A Pseudomonas stutzeri outer membrane protein inserts copper into N2O reductase

Acquisition of ionic copper by the bacterial outer membrane protein OprC through a novel binding site

Copper, while toxic in excess, is an essential micronutrient in all kingdoms of life due to its essential role in the structure and function of many proteins. Proteins mediating ionic copper import have been characterised in detail for eukaryotes, but much less so for prokaryotes. In particular, it is still unclear whether and how gram-negative bacteria acquire ionic copper. Here, we show that Pseudomonas aeruginosa OprC is an outer membrane, TonB-dependent transporter that is conserved in many Proteobacteria and which mediates acquisition of both reduced and oxidised ionic copper via an unprecedented CxxxM-HxM metal binding site. Crystal structures of wild-type and mutant OprC variants with silver and copper suggest that acquisition of Cu(I) occurs via a surface-exposed “methionine track” leading towards the principal metal binding site. Together with whole-cell copper quantitation and quantitative proteomics in a murine lung infection model, our data identify OprC as an abundant component of bacterial copper biology that may enable copper acquisition under a wide range of conditions.

How do Gram-negative bacteria acquire copper? This study shows that the outer membrane protein OprC from Pseudomonas aeruginosa is abundant during infection and mediates highly selective acquisition of both copper redox states via an extracellular "methionine track" and an unprecedented near-irreversible binding site.

Cu Homeostasis in Bacteria: The Ins and Outs

Copper (Cu) is an essential trace element for all living organisms and used as cofactor in key enzymes of important biological processes, such as aerobic respiration or superoxide dismutation. However, due to its toxicity, cells have developed elaborate mechanisms for Cu homeostasis, which balance Cu supply for cuproprotein biogenesis with the need to remove excess Cu. This review summarizes our current knowledge on bacterial Cu homeostasis with a focus on Gram-negative bacteria and describes the multiple strategies that bacteria use for uptake, storage and export of Cu. We furthermore describe general mechanistic principles that aid the bacterial response to toxic Cu concentrations and illustrate dedicated Cu relay systems that facilitate Cu delivery for cuproenzyme biogenesis. Progress in understanding how bacteria avoid Cu poisoning while maintaining a certain Cu quota for cell proliferation is of particular importance for microbial pathogens because Cu is utilized by the host immune system for attenuating pathogen survival in host cells.

Handling of nutrient copper in the bacterial envelope

The insertion of copper into bacterial cuproenzymesin vivodoes not always require a copper-binding metallochaperone – why?

Dual pathways for copper uptake by methanotrophic bacteria

Methanobactin (Mb), a 1217-Da copper chelator produced by the methanotroph Methylosinus trichosporium OB3b, is hypothesized to mediate copper acquisition from the environment, particularly from insoluble copper mineral sources. Although indirect evidence suggests that Mb provides copper for the regulation and activity of methane monooxygenase enzymes, experimental data for direct uptake of copper loaded Mb (Cu-Mb) are lacking. Uptake of intact Cu-Mb by M. trichosporium OB3b was demonstrated by isotopic and fluorescent labeling experiments. Confocal microscopy data indicate that Cu-Mb is localized in the cytoplasm. Both Cu-Mb and unchelated Cu are taken up by M. trichosporium OB3b, but by different mechanisms. Uptake of unchelated Cu is inhibited by spermine, suggesting a porin-dependent passive transport process. By contrast, uptake of Cu-Mb is inhibited by the uncoupling agents carbonyl cyanide m-chlorophenylhydrazone and methylamine, but not by spermine, consistent with an active transport process. Cu-Mb from M. trichosporium OB3b can also be internalized by other strains of methanotroph, but not by Escherichia coli, suggesting that Cu-Mb uptake is specific to methanotrophic bacteria. These findings are consistent with a key role for Cu-Mb in copper acquisition by methanotrophs and have important implications for further investigation of the copper uptake machinery.

New substrates for TonB-dependent transport: do we only see the 'tip of the iceberg'?

TonB-dependent transport is a mechanism for active uptake across the outer membrane of Gram-negative bacteria. The system promotes transport of rare nutrients and was thought to be restricted to iron complexes and vitamin B12. Recent experimental evidence of TonB-energized transport of nickel and different carbohydrates, in addition to bioinformatic-based predictions, challenges this notion and reveals that the number and variety of TonB-dependent substrates is underestimated. It is becoming clear that the chemical nature of the substrates, the energetic requirements for transport and the subsequent translocation across the cytoplasmic membrane can differ from those of the well-studied systems for iron complexes and vitamin B12. These findings question the understanding of TonB-dependent uptake and provide insights into the adaptation of bacteria to their environments.

Biogenesis of the bacterial respiratory CuA, Cu-S enzyme nitrous oxide reductase

Nitrous oxide reductase (NosZ, EC 1.7.99.6) is the terminal oxidoreductase of a respiratory electron transfer chain that transforms nitrous oxide to dinitrogen. The enzyme carries six Cu atoms. Two are arranged in the mixed-valent binuclear CuA site, and four make up the mu4-sulfide-bridged Cu cluster, CuZ. The biogenesis of a catalytically active NosZ requires auxiliary functions for metal center assembly in the periplasm. Both Tat and Sec pathways share the task to transport the various Nos proteins to their functional sites. Biogenesis of NosZ requires an ABC transporter complex and the periplasmic Cu chaperone NosL. Sustaining whole-cell NosZ function depends on the periplasmic, FAD-containing protein NosX, and the membrane-bound iron-sulfur flavoprotein NosR. Most components with a biogenetic function are now amenable to structural studies.

Identification of an operon involved in tyrosinase activity and melanin synthesis in Marinomonas mediterranea

The genomic region of Marinomonas mediterranea containing the genes required for tyrosinase activity and melanin synthesis has been cloned by marker rescue using the transposon-generated, amelanogenic strain T105. Five ORFs, two incomplete and three complete, have been sequenced in the genomic region where the transposon was inserted. RT-PCR analysis indicates that ORF 3, coding for tyrosinase, and ORF4, coding for a protein of 250 amino acids, are in the same transcriptional unit, constituting an operon whose promoter region has been determined by 5'-RACE. This operon has been sequenced in the wild-type and several mutant strains, indicating that both ORFs are required for expression of tyrosinase activity and melanin synthesis. The nitrosoguanidine generated, amelanogenic mutant ng56, shows a nonsense mutation in ORF3 coding for the tyrosinase. On the other hand, in the strain T105 the transposon is inserted in ORF4. The product of this gene is related to copper metabolism, since the addition of this metal ion to cell extracts or culture media partially restores melanin synthesis and tyrosinase activity in the strain T105. However, it does not show significant sequence similarity to previously characterized metallochaperones and hence may be an example of a new kind of those proteins. The operon has been denoted as ppoB, taking into consideration that ppoA denotes the M. mediterranea gene coding for the previously cloned polyphenol oxidase with laccase activity. This is the first demonstration of the tyrosinase gene forming part of an operon in a Gram-negative bacterium.

Escherichia coli mechanisms of copper homeostasis in a changing environment

Escherichia coli is equipped with multiple systems to ensure safe copper handling under varying environmental conditions. The Cu(I)-translocating P-type ATPase CopA, the central component in copper homeostasis, is responsible for removing excess Cu(I) from the cytoplasm. The multi-copper oxidase CueO and the multi-component copper transport system CusCFBA appear to safeguard the periplasmic space from copper-induced toxicity. Some strains of E. coli can survive in copper-rich environments that would normally overwhelm the chromosomally encoded copper homeostatic systems. Such strains possess additional plasmid-encoded genes that confer copper resistance. The pco determinant encodes genes that detoxify copper in the periplasm, although the mechanism is still unknown. Genes involved in copper homeostasis are regulated by MerR-like activators responsive to cytoplasmic Cu(I) or two-component systems sensing periplasmic Cu(I). Pathways of copper uptake and intracellular copper handling are still not identified in E. coli.

Requirements for Cu(A) and Cu-S center assembly of nitrous oxide reductase deduced from complete periplasmic enzyme maturation in the nondenitrifier Pseudomonas putida

Bacterial nitrous oxide (N(2)O) reductase is the terminal oxidoreductase of a respiratory process that generates dinitrogen from N(2)O. To attain its functional state, the enzyme is subjected to a maturation process which involves the protein-driven synthesis of a unique copper-sulfur cluster and metallation of the binuclear Cu(A) site in the periplasm. There are seven putative maturation factors, encoded by nosA, nosD, nosF, nosY, nosL, nosX, and sco. We wanted to determine the indispensable proteins by expressing nos genes from Pseudomonas stutzeri in the nondenitrifying organism Pseudomonas putida. An in silico study of denitrifying bacteria revealed that nosL, nosX (or a homologous gene, apbE), and sco, but not nosA, coexist consistently with the N(2)O reductase structural gene and other maturation genes. Nevertheless, we found that expression of only three maturation factors (periplasmic protein NosD, cytoplasmic NosF ATPase, and the six-helix integral membrane protein NosY) together with nosRZ in trans was sufficient to produce catalytically active holo-N(2)O reductase in the nondenitrifying background. We suggest that these obligatory factors are required for Cu-S center assembly. Using a mutational approach with P. stutzeri, we also studied NosA, the Cu-containing outer membrane protein previously thought to have Cu insertase function, and ScoP, a putative membrane-anchored chaperone for Cu(A) metallation. Both of these were found to be dispensable elements for N(2)O reductase biosynthesis. Our experimental and in silico data were integrated in a model of N(2)O reductase maturation.

Localization of denitrification genes on the chromosomal map of Pseudomonas aeruginosa

Cleavage of chromosomal DNA from Pseudomonas aeruginosa PAO by Spel and Dpnl has been used together with PFGE and Southern hybridization to establish the map location of the following principal denitrification genes: narGH (encoding the large and small subunits of respiratory nitrate reductase), nirS (cytochrome-cd1 nitrite reductase), nirE (uroporphyrinogen-III methyltransferase for haem d1 biosynthesis), norCB (nitric-oxide reductase complex), nosZ (nitrous-oxide reductase) and nosA (an outer-membrane protein and OprC homologue). The study also included several genes related to anaerobic or microaerophilic metabolism: napA (encoding the catalytic subunit of the periplasmic nitrate reductase), ccoN (catalytic subunit of the cytochrome-cbb3 oxidase), hemN (oxygen-independent coproporphyrinogen-III oxidase), an fnr-like regulatory gene, and azu and fdxA (electron carriers azurin and ferredoxin, respectively). Genes necessary for denitrification are concentrated at 20 to 36 min on the P. aeruginosa chromosome, where they form three separate loci, the nir-nor, nar and nos gene clusters. Genomic DNA of Pseudomonas stutzeri ZoBell was also subjected to Spel restriction and Southern analysis to assign denitrification genes to individual fragments. A homologue of nosA encoding a putative component of the Cu-processing apparatus for nitrous-oxide reductase was identified. In both P. aeruginosa and P. stutzeri there is evidence for the linkage of anr (fnrA) with hemN and ccoN; and for the presence of a napA gene.

Cell biology and molecular basis of denitrification

Denitrification is a distinct means of energy conservation, making use of N oxides as terminal electron acceptors for cellular bioenergetics under anaerobic, microaerophilic, and occasionally aerobic conditions. The process is an essential branch of the global N cycle, reversing dinitrogen fixation, and is associated with chemolithotrophic, phototrophic, diazotrophic, or organotrophic metabolism but generally not with obligately anaerobic life. Discovered more than a century ago and believed to be exclusively a bacterial trait, denitrification has now been found in halophilic and hyperthermophilic archaea and in the mitochondria of fungi, raising evolutionarily intriguing vistas. Important advances in the biochemical characterization of denitrification and the underlying genetics have been achieved with Pseudomonas stutzeri, Pseudomonas aeruginosa, Paracoccus denitrificans, Ralstonia eutropha, and Rhodobacter sphaeroides. Pseudomonads represent one of the largest assemblies of the denitrifying bacteria within a single genus, favoring their use as model organisms. Around 50 genes are required within a single bacterium to encode the core structures of the denitrification apparatus. Much of the denitrification process of gram-negative bacteria has been found confined to the periplasm, whereas the topology and enzymology of the gram-positive bacteria are less well established. The activation and enzymatic transformation of N oxides is based on the redox chemistry of Fe, Cu, and Mo. Biochemical breakthroughs have included the X-ray structures of the two types of respiratory nitrite reductases and the isolation of the novel enzymes nitric oxide reductase and nitrous oxide reductase, as well as their structural characterization by indirect spectroscopic means. This revealed unexpected relationships among denitrification enzymes and respiratory oxygen reductases. Denitrification is intimately related to fundamental cellular processes that include primary and secondary transport, protein translocation, cytochrome c biogenesis, anaerobic gene regulation, metalloprotein assembly, and the biosynthesis of the cofactors molybdopterin and heme D1. An important class of regulators for the anaerobic expression of the denitrification apparatus are transcription factors of the greater FNR family. Nitrate and nitric oxide, in addition to being respiratory substrates, have been identified as signaling molecules for the induction of distinct N oxide-metabolizing enzymes.

Lack of copper insertion into unprocessed cytoplasmic nitrous oxide reductase generated by an R20D substitution in the arginine consensus motif of the signal peptide

Metal insertion into an engineered cytoplasmic form of the multicopper enzyme N2O reductase (N2OR) (EC 1.7.99.6) of Pseudomonas stutzeri was studied. The reductase has an unusually long presequence of 50 amino acids for translocation into the periplasm. The signal peptide of N2OR shares a conserved twin-arginine sequence motif with the signal peptides of other N2O reductases and a sizeable group of periplasmic or membrane-bound enzymes, requiring cofactor insertion or processing. A catalytically inactive reductase, N2ORR20D, that lacked Cu, accumulated in the cytoplasm on mutation of the first arginine of this motif. The CuA site of N2ORR20D could be reconstituted in vitro indicating that the lack of metal was not due to a serious conformational restraint. Our findings locate the event of in vivo Cu insertion into N2OR in the periplasm or allow it to take place concomitant with protein translocation.

Molecular mechanisms of copper resistance and accumulation in bacteria

An unusual mechanism of metal resistance is found in certain plant pathogenic strains of Pseudomonas syringae that are exposed to high levels of copper compounds used in disease control on agricultural crops. These bacteria accumulate blue Cu2+ ions in the periplasm and outer membrane. At least part of this copper sequestering activity is determined by copper-binding protein products of the copper resistance operon (cop). Potential copper-binding sites of the periplasmic CopA protein show conservation with type-1, type-2, and type-3 copper sites of several eukaryotic multi-copper oxidases. In addition to compartmentalization of copper in the periplasm, two components of the cop operon, copC and copD, appear to function in copper uptake into the cytoplasm. Copper resistance operons related to cop have been described in the related plant pathogen Xanthomonas campestris and in Escherichia coli, but these resistance systems may differ functionally from the Pseudomonas syringae system.

Periplasmic location of nitrous oxide reductase and its apoform in denitrifying Pseudomonas stutzeri

Immunogold labelling techniques on ultrathin sections of low temperature embedded cells yielded evidence for the periplasmic location of the respiratory enzymes N2O reductase and nitrite reductase (cytochrome cd1) in Pseudomonas stutzeri strain ZoBell. Cell fractionation by spheroplast preparation and two-dimensional electrophoresis showed the absence of a membrane association of these enzymes. Immunocytochemical localization of N2O reductase in a mutant strain deficient in the chromophore of N2O reductase showed the gold label at the cell periphery, indicating that the copper chromophore processing takes place after export of this protein's apoform.

Metabolic pathways in Paracoccus denitrificans and closely related bacteria in relation to the phylogeny of prokaryotes

Denitrification and methylotrophy in Paracoccus denitrificans are discussed. The properties of the enzymes of denitrification: the nitrate-nitrite antiporter, nitrate reductase, nitrite reductase, nitric oxide reductase and nitrous oxide reductase are described. The genes for none of these proteins have yet been cloned and sequenced from P. denitrificans. A number of sequences are available for enzymes from Escherichia coli, Pseudomonas stutzeri and Pseudomonas aeruginosa. It is concluded that pathway specific c-type cytochromes are involved in denitrification. At least 40 genes are involved in denitrification. In methanol oxidation at least 20 genes are involved. In this case too pathway specific c-type cytochromes are involved. The sequence homology between the quinoproteins methanol dehydrogenase, alcoholde-hydrogenase and glucose dehydrogenase is discussed. This superfamily of proteins is believed to be derived from a common ancestor. The moxFJGI operon determines the structural components of methanol dehydrogenase and the associated c-type cytochrome. Upstream of this operon 3 regulatory proteins were found. The moxY protein shows the general features of a sensor protein and the moxX protein those of a regulatory protein. Thus a two component regulatory system is involved in both denitrification and methylotrophy. The phylogeny of prokaryotes based on 16S rRNA sequence is discussed. It is remarkable that the 16S rRNA of Thiosphaera pantotropha is identical to that of P. denitrificans. Still these bacteria show a number of differences. T. pantotropha is able to denitrify under aerobic circumstances and it shows heterotrophic nitrification. Nitrification and heterotrophic nitrification are found in species belonging to the beta-and gamma-subdivisions of purple non-sulfur bacteria. Thus the occurrence of heterotrophic nitrification in T. pantotropha, which belongs to the alpha-subdivision of purple non-sulfur bacteria is a remarkable property. Furthermore T. pantotropha contains two nitrate reductases of which the periplasmic one is supposed to be involved in aerobic denitrification. The nitrite reductase is of the Cu-type and not of the cytochrome cd1 type as in P. denitrificans. Also the cytochrome b of the Qbc complex of T. pantotropha is highly similar to its counterpart in P. denitrificans. It is hypothesized that the differences between these two organisms which both contain large megaplasmids is due to a combination of loss of genetic information and plasmid-coded properties. The distribution of a number of complex metabolic systems in eubacteria and in a number of species belonging to the alpha-group of purple non sulphur bacteria is reviewed.(ABSTRACT TRUNCATED AT 400 WORDS)

Molecular characterization of nosA, a Pseudomonas stutzeri gene encoding an outer membrane protein required to make copper-containing N2O reductase

A Pseudomonas stutzeri gene (nosA) encoding an outer membrane protein was cloned into the broad-host-range vector pRK290 and expressed in a mutant lacking the protein. Deletion analysis identified the approximate extent of the nosA region which was sequenced, and it was found to contain an open reading frame encoding 683 amino acids including a presumed signal sequence of 44 amino acids. The putative processed form had a molecular weight of 70,218, characteristics typical of outer membrane proteins, and considerable amino acid sequence homology with Escherichia coli BtuB. A short stretch of amino acids was homologous with the E. coli TonB-dependent outer membrane proteins, BtuB, IutA, FepA, and FhuA, suggesting a homologous function: interaction with a periplasmic protein or uptake of a specific substrate.

A cloned ompR-like gene of Streptomyces lividans 66 suppresses defective melC1, a putative copper-transfer gene

Expression of tyrosinase in Streptomyces requires functional MelC1 protein, which is postulated to transfer copper to apotyrosinase. We have previously isolated a mutant of Streptomyces lividans, HT32, that phenotypically suppressed mutations in cloned melC1 (H.-C. Tseng and C. W. Chen, in preparation). Plasmid pLUS132, containing an ATG to ATA transition at the initiation codon of melC1, was used for cloning the suppressor gene from HT32. A 1687 bp suppressor DNA was isolated that contained two characteristic Streptomyces coding sequences: a 217-amino-acid open reading frame (cutR) and a truncated open reading frame (cutS) downstream. Subcloning analysis attributed the phenotypic suppression activity to the putative cutR gene from HT32. The putative CutR exhibited similarity to the response regulator OmpR of the osmoregulatory signal-transduction system in Escherichia coli. The truncated CutS resembled, to a lesser degree, the N-terminus of EnvZ, the histidine protein kinase counterpart of OmpR. DNA hybridizing to the cloned cutR-cutS sequence was detected in 16 other Streptomyces species. We postulate that the putative cutR-cutS operon regulates copper metabolism in Streptomyces.

Metabolic regulation including anaerobic metabolism in Paracoccus denitrificans

Under anaerobic circumstances in the presence of nitrate Paracoccus denitrificans is able to denitrify. The properties of the reductases involved in nitrate reductase, nitrite reductase, nitric oxide reductase, and nitrous oxide reductase are described. For that purpose not only the properties of the enzymes of P. denitrificans are considered but also those from Escherichia coli, Pseudomonas aeruginosa, and Pseudomonas stutzeri. Nitrate reductase consists of three subunits: the alpha subunit contains the molybdenum cofactor, the beta subunit contains the iron sulfur clusters, and the gamma subunit is a special cytochrome b. Nitrate is reduced at the cytoplasmic side of the membrane and evidence for the presence of a nitrate-nitrite antiporter is presented. Electron flow is from ubiquinol via the specific cytochrome b to the nitrate reductase. Nitrite reductase (which is identical to cytochrome cd1) and nitrous oxide reductase are periplasmic proteins. Nitric oxide reductase is a membrane-bound enzyme. The bc1 complex is involved in electron flow to these reductases and the whole reaction takes place at the periplasmic side of the membrane. It is now firmly established that NO is an obligatory intermediate between nitrite and nitrous oxide. Nitrous oxide reductase is a multi-copper protein. A large number of genes is involved in the acquisition of molybdenum and copper, the formation of the molybdenum cofactor, and the insertion of the metals. It is estimated that at least 40 genes are involved in the process of denitrification. The control of the expression of these genes in P. denitrificans is totally unknown. As an example of such complex regulatory systems the function of the fnr, narX, and narL gene products in the expression of nitrate reductase in E. coli is described. The control of the effects of oxygen on the reduction of nitrate, nitrite, and nitrous oxide are discussed. Oxygen inhibits reduction of nitrate by prevention of nitrate uptake in the cell. In the case of nitrite and nitrous oxide a competition between reductases and oxidases for a limited supply of electrons from primary dehydrogenases seems to play an important role. Under some circumstances NO formed from nitrite may inhibit oxidases, resulting in a redistribution of electron flow from oxygen to nitrite. P. denitrificans contains three main oxidases: cytochrome aa3, cytochrome o, and cytochrome co. Cytochrome o is proton translocating and receives its electrons from ubiquinol. Some properties of cytochrome co, which receives its electrons from cytochrome c, are reported.(ABSTRACT TRUNCATED AT 400 WORDS)

Nitrous oxide reductase from denitrifying Pseudomonas stutzeri. Genes for copper-processing and properties of the deduced products, including a new member of the family of ATP/GTP-binding proteins

Nitrous oxide (N2O) respiration by the denitrifying bacterium Pseudomonas stutzeri requires the synthesis of the multi-copper enzyme N2O reductase, nosZ, the structural gene for this enzyme, is part of a DNA region of approximately 8 kbp that carries several essential genes. Insertional mutagenesis of the region downstream of nosZ generates apoenzyme-synthesizing strains, which argues for the existence of functions for copper acquisition or copper processing, or both, for N2O reductase. The relevant DNA region of approximately 3.2 kbp was sequenced and found to consist of three genes, nosDFY, presumably within a single transcriptional unit. Cellular concentration, copper content, and location of the N2O reductase protein were studied with mutants which were affected in the three genes. Interactions of the deduced gene products among each other and with the cytoplasmic membrane appear to be analogous to those of the components of osmotic-shock-sensitive bacterial transport systems. The 33.8-kDa product of the nosF gene belongs to the family of nucleotide-binding proteins [C. Higgins et al. (1986) Nature 323, 448-450]. Its amino acid sequence shows two canonical nucleotide-binding motifs, and the positional identity of amino acids to members of this family is around 30%. The 29.4-kDa product of the nosY gene is a hydrophobic protein with six predicted transmembrane helices and an export signal. The 48.2-kDa product of the nosD gene is a periplasmic component; it carries an export signal and is a hydrophilic protein. N2O reductase itself is a periplasmic enzyme. Our results provide evidence for an auxiliary system of Cu processing and suggest its involvement in the periplasmic biosynthesis of the Cu centers of N2O reductase.

Pseudomonas stutzeri N2O reductase contains CuA-type sites

N2O reductase (N2O----N2) is the terminal enzyme in the energy-conserving denitrification pathway of soil and marine denitrifying bacteria. The protein is composed of two identical subunits and contains eight copper ions per enzyme molecule. The magnetic circular dichroism spectrum of resting (oxidized) N2O reductase is strikingly similar to the magnetic circular dichroism spectrum of the CuA site in mammalian cytochrome c oxidase [Greenwood, C., Hull, B. C., Barber, D., Eglinton, D. G. & Thomson, A. J. (1983) Biochem. J. 215, 303-316] and is unlike the magnetic circular dichroism spectra of all other biological copper chromophores obtained to date. Sulfur (or chlorine) scatterers are required to fit the copper extended x-ray absorption fine structure data of both the oxidized and reduced forms of N2O reductase. Satisfactory fits require a Cu-N or Cu-O [denoted Cu-(N, O)] interaction at 2.0 A, a Cu-(S, Cl) interaction at 2.3 A and an additional Cu(S, Cl) interaction at approximately 2.6 A (oxidized) or approximately 2.7 A (reduced). Approximately eight sulfur ions (per eight copper ions) at approximately 2.3 A are required to fit the extended x-ray absorption fine structure data for both the oxidized and reduced N2O reductase. The 2.3-A Cu-(S, Cl) distance is nearly identical to that previously determined for the CuA site in cytochrome c oxidase. A 2.6-2.7 A Cu-(S, Cl) interaction is also present in resting and fully reduced cytochrome c oxidase. Comparison of the N2O reductase sequence, determined by translating the structural NosZ gene, with cytochrome c oxidase subunit II sequences from several sources indicates that a Gly-Xaa-Xaa-Xaa-Xaa-Xaa-Cys-Ser-Xaa-Xaa-Cys-Xaa-Xaa-Xaa-His stretch is highly conserved. This sequence contains three of the probable ligands (two cysteines and one histidine) in a CuA-type site. Collectively these data establish that Pseudomonas stutzeri N2O reductase contains CuA-type sites.

Properties of a Pseudomonas stutzeri outer membrane channel-forming protein (NosA) required for production of copper-containing N2O reductase

A protein (NosA) in the outer membrane of Pseudomonas stutzeri that is required for copper to be inserted into N2O reductase has been extracted and purified to homogeneity. The purified protein could form channels in black lipid bilayers. Like N2O reductase, NosA contained copper and was only made anaerobically. In contrast to N2O reductase, its synthesis was repressed by exogenous copper (but not by Mn, Co, Ni, Zn, or Fe). Also in contrast to N2O reductase, NosA homologs were not immunologically detectable in Pseudomonas aeruginosa, Pseudomonas mendocina, Pseudomonas alcaligenes, or other strains of P. stutzeri.

Molecular cloning, heterologous expression, and primary structure of the structural gene for the copper enzyme nitrous oxide reductase from denitrifying Pseudomonas stutzeri

The nos genes of Pseudomonas stutzeri are required for the anaerobic respiration of nitrous oxide, which is part of the overall denitrification process. A nos-coding region of ca. 8 kilobases was cloned by plasmid integration and excision. It comprised nosZ, the structural gene for the copper-containing enzyme nitrous oxide reductase, genes for copper chromophore biosynthesis, and a supposed regulatory region. The location of the nosZ gene and its transcriptional direction were identified by using a series of constructs to transform Escherichia coli and express nitrous oxide reductase in the heterologous background. Plasmid pAV5021 led to a nearly 12-fold overexpression of the NosZ protein compared with that in the P. stutzeri wild type. The complete sequence of the nosZ gene, comprising 1,914 nucleotides, together with 282 nucleotides of 5'-flanking sequences and 238 nucleotides of 3'-flanking sequences was determined. An open reading frame coded for a protein of 638 residues (Mr, 70,822) including a presumed signal sequence of 35 residues for protein export. The presequence is in conformity with the periplasmic location of the enzyme. Another open reading frame of 2,097 nucleotides, in the opposite transcriptional direction to that of nosZ, was excluded by several criteria from representing the coding region for nitrous oxide reductase. Codon usage for nosZ of P. stutzeri showed a high G + C content in the degenerate codon position (83.9% versus an average of 60.2%) and relaxed codon usage for the Glu codon, characteristic features of Pseudomonas genes from other species. E. coli nitrous oxide reductase was purified to homogeneity. It had the Mr of the P. stutzeri enzyme but lacked the copper chromophore.