1.

The Legionella pneumophila hel locus encodes intracellularly induced homologs of heavy-metal ion transporters of Alcaligenes spp

We continued characterization of the Legionella pneumophila hel locus. Mutagenesis and DNA sequencing identified three genes similar to the czc and cnr loci of Alcaligenes eutrophus and the ncc locus of Alcaligenes xylosoxidans. On the basis of their similarity to these loci, we designated the L. pneumophila genes helC, helB, and helA. Mutations in the hel genes led to reduced cytopathicity towards U937 cells, although the mutant strains did not appear defective in other assays of virulence. Transcription of the hel locus was induced by the intracellular environment but was not induced by any of a variety of in vitro stress conditions. The function of the hel gene products remains to be determined.

The amyloid precursor protein of Alzheimer's disease in the reduction of copper(II) to copper(I)

The transition metal ion copper(II) has a critical role in chronic neurologic diseases. The amyloid precursor protein (APP) of Alzheimer's disease or a synthetic peptide representing its copper-binding site reduced bound copper(II) to copper(I). This copper ion-mediated redox reaction led to disulfide bond formation in APP, which indicated that free sulfhydryl groups of APP were involved. Neither superoxide nor hydrogen peroxide had an effect on the kinetics of copper(II) reduction. The reduction of copper(II) to copper(I) by APP involves an electron-transfer reaction and could enhance the production of hydroxyl radicals, which could then attack nearby sites. Thus, copper-mediated toxicity may contribute to neurodegeneration in Alzheimer's disease.

Cloning and expression of the unique Ca2+-ATPase from Flavobacterium odoratum

The 60-kDa Ca2+-ATPase from Flavobacterium odoratum is kinetically and mechanistically similar to other P-type ATPases, suggesting its use as a model system for structure-function studies of ion transport. A portion of the gene was amplified by polymerase chain reaction of genomic DNA with degenerate oligonucleotide primers, one based on the N-terminal amino acid sequence of the purified protein and the other based on a consensus sequence for the phosphorylation site of P-type ATPases. This gene fragment was used to screen a lambda library of F. odoratum 29979 DNA. Clone "C" is 3.3 kilobases in length and contains one complete and part of a second open reading frame, the first of which encodes a 58-kDa protein containing the exact N-terminal amino acid sequence of the purified protein. We have named this gene cda, for calcium-dependent ATPase. Escherichia coli, transformed with clone C, demonstrates high levels of calcium-dependent and vanadate-sensitive ATP hydrolysis activity, forms a 60-kDa phosphointermediate, and cross-reacts with antibodies to the purified Ca2+-ATPase. The gene has almost no sequence homology to even the highly conserved regions characteristic of P-type ATPases but does possess significant homology to a protein with alkaline phosphatase activity (PhoD) from Zymomonas mobilis. The putative phosphorylation site is a Walker A (P-loop) ATP binding sequence and is modified relative to P-type ATPases, suggesting that the F. odoratum Ca2+-ATPase may represent an ancestral link between the F- and the P-type ATPases or perhaps a new class of ATPases.

Cloning and membrane topology of a P type ATPase from Helicobacter pylori

Southern blot screening of a genomic Helicobacter pylori library was employed to find a P type ATPase using a mixture of 16 DNA oligonucleotides coding for the DKTGT(I/L)T consensus sequence specific for the phosphorylation site of this family of ATPases. A positive clone, pRH439, was isolated and sequenced. The inserted 3.4-kb H. pylori DNA contained an intact open reading frame encoding a protein of 686 amino acids carrying the consensus sites for phosphorylation and ATP binding. The amino acid sequence exhibits a 25-30% identity with bacterial Cd2+ and Cu2+ ATPases. Genomic Southern blot analysis showed that this ATPase was present in all H. pylori strains examined, whereas it was not detectable in Campylobacter jejuni and other bacteria. The membrane topology of this ATPase was investigated using in vitro transcription/translation of fusion vectors to find signal anchor and/or stop transfer sequences. Eight regions of the H. pylori ATPase acted as signal anchor and/or stop transfer sequences and were ordered pairwise along the polypeptide chain placing the N and C-terminal amino acids in the cytoplasm. These transmembrane segments are contained between positions 73 and 92 (H1), 98 and 125 (H2), 128 and 148 (H3), 149 and 176 (H4), 309 and 327 (H5), 337 and 371 (H6), 637 and 658 (H7), and 659 and 685 (H8). The membrane domain of the ATPase, therefore, consists of at least four pairs of transmembrane segments with the phosphorylation site and ATP binding domain located in the large cytoplasmic loop between H6 and H7. The cytoplasmic domain contains several histidines and cysteines, perhaps indicative of divalent cation binding sites. There are several charged amino acids (3 Lys, 2 Glu, 2 Asp), predicted to be in the membrane domain mainly in H2, H3, and H4 and a Cys-Pro-Cys putative metal ion site in H6. The extracytoplasmic domain also has several charged amino acids (5 Glu, 1 Asp, 1 Lys, 1 Arg). It is likely that this novel protein is a heavy metal cation transporting ATPase and belongs to a family of P type ATPases containing eight transmembrane segments.

Identification of a major hepatic copper binding protein as S-adenosylhomocysteine hydrolase

The properties of a mouse liver copper binding protein (CuBP) and human placental S-adenosylhomocysteine hydrolase (SAHH) were compared to test the hypothesis that CuBP is SAHH. CuBP and SAHH migrated identically on SDS-polyacrylamide gel electrophoresis gels, and their 48-kDa monomers both self-associate to tetramers. Human placental SAHH cross-reacted with polyclonal antibodies to mouse liver CuBP, and CuBP from mouse liver cross-reacted with two monoclonal antibodies to human placental SAHH. A third monoclonal antibody to human placenta SAHH reacted weakly with the mouse liver protein but well with CuBP from human lymphoblasts. NAD(+)-activated CuBP has high SAHH enzymatic activity. Moreover, human placental SAHH, like mouse liver CuBP, has a single high affinity copper binding site per 48-kDa subunit. Thus, the data confirm that CuBP is SAHH, and SAHH is proposed to be a bifunctional protein with roles in sulfur-amino acid metabolism and copper metabolism. The copper binding activity of SAHH is proposed to play a significant role in the intracellular distribution of copper, and SAHH enzymatic activity may influence copper metabolism through its role in cysteine biosynthesis from methionine.

Copper binding to mouse liver S-adenosylhomocysteine hydrolase and the effects of copper on its levels

The dissociation constant and stoichiometry of copper binding to mouse liver S-adenosylhomocysteine hydrolase (SAHH) was determined as part of characterizing the possible roles of SAHH in copper metabolism. Copper (64Cu(II)) binding was measured by an ultrafiltration method in the presence of EDTA as a competing ligand. The KD was 3.9 +/- 0.7 x 10(-16) M, and the stoichiometry was one g atom of copper per 48-kDa subunit. Western blots indicated that the liver contains approximately 12 times more SAHH than the kidney, which in turn contains approximately 5 times more SAHH than the brain. The high concentration and copper affinity of SAHH in the liver may contribute to the liver's ability to preferentially accumulate copper, and the low levels of SAHH in the brain may contribute to the sensitivity of the brain to copper deficiency. The effects of genetic defects of copper metabolism and copper deficiency on SAHH were also determined. Normal SAHH levels were detected in brindled mouse liver, kidney, and brain. However, SAHH from brindled mouse liver eluted abnormally from phenyl Superose columns implying an effect of the brindled mouse defect on SAHH protein structure. Hepatic cytosols from the toxic milk mouse contained approximately 42% the amount of SAHH detected in controls, and hepatic levels of SAHH were also decreased by approximately 45% in copper-deficient mice. The binding properties of SAHH and the effects of abnormal states of copper metabolism on its levels are consistent with significant roles for SAHH in normal and abnormal copper metabolism. SAHH may have roles in regulating tissue copper levels and the distribution of intracellular copper.

CadC, the transcriptional regulatory protein of the cadmium resistance system of Staphylococcus aureus plasmid pI258

The CadC protein from the cadA cadmium resistance operon of Staphylococcus aureus plasmid pI258 regulates transcription of this system in vitro. The CadC protein was overproduced in Escherichia coli cells and partially purified. Gel shift assays of the proposed cadA operator/promoter region DNA showed specific association with the CadC protein. Control arsenic resistance operator/promoter DNA from the same plasmid was not shifted by the CadC protein. Cd2+, Bi3+, and Pb2+ caused the release of CadC from DNA in gel retardation assays. DNase I footprinting measurements showed that the CadC protein specifically associated with and protected a region of operator/promoter DNA from nucleotide positions -7 to +14 relative to the start point of mRNA synthesis. Runoff transcription assays with the operator/promoter region of DNA (plus the first 69 nucleotides of the cadC gene) and purified E. coli RNA polymerase gave an mRNA product of the predicted size. Added CadC protein inhibited transcription in vitro.

The ATX1 gene of Saccharomyces cerevisiae encodes a small metal homeostasis factor that protects cells against reactive oxygen toxicity

In aerobic organisms, protection against oxidative damage involves the combined action of highly specialized antioxidant enzymes, such as superoxide dismutase (SOD) and catalase. Here we describe the isolation and characterization of another gene in the yeast Saccharomyces cerevisiae that plays a critical role in detoxification of reactive oxygen species. This gene, named ATX1, was originally isolated by its ability to suppress oxygen toxicity in yeast lacking SOD. ATX1 encodes a 8.2-kDa polypeptide exhibiting significant similarity and identity to various bacterial metal transporters. Potential ATX1 homologues were also identified in multicellular eukaryotes, including the plants Arabidopsis thaliana and Oryza sativa and the nematode Caenorhabditis elegans. In yeast cells, ATX1 evidently acts in the transport and/or partitioning of copper, and this role in copper homeostasis appears to be directly relevant to the ATX1 suppression of oxygen toxicity: ATX1 was incapable of compensating for SOD when cells were depleted of exogenous copper. Strains containing a deletion in the chromosomal ATX1 locus were generated. Loss of ATX1 function rendered both mutant and wild-type SOD strains hypersensitive toward paraquat (a generator of superoxide anion) and was also associated with an increased sensitivity toward hydrogen peroxide. Hence, ATX1 protects cells against the toxicity of both superoxide anion and hydrogen peroxide.

Copper and silver transport by CopB-ATPase in membrane vesicles of Enterococcus hirae

The P-type ATPase, CopB, of Enterococcus hirae is required for the copper resistance displayed by this organism and thus was postulated to be a copper pump. Using 64Cu+ and 110mAg+, we here show ATP-driven copper and silver accumulation catalyzed by CopB in native inside-out membrane vesicles of E. hirae. CopB ATPase exhibited an apparent Km for Cu+ and Ag+ of 1 microM and for ATP of 10 microM. Transport was maximal at pH 6 and had an apparent Vmax of 0.07 nmol.min-1.mg-1 for both copper and silver transport. Vanadate displayed a biphasic effect on transport: maximal inhibition was observed at 40 microM vanadate for copper transport and 60 microM for silver transport, respectively. At higher vanadate concentrations, these inhibitions were reversed. The CopB ATPase of E. hirae is thus a pump for the extrusion of monovalent copper and silver ions, with copper probably being the natural substrate.

Characterization of the exon structure of the Menkes disease gene using vectorette PCR

The gene defective in Menkes disease, an X-linked recessive disturbance of copper metabolism, has been isolated and predicted to encode a copper-binding P-type ATPase. We determined the complete exon-intron structure of the Menkes disease gene, which spans about 150 kb of genomic DNA. The gene contains 23 exons, and the ATG start codon is in the second exon. All of the exon-intron boundaries were sequenced and conformed to the GT/AT rule, except for the 5' splice site of intron 9. A preliminary comparison demonstrated a striking similarity between the exon structures of the Menkes and Wilson disease genes, giving insight into their evolution.

Understanding cellular responses to toxic agents: a model for mechanism-choice in bacterial metal resistance

Bacterial resistances to metals are heterogeneous in both their genetic and biochemical bases. Metal resistance may be chromosomally-, plasmid- or transposon-encoded, and one or more genes may be involved: at the biochemical level at least six different mechanisms are responsible for resistance. Various types of resistance mechanisms can occur singly or in combination and for a particular metal different mechanisms of resistance can occur in the same species. To understand better the diverse responses of bacteria to metal ion challenge we have constructed a qualitative model for the selection of metal resistance in bacteria. How a bacterium becomes resistant to a particular metal depends on the number and location of cellular components sensitive to the specific metal ion. Other important selective factors include the nature of the uptake systems for the metal, the role and interactions of the metal in the normal metabolism of the cell and the availability of plasmid (or transposon) encoded resistance mechanisms. The selection model presented is based on the interaction of these factors and allows predictions to be made about the evolution of metal resistance in bacterial populations. It also allows prediction of the genetic basis and of mechanisms of resistance which are in substantial agreement with those in well-documented populations. The interaction of, and selection for resistance to, toxic substances in addition to metals, such as antibiotics and toxic analogues, involve similar principles to those concerning metals. Potentially, models for selection of resistance to any substance can be derived using this approach.

Bacterial resistance mechanisms for heavy metals of environmental concern

Bacterial species have genetically-determined systems for resistances to toxic heavy metals. Those for metals of environmental concern including mercury cadmium, arsenic and others are briefly summarized, considering the genes of the systems and the biochemical mechanisms by which the resistance proteins function.

Ion efflux systems involved in bacterial metal resistances

Studying metal ion resistance gives us important insights into environmental processes and provides an understanding of basic living processes. This review concentrates on bacterial efflux systems for inorganic metal cations and anions, which have generally been found as resistance systems from bacteria isolated from metal-polluted environments. The protein products of the genes involved are sometimes prototypes of new families of proteins or of important new branches of known families. Sometimes, a group of related proteins (and presumedly the underlying physiological function) has still to be defined. For example, the efflux of the inorganic metal anion arsenite is mediated by a membrane protein which functions alone in Gram-positive bacteria, but which requires an additional ATPase subunit in some Gram-negative bacteria. Resistance to Cd2+ and Zn2+ in Gram-positive bacteria is the result of a P-type efflux ATPase which is related to the copper transport P-type ATPases of bacteria and humans (defective in the human hereditary diseases Menkes' syndrome and Wilson's disease). In contrast, resistance to Zn2+, Ni2+, Co2+ and Cd2+ in Gram-negative bacteria is based on the action of proton-cation antiporters, members of a newly-recognized protein family that has been implicated in diverse functions such as metal resistance/nodulation of legumes/cell division (therefore, the family is called RND). Another new protein family, named CDF for 'cation diffusion facilitator' has as prototype the protein CzcD, which is a regulatory component of a cobalt-zinc-cadmium resistance determinant in the Gram-negative bacterium Alcaligenes eutrophus. A family for the ChrA chromate resistance system in Gram-negative bacteria has still to be defined.

Nucleotide sequence and mutational analysis indicate that two Helicobacter pylori genes encode a P-type ATPase and a cation-binding protein associated with copper transport

A 2.7 kb fragment of Helicobacter pylori UA802 chromosomal DNA was cloned and sequenced. Three open reading frames (designated ORF1, ORF2 and ORF3, respectively) were predicted from the DNA sequence, of which ORF1 and ORF2 appeared to be located within the same operon. The deduced 611-amino-acid sequence of ORF1, a P-type ATPase (designated hpCopA), had striking homology (29-38%) with several bacterial P-type ATPase and contained the potential functional domains conserved in P-type ATPases from various sources ranging from bacterial to human. A protein of 66 amino acids (designated hpCopP) encoded by ORF2 shared extensive sequence similarity with MerP, a periplasmic mercuric ion-transporting protein, and contains the heavy metal-binding motif. Disruption of ORF1 with a chloramphenicol-resistance cassette (CAT) rendered the H. pylori mutants more susceptible to cupric ion than their parental strains, whereas there is no significant alteration of susceptibility to Ni2+, Cd2d+ and Hg2+ between the mutants and the parental strains. The results obtained indicate that ORF1 and ORF2 comprise a cation-transporting system which is associated with copper export out of the H. pylori cells.

Combined nickel-cobalt-cadmium resistance encoded by the ncc locus of Alcaligenes xylosoxidans 31A

The nickel-cobalt-cadmium resistance genes carried by plasmid pTOM9 of Alcaligenes xylosoxidans 31A are located on a 14.5-kb BamHI fragment. By random Tn5 insertion mutagenesis, the fragment was shown to contain two distinct nickel resistance loci, ncc and nre. The ncc locus causes a high-level combined nickel, cobalt, and cadmium resistance in strain AE104, which is a cured derivative of the metal-resistant bacterium Alcaligenes eutrophus CH34. ncc is not expressed in Escherichia coli. The nre locus causes low-level nickel resistance in both Alcaligenes and E. coli strains. The nucleotide sequence of the ncc locus revealed seven open reading frames designated nccYXHCBAN. The corresponding predicted proteins share strong similarities with proteins encoded by the metal resistance loci cnr (cnrYXHCBA) and czc (czcRCBAD) of A. eutrophus CH34. When different DNA fragments carrying ncc genes were heterologously expressed under the control of the bacteriophage T7 promoter, five protein bands representing NccA (116 kDa), NccB (40 kDa), NccC (46 kDa), NccN (23.5 kDa), and NccX (16.5 kDa) were detected.

P-type ATPase from the cyanobacterium Synechococcus 7942 related to the human Menkes and Wilson disease gene products

DNA encoding a P-type ATPase was cloned from the cyanobacterium Synechococcus 7942. The cloned ctaA gene encodes a 790-amino acid polypeptide related to the CopA Cu(2+)-uptake ATPase of Enterococcus hirae, to other known P-type ATPases, and to the candidate gene products for the human diseases of copper metabolism, Menkes disease and Wilson disease. Disruption of the single chromosomal gene in Synechococcus 7942 by insertion of an antibiotic-resistance cassette results in a mutant cell line with increased tolerance to Cu2+ compared with the wild type.

A putative P-type Cu(2+)-transporting ATPase gene on chromosome II of Saccharomyces cerevisiae

We have sequenced a gene on the right arm near the telomere of chromosome II of Saccharomyces cerevisiae which codes for a putative P-type cation-transporting ATPase (PCA1). The gene codes for a 1216 amino acids protein. The PCA1 gene expresses a 3.5 kb message in both haploid and diploid cells when grown in glucose-based rich medium YPD. The gene product is most similar at the C-terminal region to a human copper-transporting ATPase and Enterococcus hirae copper-transporting ATPases and also an N-terminal dithiol region that was proposed to be a 'metal-binding motif'. Cells lacking PCA1 display no obvious phenotype when tested under standard conditions: whereas they cease growth much earlier than the isogenic wild-type cells in a minimal medium with high copper concentration. Overexpression of PCA1 under GAL1/10 promoter in yeast cells causes poor growth. We also show that yeast strains carrying PCA1 in multiple copies grow slower than isogenic wild-type strains in a minimal synthetic medium containing 0.3 mM-CuSO4.