A topological model for the high-affinity nickel transporter of Alcaligenes eutrophus

Metal selectivity and translocation mechanism characterization in proteoliposomes of the transmembrane NiCoT transporter NixA from Helicobacter pylori

Essential trace metals play key roles in the survival, replication, and virulence of bacterial pathogens. Helicobacter pylori (H. pylori), the main bacterial cause of gastric ulcers, requires Ni(ii) to colonize and persist in the acidic environment inside the stomach, exploiting the nickel-containing enzyme urease to catalyze the hydrolysis of urea to ammonia and bicarbonate and create a pH-buffered microenvironment. Urease utilizes Ni(ii) as a catalytic cofactor for its activity. In ureolytic bacteria, unique transmembrane (TM) transporters evolved to guarantee the selective uptake and efflux of Ni(ii) across cellular membranes to meet the cellular requirements. NixA is an essential Ni(ii) transporter expressed by H. pylori when the extracellular environment experiences a drop in pH. This Class I nickel-cobalt transporter of the NiCoT family catalyzes the uptake of Ni(ii) across the inner membrane from the periplasm. In this study, we characterized NixA using a platform whereby, for the first time on a NiCoT transporter, recombinantly expressed and purified NixA and key mutants in the translocation pathway have been reconstituted in artificial lipid bilayer vesicles (proteoliposomes). Fluorescent sensors responsive to Ni(ii) transport (Fluozin-3-Zn(ii)), luminal pH changes (pyranine), and membrane potential (oxonol VI) were encapsulated in the proteoliposomes lumen to monitor, in real-time, NixA transport properties and translocation mechanism. Kinetic transport analysis revealed that NixA is highly selective for Ni(ii) with no substrate promiscuity towards Co(ii), the other putative metal substrate of the NiCoT family, nor Zn(ii). NixA-mediated Ni(ii) transport exhibited a Michaelis-Menten-type saturable substrate concentration dependence, with an experimental KM, Ni(ii) = 31.0 ± 1.2 μM. Ni(ii) transport by NixA was demonstrated to be electrogenic, and metal translocation did not require a proton motive force, resulting in the generation of a positive-inside transmembrane potential in the proteoliposome lumen. Mutation analysis characterized key transmembrane residues for substrate recognition, binding, and/or transport, suggesting the presence of a three-step transmembrane translocation conduit. Taken together, these investigations reveal that NixA is a Ni(ii)-selective Class I NiCoT electrogenic uniporter. The work also provides an in vitro approach to characterize the transport properties of metal transporters responsible for Ni(ii) acquisition and extrusion in prokaryotes.

Mechanisms of nickel toxicity in microorganisms

Nickel has long been known to be an important human toxicant, including having the ability to form carcinomas, but until recently nickel was believed to be an issue only to microorganisms living in nickel-rich serpentine soils or areas contaminated by industrial pollution. This assumption was overturned by the discovery of a nickel defense system (RcnR/RcnA) found in microorganisms that live in a wide range of environmental niches, suggesting that nickel homeostasis is a general biological concern. To date, the mechanisms of nickel toxicity in microorganisms and higher eukaryotes are poorly understood. In this review, we summarize nickel homeostasis processes used by microorganisms and highlight in vivo and in vitro effects of exposure to elevated concentrations of nickel. On the basis of this evidence we propose four mechanisms of nickel toxicity: (1) nickel replaces the essential metal of metalloproteins, (2) nickel binds to catalytic residues of non-metalloenzymes; (3) nickel binds outside the catalytic site of an enzyme to inhibit allosterically and (4) nickel indirectly causes oxidative stress.

Identification of the transcriptional regulator NcrB in the nickel resistance determinant of Leptospirillum ferriphilum UBK03

The nickel resistance determinant ncrABCY was identified in Leptospirillum ferriphilum UBK03. Within this operon, ncrA and ncrC encode two membrane proteins that form an efflux system, and ncrB encodes NcrB, which belongs to an uncharacterized family (DUF156) of proteins. How this determinant is regulated remains unknown. Our data indicate that expression of the nickel resistance determinant is induced by nickel. The promoter of ncrA, designated pncrA, was cloned into the promoter probe vector pPR9TT, and co-transformed with either a wild-type or mutant nickel resistance determinant. The results revealed that ncrB encoded a transcriptional regulator that could regulate the expression of ncrA, ncrB, and ncrC. A GC-rich inverted repeat sequence was identified in the promoter pncrA. Electrophoretic mobility shift assays (EMSAs) and footprinting assays showed that purified NcrB could specifically bind to the inverted repeat sequence of pncrA in vitro; this was confirmed by bacterial one-hybrid analysis. Moreover, this binding was inhibited in the presence of nickel ions. Thus, we classified NcrB as a transcriptional regulator that recognizes the inverted repeat sequence binding motif to regulate the expression of the key nickel resistance gene, ncrA.

Genomic analyses of transport proteins in Ralstonia metallidurans

Ralstonia (Wautersia, Cupriavidus) metallidurans (Rme) is better able to withstand high concentrations of heavy metals than any other well-studied organism. This fact renders it a potential agent of bioremediation as well as an ideal model organism for understanding metal resistance phenotypes. We have analysed the genome of Rme for genes encoding homologues of established and putative transport proteins; 13% of all genes in Rme encode such homologues. Nearly one-third of the transporters identified (32%) appear to function in inorganic ion transport with three-quarters of these acting on cations. Transporters specific for amino acids outnumber sugar transporters nearly 3 : 1, and this fact plus the large number of uptake systems for organic acids indicates the heterotrophic preferences of these bacteria. Putative drug efflux pumps comprise 10% of the encoded transporters, but numerous efflux pumps for heavy metals, metabolites and macromolecules were also identified. The results presented should facilitate genetic manipulation and mechanistic studies of transport in this remarkable bacterium.

Nickel-resistant determinant from Leptospirillum ferriphilum

Leptospirillum ferriphilum strain UBK03 isolated from a mine in Jiangxi, China, is resistant to Ni(2+) (30 to 40 mM). A four-gene nickel resistance cluster was identified and, when transformed into Escherichia coli, enabled growth in 6 mM nickel. Mutation experiments revealed that the genes ncrA, ncrB, and ncrC could confer nickel resistance in Escherichia coli, whereas the gene ncrY could have a negative effect on nickel resistance.

Nickelophilous plants and their significance in phytotechnologies

Nickeliferous soils are invaded predominantly by members of the Brassicaceae, Cyperaceae, Cunoniaceae, Caryophyllaceae, Fabaceae, Flacourtiaceae, Euphorbiaceous, Lamiaceae, Poaceae and Violaceae, and many of these plants are metal tolerant. About 300 Ni hyperaccumulating plants been identified. These members exhibit unusual appetite for toxic metals and elemental defense. Hyperaccumulators provide protection against fungal and insect attack. Investigations suggested that Ni-hyperaccumulation has a protective function against fungal and bacterial pathogens in Streptanthus polygaloides and Thlaspi montanum. Significance of nickelophilous plants and their significance in phytotechnologies are discussed in this paper.

Identification and characterization of the nickel uptake system for urease biogenesis in Streptococcus salivarius 57.I

Ureases are multisubunit enzymes requiring Ni(2+) for activity. The low pH-inducible urease gene cluster in Streptococcus salivarius 57.I is organized as an operon, beginning with ureI, followed by ureABC (structural genes), and ureEFGD (accessory genes). Urease biogenesis also requires a high-affinity Ni(2+) uptake system. By searching the partial genome sequence of a closely related organism, Streptococcus thermophilus LMG18311, three open reading frame (ORFs) homologous to those encoding proteins involved in cobalamin biosynthesis and cobalt transport (cbiMQO) were identified immediately 3' to the ure operon. To determine whether these genes were involved in urease biogenesis by catalyzing Ni(2+) uptake in S. salivarius, regions 3' to ureD were amplified by PCRs from S. salivarius by using primers identical to the S. thermophilus sequences. Sequence analysis of the products revealed three ORFs. Reverse transcriptase PCR was used to demonstrate that the ORFs are transcribed as part of the ure operon. Insertional inactivation of ORF1 with a polar kanamycin marker completely abolished urease activity and the ability to accumulate (63)Ni(2+) during growth. Supplementation of the growth medium with NiCl(2) at concentrations as low as 2.5 micro M partially restored urease activity in the mutant. Both wild-type and mutant strains showed enhanced urease activity when exogenous Ni(2+) was provided at neutral pH. Enhancement of urease activity by adding nickel was regulated at the posttranslational level. Thus, ORF1, ORF2, and ORF3 are part of the ure operon, and these genes, designated ureM, ureQ, and ureO, respectively, likely encode a Ni(2+)-specific ATP-binding cassette transporter.

Nickel uptake and utilization by microorganisms

Nickel is an essential nutrient for selected microorganisms where it participates in a variety of cellular processes. Many microbes are capable of sensing cellular nickel ion concentrations and taking up this nutrient via nickel-specific permeases or ATP-binding cassette-type transport systems. The metal ion is specifically incorporated into nickel-dependent enzymes, often via complex assembly processes requiring accessory proteins and additional non-protein components, in some cases accompanied by nucleotide triphosphate hydrolysis. To date, nine nickel-containing enzymes are known: urease, NiFe-hydrogenase, carbon monoxide dehydrogenase, acetyl-CoA decarbonylase/synthase, methyl coenzyme M reductase, certain superoxide dismutases, some glyoxylases, aci-reductone dioxygenase, and methylenediurease. Seven of these enzymes have been structurally characterized, revealing distinct metallocenter environments in each case.

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.

Substrate specificity of nickel/cobalt permeases: insights from mutants altered in transmembrane domains I and II

HoxN, a high-affinity, nickel-specific permease of Ralstonia eutropha H16, and NhlF, a nickel/cobalt permease of Rhodococcus rhodochrous J1, are structurally related members of the nickel/cobalt transporter (NiCoT) family. These transporters have an eight-helix structure and are characterized by highly conserved segments with polar or charged amino acid residues in transmembrane domains (TMDs) II, III, V, and VI. Two histidine residues in a Ni2+ binding motif, the signature sequence of NiCoTs, in TMD II of HoxN have been shown to be crucial for activity. Replacement of the corresponding His residues in NhlF affected both Co2+ and Ni2+ uptake, demonstrating that NhlF employs a HoxN-like mechanism for transport of the two cations. Multiple alignments of bacterial NiCoT sequences identified a striking correlation between a hydrophobic residue (Val or Phe) in TMD II and a position in the center of TMD I occupied by either an Asn (as in HoxN) or a His (as in NhlF). Introducing an isoleucine residue at the latter position strongly reduced HoxN activity and abolished NhlF activity, suggesting that a Lewis base N-donor moiety is important. The Asn-to-His exchange had no effect on HoxN, whereas the converse replacement reduced NhlF-mediated Ni2+ uptake significantly. Replacement of the entire TMD I of HoxN by the respective NhlF segment resulted in a chimera that transported Ni2+ and Co2+ with low capacity. The Val-to-Phe exchange in TMD II of HoxN led to a considerable rise in Ni2+ uptake capacity and conferred to the variant the ability to transport Co2+. NhlF activity dropped in response to the converse mutation. Our data predict that TMDs I and II in NiCoTs spatially interact to form a critical part of the selectivity filter. As seen for the V64F variant of HoxN, modification of this site can increase the velocity of transport and concomitantly reduce the specificity.

Conserved low-affinity nickel-binding amino acids are essential for the function of the nickel permease NixA of Helicobacter pylori

Nickel acquisition is necessary for urease activity, a major virulence factor of the human gastric pathogen Helicobacter pylori. The nickel permease NixA of H. pylori is a member of the single-component nickel-cobalt transporter family. To identify functionally relevant amino acids of NixA, single-site exchanges were introduced into NixA via PCR-based mutagenesis. This study investigated one of the recognition motifs for this family in transmembrane segment III and other conserved amino acids, mostly with possible nickel-binding capacities. The mutant alleles were expressed in Escherichia coli, and activity of the altered permeases was analyzed by measuring nickel accumulation and urease activity. Expression was checked by immunoblotting after sodium dodecyl sulfate-polyacrylamide gel electrophoresis with a NixA-specific antibody. Replacement of Phe-75 and His-79-both part of the characteristic sequence motif-and of Asn-127, Thr-195, and Ser-197 with alanine abolished nickel uptake in the E. coli system. The results were unchanged if these amino acids were replaced with residues more similar to the original amino acid. The phenotype of the null mutants was independent of the culture medium. Mutation of Val-82, Tyr-242, Thr-260, His-181, and His-15 strongly affected uptake activity under nickel limitation on complex Luria-Bertani medium but had little effect in minimal medium. Eight other conserved amino acids (Ser-80, Ser-81, Phe-119, Trp-180, Tyr-183, Trp-244, Pro-249, and Asn-256) were found to be dispensable for the function of NixA. These results show that atypical nickel-binding amino acids play an important function in nickel uptake and that most of the essential amino acids are clustered in conserved motifs.

Metals and the rhizobial-legume symbiosis--uptake, utilization and signalling

In this review, we consider how the nitrogen-fixing root nodule bacteria, the 'rhizobia', acquire various metals, paying particular attention to the uptake of iron. We also review the literature pertaining to the roles of molybdenum and nickel in the symbiosis with legumes. We highlight some gaps in our knowledge, for example the lack of information on how rhizobia acquire molybdenum. We examine the means whereby different metals affect rhizobial physiology and the role of metals as signals for gene regulation. We describe the ways in which genetics has shown (or not) if, and how, particular metal uptake and/or metal-mediated signalling pathways are required for the symbiotic interaction with legumes.

Membrane topology of the NixA nickel transporter of Helicobacter pylori: two nickel transport-specific motifs within transmembrane helices II and III

NixA, the high-affinity cytoplasmic membrane nickel transport protein of Helicobacter pylori, imports Ni(2+) into the cell for insertion into the active site of the urease metalloenzyme, which is required for gastric colonization. NixA fractionates with the cytoplasmic membrane, and protein cross-linking studies suggest that NixA functions as a monomer. A preliminary topological model of NixA with seven transmembrane domains was previously proposed based on hydropathy, charge dispersion, and homology to other transporters. To test the proposed topology of NixA and relate critical residues to specific structural elements, a series of 21 NixA-LacZ and 21 NixA-PhoA fusions were created along the entire length of the protein. Expression of reporter fusions was confirmed by Western blotting with beta-galactosidase- and alkaline phosphatase-specific antisera. The activities of reporter fusions near to and upstream of the predicted translational initiation demonstrated the presence of an additional amino-terminal transmembrane domain including a membrane localization signal. Activities of fusions immediately adjacent to motifs which have been shown to be requisite for Ni(2+) transport localized these motifs entirely within transmembrane domains II and III. Fusion activities localized six additional Asp and Glu residues which reduced Ni(2+) transport by >90% when mutated within or immediately adjacent to transmembrane domains II, V, VI, and VII. All fusions strongly support a model of NixA in which the amino and carboxy termini are located in the cytoplasm and the protein possesses eight transmembrane domains.

Membrane topology and insertion of membrane proteins: search for topogenic signals

Integral membrane proteins are found in all cellular membranes and carry out many of the functions that are essential to life. The membrane-embedded domains of integral membrane proteins are structurally quite simple, allowing the use of various prediction methods and biochemical methods to obtain structural information about membrane proteins. A critical step in the biosynthetic pathway leading to the folded protein in the membrane is its insertion into the lipid bilayer. Understanding of the fundamentals of the insertion and folding processes will significantly improve the methods used to predict the three-dimensional membrane protein structure from the amino acid sequence. In the first part of this review, biochemical approaches to elucidate membrane protein topology are reviewed and evaluated, and in the second part, the use of similar techniques to study membrane protein insertion is discussed. The latter studies search for signals in the polypeptide chain that direct the insertion process. Knowledge of the topogenic signals in the nascent chain of a membrane protein is essential for the evaluation of membrane topology studies.

Phylogenetic characterization of novel transport protein families revealed by genome analyses

As a result of recent genome sequencing projects as well as detailed biochemical, molecular genetic and physiological experimentation on representative transport proteins, we have come to realize that all organisms possess an extensive but limited array of transport protein types that allow the uptake of nutrients and excretion of toxic substances. These proteins fall into phylogenetic families that presumably reflect their evolutionary histories. Some of these families are restricted to a single phylogenetic group of organisms and may have arisen recently in evolutionary time while others are found ubiquitously and may be ancient. In this study we conduct systematic phylogenetic analyses of 26 families of transport systems that either had not been characterized previously or were in need of updating. Among the families analyzed are some that are bacterial-specific, others that are eukaryotic-specific, and others that are ubiquitous. They can function by either a channel-type or a carrier-type mechanism, and in the latter case, they are frequently energized by coupling solute transport to the flux of an ion down its electrochemical gradient. We tabulate the currently sequenced members of the 26 families analyzed, describe the properties of these families, and present partial multiple alignments, signature sequences and phylogenetic trees for them all.

Conserved residues and motifs in the NixA protein of Helicobacter pylori are critical for the high affinity transport of nickel ions

NixA, the high affinity nickel transport protein of Helicobacter pylori, imports Ni2+ ions across the cytoplasmic membrane for insertion into the active site of the urease metalloenzyme, which is essential for colonization of the gastric mucosa. Twelve conserved aspartate (aspartates 47, 49, 55, 194, 231, and 234), glutamate (glutamates 106, 198, and 274), and histidine (histidines 44, 50, and 79) residues were identified by alignment of NixA with homologous transporters. Polymerase chain reaction-generated site-directed mutants of these residues were expressed in E. coli along with the H. pylori urease gene cluster. Mutations in residues within the predicted periplasmic domains of NixA maintained near wild type levels of Ni2+ uptake and urease activity, as did control mutations of conserved positively charged residues (lysines 140 and 268; arginines 162 and 167). Mutations in highly conserved motifs in predicted helices II and III of NixA abolished Ni2+ uptake and urease activity. Mutations in helices V and VI and the cytoplasmic domains decreased Ni2+ transport rates by >/=90%. Reduction in rates of Ni2+ transport correlated with reduction in urease activities (r = 0.77). Ni2+ transport was inhibited in the presence of Co2+, Cu2+, and Zn2+, indicating that these ions may also be bound or transported by NixA. We conclude that conserved Asp, Glu, and His residues in the transmembrane domains of NixA are critical for the transport of the divalent cations Ni2+, Co2+, Cu2+, and Zn2+ into the cytoplasm of H. pylori.

A Ni2+ binding motif is the basis of high affinity transport of the Alcaligenes eutrophus nickel permease

Amino acid exchanges in the Alcaligenes eutrophus nickel permease (HoxN) were constructed by site-directed mutagenesis, and their effects on nickel ion uptake were investigated. Mutant hoxN alleles were expressed in Escherichia coli, and activity of the altered permeases was examined via a recently described physiological assay (Wolfram, L., Friedrich, B., and Eitinger, T. (1995) J. Bacteriol. 177, 1840-1843). Replacement of Cys-37, Cys-256, or Cys-318 by alanine did not severely affect nickel ion uptake. This activity of a C331A mutant was diminished by 60%, and a similar phenotype was obtained with a cysteine-less mutant harboring four Cys to Ala exchanges. Alterations in a histidine-containing sequence motif (His-62, Asp-67, His-68), which is conserved in microbial nickel transport proteins, strongly affected or completely abolished transport activity in the E. coli system. The analysis of HoxN alkaline phosphatase fusion proteins implied that His-62, Asp-67, and His-68 exchanges did not interfere with overall membrane topology or stability of the nickel permease. These mutations were reintroduced into the A. eutrophus wild-type strain. Analyses of the resulting HoxN mutants indicated that exchanges in the histidine motif led to a clearly decreased affinity of the permease for nickel ion.

Allelic exchange mutagenesis of nixA in Helicobacter pylori results in reduced nickel transport and urease activity

Helicobacter pylori, an etiologic agent of gastritis and peptic ulceration in humans, synthesizes urease, a nickel metalloenzyme, as its most abundant protein. NixA, a high-affinity nickel transport protein, allows synthesis of catalytically active urease when coexpressed with H. pylori urease in an Escherichia coli host. To determine whether NixA is essential for the production of active urease in H. pylori, nixA was insertionally inactivated with a kanamycin resistance cassette (aphA) and this construct was electroporated into H. pylori ATCC 43504; allelic exchange mutants were selected on kanamycin-containing medium. The nixA mutation, confirmed by PCR, reduced urease activity by 42% (140 +/- 70 micromol of NH3/min/mg of protein in the mutant versus 240 +/- 100 micromol of NH3/min/mg of protein in the parent (P = 0.037). Rates of nickel transport were dramatically reduced (P = 0.0002) in the nixA mutant (9.3 +/- 3.7 pmol of Ni2+/min/10(8) bacteria) of H. pylori as compared with the parent strain (30.2 +/- 8.1 pmol of Ni2+/min/10(8) bacteria). We conclude that NixA is an important mediator of nickel transport in H. pylori. That residual nickel transport and urease activity remain in the nixA mutant, however, provides evidence for the presence of a redundant transport system in this species.

hyp gene products in Alcaligenes eutrophus are part of a hydrogenase-maturation system

In Alcaligenes eutrophus H16 the hyp gene complex consists of six open reading frames hypA1, B1, F1, C, D and E whose products are involved in maturation of the two NiFe hydrogenases: an NAD-reducing cytoplasmic enzyme (SH) and a membrane-bound electron-transport-coupled protein (MBH). hypB1 and hypF1 were originally considered to form a single open reading frame designated hypB [Dernedde, J., Eitinger, M. & Friedrich, B. (1993) Arch. Microbiol. 159, 545-553]. Re-examination of the relevant sequence identified hypB1 and hypF1 as two distinct genes. Non-polar in-frame deletions in the individual hyp genes were constructed in vitro and transferred via gene replacement to the wild-type strain. The resulting mutants fall into two classes. Deletions in hypC, D and E (class I) gave a clear negative phenotype, while hypA1, B1 and F1 deletion mutants (class II) were not impaired in hydrogen metabolism. Class I mutants were unable to grow on hydrogen under autotrophic conditions. The enzymatic activities of SH and MBH were disrupted in all three class I mutants. Immunoblot analysis showed the presence of the H2-activating SH subunit (HoxH) at levels comparable to those observed in the wild-type strain whereas the other three subunits (HoxF, U and Y) were only detectable in trace amounts, probably due to proteolytic degradation. Likewise, MBH was less stable in hypC, D and E deletion mutants and was not attached to the cytoplasmic membrane. In the wild-type strain, HoxH and the MBH large subunit (HoxG) undergo C-terminal proteolytic processing before attaining enzymatic activity. In class I mutants this maturation was blocked. 63Ni-incorporation experiments identified both hydrogenases as nickel-free apoproteins in these mutants. Although class II mutants bearing deletions in hypA1, B1 and F1 showed no alteration of the wild-type phenotype, a role for these genes in the incorporation of nickel and hence hydrogenase maturation cannot be excluded, since there is experimental evidence that this set of genes is duplicated in A. eutrophus.

Helicobacter pylori nickel-transport gene nixA: synthesis of catalytically active urease in Escherichia coli independent of growth conditions

Urease is a virulence determinant, a taxonomic and diagnostic marker, and immunogen for Helicobacter pylori, an aetiologic agent of gastritis and peptic ulceration. This enzyme requires Ni2+ ions in the active site for successful hydrolysis of urea. When expressed in Escherichia coli, recombinant urease is only weakly active unless urease structural subunits are overexpressed, exogenous NiCl2 is added, and the host strain is grown in medium that does not chelate free Ni2+. As wild-type H. pylori does not require such conditions for very high levels of urease expression, we reasoned that additional genes were required to accumulate the metal ion. To isolate such genes, E. coli SE5000 (pHP808), which carries the H. pylori urease gene cluster, was complemented with a lambda ZAP-derived plasmid library of the H. pylori chromosome. One of 1000 ampicillin-resistant clones, plated onto urea segregation agar, produced detectable urease. Urease activity of this co-transformant, grown in Luria broth containing 1 microM NiCl2, was 36 mumol NH3 min-1 mg-1 protein. Urease-enhancing activity, which is not directly linked to the urease gene cluster, was localized by subcloning and nucleotide sequencing. The largest open reading frame, designated nixA, predicted a polypeptide of 34,317 Da that displayed characteristics of an integral membrane protein. In vitro transcription-translation of nixA sequences yielded a polypeptide estimated to be 32 kDa in size. An in-frame Bal31 deletion within nixA abolished urease-enhancing activity. At 50 nM NiCl2, E. coli containing the nixA clone transported 1250 +/- 460 pmol Ni2+ min-1 10(-8) cells, whereas the vector control transported only 140 +/- 85 pmol Ni2+ min-1 10(8) cells, i.e. significantly less (P = 0.01). We conclude that NixA confers upon E. coli a high-affinity nickel-transport system (KT = 11.3 +/- 2.4 nM; Vmax = 1750 +/- 220 pmol Ni2+ min-1 10(-8) cells) and is necessary for expression of catalytically active urease, regardless of growth conditions.

The Alcaligenes eutrophus protein HoxN mediates nickel transport in Escherichia coli

HoxN, an integral membrane protein with seven transmembrane helices and a molecular mass of 33.1 kDa, is involved in high-affinity nickel transport in Alcaligenes eutrophus H16. From genetic analyses, it has been concluded that HoxN is a single-component ion carrier. To investigate this assumption, hoxN was introduced into Escherichia coli. The recombinant strain showed significantly enhanced nickel uptake in a short-interval assay. Likewise, growth in the presence of 63NiCl2 yielded a more than 15-fold-increased cellular nickel content. The HoxN-based nickel transport activity could also be demonstrated in a physiological assay: an E. coli strain coexpressing hoxN and the urease operon of Klebsiella aerogenes exhibited urease activity 10-fold greater than that in the strain lacking a functional hoxN. These results strongly suggest that HoxN is sufficient to operate as a nickel permease. Multiple sequence alignment of HoxN and four other bacterial membrane proteins implicated in nickel metabolism revealed two conserved signatures which may play a role in the nickel translocation process.

A family of homologous substrate-binding proteins with a broad range of substrate specificity and dissimilar biological functions

The uptake of peptides is accomplished mainly by a family of homologous oligopeptide or dipeptide transporters in bacteria. Computer-aided sequence analyses expand members of the oligopeptide-binding protein family to nickel and heme permeases and other proteins, including an enzyme hyaluronate synthase. They are involved in human pathogenicity, bacterial virulence, substrate-sensing, bacterial conjugation and bacterial metabolic reactions distinct from nutrient uptake. These homologous proteins are found in both purple bacteria and Gram-positive bacteria, indicating the presence of a common ancestor before the appearance of the two eubacterial phyla. Nevertheless, the pheromone-binding proteins, involved in bacterial conjugation, and the hyaluronate synthase are present only in the low G-C Gram-positive eubacteria subdivision, which suggests that these proteins diverged from the common ancestor after the appearance of this subdivision.