Use of rosy mutant strains of Drosophila melanogaster to probe the structure and function of xanthine dehydrogenase

Iron Sulfur and Molybdenum Cofactor Enzymes Regulate the Drosophila Life Cycle by Controlling Cell Metabolism

Iron sulfur (Fe-S) clusters and the molybdenum cofactor (Moco) are present at enzyme sites, where the active metal facilitates electron transfer. Such enzyme systems are soluble in the mitochondrial matrix, cytosol and nucleus, or embedded in the inner mitochondrial membrane, but virtually absent from the cell secretory pathway. They are of ancient evolutionary origin supporting respiration, DNA replication, transcription, translation, the biosynthesis of steroids, heme, catabolism of purines, hydroxylation of xenobiotics, and cellular sulfur metabolism. Here, Fe-S cluster and Moco biosynthesis in Drosophila melanogaster is reviewed and the multiple biochemical and physiological functions of known Fe-S and Moco enzymes are described. We show that RNA interference of Mocs3 disrupts Moco biosynthesis and the circadian clock. Fe-S-dependent mitochondrial respiration is discussed in the context of germ line and somatic development, stem cell differentiation and aging. The subcellular compartmentalization of the Fe-S and Moco assembly machinery components and their connections to iron sensing mechanisms and intermediary metabolism are emphasized. A biochemically active Fe-S core complex of heterologously expressed fly Nfs1, Isd11, IscU, and human frataxin is presented. Based on the recent demonstration that copper displaces the Fe-S cluster of yeast and human ferredoxin, an explanation for why high dietary copper leads to cytoplasmic iron deficiency in flies is proposed. Another proposal that exosomes contribute to the transport of xanthine dehydrogenase from peripheral tissues to the eye pigment cells is put forward, where the Vps16a subunit of the HOPS complex may have a specialized role in concentrating this enzyme within pigment granules. Finally, we formulate a hypothesis that (i) mitochondrial superoxide mobilizes iron from the Fe-S clusters in aconitase and succinate dehydrogenase; (ii) increased iron transiently displaces manganese on superoxide dismutase, which may function as a mitochondrial iron sensor since it is inactivated by iron; (iii) with the Krebs cycle thus disrupted, citrate is exported to the cytosol for fatty acid synthesis, while succinyl-CoA and the iron are used for heme biosynthesis; (iv) as iron is used for heme biosynthesis its concentration in the matrix drops allowing for manganese to reactivate superoxide dismutase and Fe-S cluster biosynthesis to reestablish the Krebs cycle.

Purification, characterization, and cloning of a bifunctional molybdoenzyme with hydratase and alcohol dehydrogenase activity

A bifunctional hydratase/alcohol dehydrogenase was isolated from the cyclohexanol degrading bacterium Alicycliphilus denitrificans DSMZ 14773. The enzyme catalyzes the addition of water to α,β-unsaturated carbonyl compounds and the subsequent alcohol oxidation. The purified enzyme showed three subunits in SDS gel, and the gene sequence revealed that this enzyme belongs to the molybdopterin binding oxidoreductase family containing molybdopterins, FAD, and iron-sulfur clusters.

Spectroscopic and biochemical studies on protein variants of quinaldine 4-oxidase: Role of E736 in catalysis and effects of serine ligands on the FeSI and FeSII clusters

Quinaldine 4-oxidase (Qox), which catalyzes the hydroxylation of quinaldine to 1H-4-oxoquinaldine, is a heterotrimeric (LMS)2 molybdo-iron/sulfur flavoprotein belonging to the xanthine oxidase family. Variants of Qox were generated by site-directed mutagenesis. Replacement in the large subunit at E736, which is presumed to be located close to the molybdenum, by aspartate (QoxLE736D) resulted in a marked decrease in kcat app for quinaldine, while Km app was largely unaffected. Although a minor reduction of the glutamine substituted variant QoxLE736Q by quinaldine occurred, its activity was below detection, indicating that the carboxylate group of E736 is crucial for catalysis. Replacement of cysteine ligands C40, C45, or C60 (FeSII) and of the C120 or C154 ligands to FeSI in the small subunit of Qox by serine led to decreased iron contents of the protein preparations. Substitutions C40S and C45S (Fe1 of FeSII) suppressed the characteristic FeSII EPR signals and significantly reduced catalytic activity. In QoxSC154S (Fe1 of FeSI), the g-factor components of FeSI were drastically changed. In contrast, Qox proteins with substitutions of C48 and C60 (Fe2 of FeSII), and of the C120 ligand at Fe2 of FeSI, retained considerable activity and showed less pronounced changes in their EPR parameters. Taken together, the properties of the Qox variants suggest that Fe1 of both FeSI and FeSII are the reducible iron sites, whereas the Fe2 ions remain in the ferric state. The location of the reducible iron sites of FeSI and FeSII appears to be conserved in enzymes of the xanthine oxidase family.

Gene cluster of Arthrobacter ilicis Ru61a involved in the degradation of quinaldine to anthranilate: characterization and functional expression of the quinaldine 4-oxidase qoxLMS genes

A genetic analysis of the anthranilate pathway of quinaldine degradation was performed. A 23-kb region of DNA from Arthrobacter ilicis Rü61a was cloned into the cosmid pVK100. Although Escherichia coli clones containing the recombinant cosmid did not transform quinaldine, cosmids harboring the 23-kb region, or a 10.8-kb stretch of this region, conferred to Pseudomonas putida KT2440 the ability to cometabolically convert quinaldine to anthranilate. The 10.8-kb fragment thus contains the genes coding for quinaldine 4-oxidase (Qox), 1H-4-oxoquinaldine 3-monooxygenase, 1H-3-hydroxy-4-oxoquinaldine 2,4-dioxygenase, and N-acetylanthranilate amidase. The qoxLMS genes coding for the molybdopterin cytosine dinucleotide-(MCD-), FeSI-, FeSII-, and FAD-containing Qox were inserted into the expression vector pJB653, generating pKP1. Qox is the first MCD-containing enzyme to be synthesized in a catalytically fully competent form by a heterologous host, P. putida KT2440 pKP1; the catalytic properties and the UV-visible and EPR spectra of Qox purified from P. putida KT2440 pKP1 were essentially like those of wild-type Qox. This provides a starting point for the construction of protein variants of Qox by site-directed mutagenesis. Downstream of the qoxLMS genes, a putative gene whose deduced amino acid sequence showed 37% similarity to the cofactor-inserting chaperone XdhC was located. Additional open reading frames identified on the 23-kb segment may encode further enzymes (a glutamyl tRNA synthetase, an esterase, two short-chain dehydrogenases/reductases, an ATPase belonging to the AAA family, a 2-hydroxyhepta-2,4-diene-1,7-dioate isomerase/5-oxopent-3-ene-1,2,5-tricarboxylate decarboxylase-like protein, and an enzyme of the mandelate racemase group) and hypothetical proteins involved in transcriptional regulation, and metabolite transport.

Molecular characterization of the amplified aldehyde oxidase from insecticide resistant Culex quinquefasciatus

Primary structural information including the complete nucleotide sequence of the first insect aldehyde oxidase (AO) was obtained from the common house mosquito Culex quinquefasciatus (Say) through cloning and sequencing of both genomic DNA and cDNA. The deduced amino-acid sequence encodes a 150-kDa protein of 1266 amino-acid residues, which is consistent with the expected monomeric subunit size of AO. The Culex AO sequence contains a molybdopterin cofactor binding domain and two iron-sulfur centres. A comparison of the partial sequences of AO from insecticide resistant and susceptible strains of C. quinquefasciatus shows two distinct alleles of this enzyme, one of which is amplified in the insecticide resistant strain on a 30-kb DNA amplicon alongside two resistance-associated esterases. The amplified AO gene results in elevated AO activity in all life stages, but activity is highest in 3rd instar larvae. The elevated enzyme can be seen as a separate band on polyacrylamide gel electrophoresis. The role of AO in xenobiotic oxidation in mammals and the partial inhibition of elevated AO activity by a range of insecticides in Culex, suggest that this AO may play a role in insecticide resistance.

Isolation and characterization of the Xanthine dehydrogenase gene of the Mediterranean fruit fly, Ceratitis capitata

Xanthine dehydrogenase (XDH) is a member of the molybdenum hydroxylase family of enzymes catalyzing the oxidation of hypoxanthine and xanthine to uric acid. The enzyme is also required for the production of one of the major Drosophila eye pigments, drosopterin. The XDH gene has been isolated in many species representing a broad cross section of the major groups of living organisms, including the cDNA encoding XDH from the Mediterranean fruit fly Ceratitis capitata (CcXDH) described here. CcXDH is closely related to other insect XDHs and is able to rescue the phenotype of the Drosophila melanogaster XDH mutant, rosy, in germline transformation experiments. A previously identified medfly mutant, termed rosy, whose phenotype is suggestive of a disruption in XDH function, has been examined for possible mutations in the XDH gene. However, we find no direct evidence that a mutation in the CcXDH gene or that a reduction in the CcXDH enzyme activity is present in rosy medflies. Conclusive studies of the nature of the medfly rosy mutant will require rescue by germline transformation of mutant medflies.

Xanthine dehydrogenase from Pseudomonas putida 86: specificity, oxidation-reduction potentials of its redox-active centers, and first EPR characterization

Xanthine dehydrogenase (XDH) from Pseudomonas putida 86, which was induced 65-fold by growth on hypoxanthine, was purified to homogeneity. It catalyzes the oxidation of hypoxanthine, xanthine, purine, and some aromatic aldehydes, using NAD+ as the preferred electron acceptor. In the hypoxanthine:NAD+ assay, the specific activity of purified XDH was 26.7 U (mg protein)(-1). Its activity with ferricyanide and dioxygen was 58% and 4%, respectively, relative to the activity observed with NAD+. XDH from P. putida 86 consists of 91.0 kDa and 46.2 kDa subunits presumably forming an alpha4beta4 structure and contains the same set of redox-active centers as eukaryotic XDHs. After reduction of the enzyme with xanthine, electron paramagnetic resonance (EPR) signals of the neutral FAD semiquinone radical and the Mo(V) rapid signal were observed at 77 K. Resonances from FeSI and FeSII were detected at 15 K. Whereas the observable g factors for FeSII resemble those of other molybdenum hydroxylases, the FeSI center in contrast to most other known FeSI centers has nearly axial symmetry. The EPR features of the redox-active centers of P. putida XDH are very similar to those of eukaryotic XDHs/xanthine oxidases, suggesting that the environment of each center and their functionality are analogous in these enzymes. The midpoint potentials determined for the molybdenum, FeSI and FAD redox couples are close to each other and resemble those of the corresponding centers in eukaryotic XDHs.

Cloning and molecular characterization of the genes for carbon monoxide dehydrogenase and localization of molybdopterin, flavin adenine dinucleotide, and iron-sulfur centers in the enzyme of Hydrogenophaga pseudoflava

Carbon monoxide dehydrogenases (CO-DH) are the enzymes responsible for the oxidation of CO to carbon dioxide in carboxydobacteria and consist of three nonidentical subunits containing molybdopterin flavin adenine dinucleotide (FAD), and two different iron-sulfur clusters (O. Meyer, K. Frunzke, D. Gadkari, S. Jacobitz, I. Hugendieck, and M. Kraut, FEMS Microbiol. Rev. 87:253-260, 1990). The three structural genes of CO-DH in Hydrogenophaga pseudoflava were cloned and characterized. The genes were clustered on the chromosome in the transcriptional order cutM-cutS-cutL. The cloned cutM, cutS, and cutL genes had open reading frames of 864, 492, and 2,412 nucleotides, coding for proteins with calculated molecular weights of 30,694, 17,752, and 87,224, respectively. The overall identities in the nucleotide sequence of the genes and the amino acid sequence of the subunits with those of other carboxydobacteria were 64.5 to 74.3% and 62.8 to 72.3%, respectively. Primer extension analysis revealed that the transcriptional start site of the genes was the nucleotide G located 47 bp upstream of the cutM start codon. The deduced amino acid sequences of the three subunits of CO-DH implied the presence of molybdenum cofactor, FAD, and iron-sulfur centers in CutL, CutM, and CutS, respectively. Fluorometric analysis coupled with denaturing polyacrylamide gel electrophoresis of fractions from hydroxyapatite column chromatography in the presence of 8 M urea of active CO-DH and from gel filtration of spontaneously inactivated enzyme revealed that the large and medium subunits of CO-DH in H. pseudoflava bind molybdopterin and FAD cofactors, respectively. Iron-sulfur centers of the enzyme were identified to be present in the small subunit on the basis of the iron content in each subunit eluted from the denaturing polyacrylamide gels.

NITRATE REDUCTASE STRUCTURE, FUNCTION AND REGULATION: Bridging the Gap between Biochemistry and Physiology

Nitrate reductase (NR; EC 1.6.6.1-3) catalyzes NAD(P)H reduction of nitrate to nitrite. NR serves plants, algae, and fungi as a central point for integration of metabolism by governing flux of reduced nitrogen by several regulatory mechanisms. The NR monomer is composed of a ~100-kD polypeptide and one each of FAD, heme-iron, and molybdenum-molybdopterin (Mo-MPT). NR has eight sequence segments: (a) N-terminal "acidic" region; (b) Mo-MPT domain with nitrate-reducing active site; (c) interface domain; (d) Hinge 1 containing serine phosphorylated in reversible activity regulation with inhibition by 14-3-3 binding protein; (e) cytochrome b domain; (f) Hinge 2; (g) FAD domain; and (h) NAD(P)H domain. The cytochrome b reductase fragment contains the active site where NAD(P)H transfers electrons to FAD. A complete three-dimensional dimeric NR structure model was built from structures of sulfite oxidase and cytochrome b reductase. Key active site residues have been investigated. NR structure, function, and regulation are now becoming understood.

Structure and expression of tandemly duplicated xanthine dehydrogenase genes of the silkworm (Bombyx mori)

Xanthine dehydrogenase (XDH) is a molybdoenzyme which catalyses oxidation of xanthine and hypoxanthine to uric acid. We isolated genomic clones of silkworm (Bombyx mori) XDH genes (BmXDH1 and BmXDH2). The BmXDH2 gene is located upstream from the BmXDH1 gene and they show a tandemly duplicated structure. Both BmXDH genes were expressed in the fat body and Malpighian tubules, whereas only the BmXDH1 gene was expressed in the midgut. Phylogenetic analysis indicates that BmXDH gene duplication occurred after the divergence of the silkworm and dipteran species. Intron insertion site comparison shows that some introns were lost during insect XDH gene evolution.

4-hydroxybenzoyl coenzyme A reductase (dehydroxylating) is required for anaerobic degradation of 4-hydroxybenzoate by Rhodopseudomonas palustris and shares features with molybdenum-containing hydroxylases

The anaerobic degradation of 4-hydroxybenzoate is initiated by the formation of 4-hydroxybenzoyl coenzyme A, with the next step proposed to be a dehydroxylation to benzoyl coenzyme A, the starting compound for a central pathway of aromatic compound ring reduction and cleavage. Three open reading frames, divergently transcribed from the 4-hydroxybenzoate coenzyme A ligase gene, hbaA, were identified and sequenced from the phototrophic bacterium Rhodopseudomonas palustris. These genes, named hbaBCD, specify polypeptides of 17.5, 82.6, and 34.5 kDa, respectively. The deduced amino acid sequences show considerable similarities to a group of hydroxylating enzymes involved in CO, xanthine, and nicotine metabolism that have conserved binding sites for [2Fe-2S] clusters and a molybdenum cofactor. Cassette disruption of the hbaB gene yielded a mutant that was unable to grow anaerobically on 4-hydroxybenzoate but grew normally on benzoate. The hbaB mutant cells did not accumulate [14C]benzoyl coenzyme A during short-term uptake of [14C]4-hydroxybenzoate, but benzoyl coenzyme A was the major radioactive metabolite formed by the wild type. In addition, crude extracts of the mutant failed to convert 4-hydroxybenzoyl coenzyme A to benzoyl coenzyme A. This evidence indicates that the hbaBCD genes encode the subunits of a 4-hydroxybenzoyl coenzyme A reductase (dehydroxylating). The sizes of the specified polypeptides are similar to those reported for 4-hydroxybenzoyl coenzyme A reductase isolated from the denitrifying bacterium Thauera aromatica. The amino acid consensus sequence for a molybdenum cofactor binding site is in HbaC. This cofactor appears to be an essential component because anaerobic growth of R. palustris on 4-hydroxybenzoate, but not on benzoate, was retarded unless 0.1 microM molybdate was added to the medium. Neither tungstate nor vanadate replaced molybdate, and tungstate competitively inhibited growth stimulation by molybdate.

TAO1, a representative of the molybdenum cofactor containing hydroxylases from tomato

Aldehyde oxidase and xanthine dehydrogenase are a group of ubiquitous hydroxylases, containing a molybdenum cofactor (MoCo) and two iron-sulfur groups. Plant aldehyde oxidase and xanthine dehydrogenase activities are involved in nitrogen metabolism and hormone biosynthesis, and their corresponding genes have not yet been isolated. Here we describe a new gene from tomato, which shows the characteristics of a MoCo containing hydroxylase. It shares sequence homology with xanthine dehydrogenases and aldehyde oxidases from various organisms, and similarly contains binding sites for two iron-sulfur centers and a molybdenum-binding region. However, it does not contain the xanthine dehydrogenase conserved sequences thought to be involved in NAD binding and in substrate specificity, and is likely to encode an aldehyde oxidase-type activity. This gene was designated tomato aldehyde oxidase 1 (TAO1). TAO1 belongs to a multigene family, whose members are shown to map to clusters on chromosomes 1 and 11. MoCo hydroxylase activity is shown to be recognized by antibodies raised against recombinant TAO1 polypeptides. Immunoblots reveal that TAO1 cross-reacting material is ubiquitously expressed in various organisms, and in plants it is mostly abundant in fruits and rapidly dividing tissues.

Synonymous substitutions in the Xdh gene of Drosophila: heterogeneous distribution along the coding region

The Xdh (rosy) region of Drosophila subobscura has been sequenced and compared to the homologous region of D. pseudoobscura and D. melanogaster. Estimates of the numbers of synonymous substitutions per site (Ks) confirm that Xdh has a high synonymous substitution rate. The distributions of both nonsynonymous and synonymous substitutions along the coding region were found to be heterogeneous. Also, no relationship has been detected between Ks estimates and codon usage bias along the gene, in contrast with the generally observed relationship among genes. This heterogeneous distribution of synonymous substitutions along the Xdh gene, which is expression-level independent, could be explained by a differential selection pressure on synonymous sites along the coding region acting on mRNA secondary structure. The synonymous rate in the Xdh coding region is lower in the D. subobscura than in the D. pseudoobscura lineage, whereas the reverse is true for the Adh gene.

Cloning, expression, and sequence analysis of the three genes encoding quinoline 2-oxidoreductase, a molybdenum-containing hydroxylase from Pseudomonas putida 86

The three genes coding for quinoline 2-oxidoreductase (Qor) of Pseudomonas putida 86 were cloned and sequenced. The qor genes are clustered in the transcriptional order medium (M) small (S), large (L) and code for three subunits of 288 (QorM), 168 (QorS), and 788 (QorL) amino acids, respectively. Formation of active quinoline 2-oxidoreductase and degradation of quinoline occurred in a recombinant P. putida KT2440 clone. The amino acid sequences of Qor show significant homology to various prokaryotic molybdenum containing hydroxylases and to eukaryotic xanthine dehydrogenases. QorS contains two conserved motifs for [2Fe-2S] clusters. The binding motif for the N-terminal [2Fe-2S] cluster corresponds to the binding site of bacterial and chloroplast-type [2Fe-2S] ferredoxins, whereas the amino acid pattern of the internal [2Fe-2S] center apparently is a distinct feature of molybdenum-containing hydroxylases, showing no homology to any other described [2Fe-2S] binding motif. The medium subunit QorM presumably contains the FAD, but no conserved sequence areas or described motifs of FAD, NAD, NADP, or ATP binding were detected. Putative binding sites of the molybdopterin cytosine dinucleotide cofactor were detected in QorL by comparison with "contacting segments" recently described in aldehyde oxidoreductase from Desulfovibrio gigas (Romão, M. J., Archer, M., Moura, I., Moura, J. J. G., LeGall, J., Engh, R., Schneider, M., Hof, P., and Huber, R. (1995) Science 270, 1170-1176).

Properties of xanthine dehydrogenase variants from rosy mutant strains of Drosophila melanogaster and their relevance to the enzyme's structure and mechanism

Xanthine dehydrogenase, a molybdenum, iron-sulfur flavoenzyme encoded in the fruit fly Drosophila melanogaster by the rosy gene, has been characterised both from the wild-type and mutant files. Enzyme assays, using a variety of different oxidising and reducing substrates were supplemented by limited molecular characterisation. Four rosy strains showed no detectable activity in any enzyme assay tried, whereas from four wild-type and three rosy mutant strains, those for the [E89K], [L127F] and [L157P]xanthine dehydrogenases (in all of which the mutation is in the iron-sulfur domain), the enzyme molecules, although present at different levels, had extremely similar or identical properties. This was confirmed by purification of one wild-type and one mutant enzyme. [E89K]xanthine dehydrogenase. These both had ultraviolet-visible absorption spectra similar to milk xanthine oxidase. Both were found to be quite stable molecules, showing very high catalytic-centre activities and with little tendency to become degraded by proteolysis or modified by conversion to oxidase or desulfo forms. In three further rosy strains, giving [G353D]xanthine dehydrogenase and [S357F]xanthine dehydrogenase mutated in the flavin domain, and [G1011E]xanthine dehydrogenase mutated in the molybdenum domain, enzyme activities were selectively diminished in certain assays. For the G353D and S357F mutant enzymes activities to NAD+ as oxidising substrate were diminished, to zero for the latter. In addition for [G353D]xanthine dehydrogenase, there was an increase in apparent Km values both for NAD+ and NADH. These findings indicate involvement of this part of the sequence in the NAD(+)-binding site. The G1011E mutation has a profound effect on the enzyme. As isolated and as present in crude extracts of the files, this xanthine dehydrogenase variant lacks activity to xanthine or pterin as reducing substrate, indicating an impairment of the functioning of its molybdenum centre. However, it retains full activity to NADH with dyes as oxidising substrate. Mild oxidation of the enzyme converts it, apparently irreversibly, to a form showing full activity to xanthine and pterin. The nature of the group that is oxidised is discussed in the light of redox potential data. It is proposed that the process involves oxidation of the pterin of the molybdenum cofactor from the tetrahydro to a dihydro oxidation state. This conclusion is fully consistent with recent information [Romäo, M. J., Archer, M., Moura, I., Moura. J.J.G., LeGall, J., Engh, R., Schneider, M., Hof, P. & Huber, R. (1995) Science 270. 1170-1176) from X-ray crystallography on the structure of a closely related enzyme from Desulfovibrio gigas. It is proposed, that apparent irreversibility of the oxidative activating process for [G1011E]xanthine dehydrogenase, is due to conversion of its pterin to the tricyclic derivative detected by these workers. The data thus provide the strongest evidence available, that the oxidation state of the pterin can have a controlling influence on the activity of a molybdenum cofactor enzyme. Implications regarding pterin incorporation into xanthine dehydrogenase and in relation to other molybdenum enzymes are discussed.

Molecular cloning of the isoquinoline 1-oxidoreductase genes from Pseudomonas diminuta 7, structural analysis of iorA and iorB, and sequence comparisons with other molybdenum-containing hydroxylases

The iorA and iorB genes from the isoquinoline-degrading bacterium Pseudomonas diminuta 7, encoding the heterodimeric molybdo-iron-sulfur-protein isoquinoline 1-oxidoreductase, were cloned and sequenced. The deduced amino acid sequences IorA and IorB showed homologies (i) to the small (gamma) and large (alpha) subunits of complex molybdenum-containing hydroxylases (alpha beta gamma/alpha 2 beta 2 gamma 2) possessing a pterin molybdenum cofactor with a monooxo-monosulfido-type molybdenum center, (ii) to the N- and C-terminal regions of aldehyde oxidoreductase from Desulfovibrio gigas, and (iii) to the N- and C-terminal domains of eucaryotic xanthine dehydrogenases, respectively. The closest similarity to IorB was shown by aldehyde dehydrogenase (Adh) from the acetic acid bacterium Acetobacter polyoxogenes. Five conserved domains of IorB were identified by multiple sequence alignments. Whereas IorB and Adh showed an identical sequential arrangement of these conserved domains, in all other molybdenum-containing hydroxylases the relative position of "domain A" differed. IorA contained eight conserved cysteine residues. The amino acid pattern harboring the four cysteine residues proposed to ligate the Fe/S I cluster was homologous to the consensus binding site of bacterial and chloroplast-type [2Fe-2S] ferredoxins, whereas the pattern including the four cysteines assumed to ligate the Fe/S II center showed no similarities to any described [2Fe-2S] binding motif. The N-terminal region of IorB comprised a putative signal peptide similar to typical leader peptides, indicating that isoquinoline 1-oxidoreductase is associated with the cell membrane.

Molecular characterization of the gene cluster coxMSL encoding the molybdenum-containing carbon monoxide dehydrogenase of Oligotropha carboxidovorans

The CO dehydrogenase structural genes (cox) and orf4 are clustered in the transcriptional order coxM--> coxS--> coxL--> orf4 on the 128-kb megaplasmid pHCG3 of the carboxidotroph Oligotropha carboxidovorans OM5. Sequence analysis suggested association of molybdopterin cytosine dinucleotide and flavin adenine dinucleotide with CoxL and of the [2Fe-2S] clusters with CoxS.

Cloning and molecular characterization of hxA, the gene coding for the xanthine dehydrogenase (purine hydroxylase I) of Aspergillus nidulans

We have cloned and sequenced the hxA gene coding for the xanthine dehydrogenase (purine hydroxylase I) of Aspergillus nidulans. The gene codes for a polypeptide of 1363 amino acids. The sequencing of a nonsense mutation, hxA5, proves formally that the clones isolated correspond to the hxA gene. The gene sequence is interrupted by three introns. Similarity searches reveal two iron-sulfur centers and a NAD/FAD-binding domain and have enabled a consensus sequence to be determined for the molybdenum cofactor-binding domain. The A. nidulans sequence is a useful outclass for the other known sequences, which are all from metazoans. In particular, it gives added significance to the missense mutations sequenced in Drosophila melanogaster and leads to the conclusion that while one of the recently sequenced human genes codes for a xanthine dehydrogenase, the other one must code for a different molybdenum-containing hydroxylase, possibly an aldehyde oxidase. The transcription of the hxA gene is induced by the uric acid analogue 2-thiouric acid and repressed by ammonium. Induction necessitates the product of the uaY regulatory gene.

DNA sequence of the cut A, B and C genes, encoding the molybdenum containing hydroxylase carbon monoxide dehydrogenase, from Pseudomonas thermocarboxydovorans strain C2

Pseudomonas thermocarboxydovorans strain C2 is capable of using carbon monoxide as the sole source of carbon and energy. The key enzyme for CO utilisation is the molybdenum containing iron-flavoprotein carbon monoxide dehydrogenase (CODH). This paper reports the DNA sequencing of a 4.7 kb region of the C2 genome which appears to encode the CODH enzyme. The genes for the three subunits of CODH, which we have named cut A, B and C, have been identified and they appear to form an operon. The predicted protein sequences of the three subunits have homology to the structurally related protein, xanthine dehydrogenase, from Drosophila melanogaster. By comparison with xanthine dehydrogenase it can be predicted that the molybdenum cofactor binds to the large subunit of CODH, the small subunit of CODH contains the iron-sulphur centers and the medium subunit binds FAD/NAD+.

Structural analysis and molybdenum-dependent expression of the pAO1-encoded nicotine dehydrogenase genes of Arthrobacter nicotinovorans

The genes of nicotine dehydrogenase (NDH) were identified, cloned and sequenced from the catabolic plasmid pAO1 of Arthrobacter nicotinovorans. In immediate proximity to this gene cluster is the beginning of the 6-hydroxy-L-niotine oxidase (6-HLNO) gene. NDH is composed of three subunits (A, B and C) of M(r) 30,011, 14,924 and 87,677. It belongs to a family of bacterial hydroxylases with a similar subunit structure; they have molybdopterin dinucleotide, FAD and Fe-S clusters as cofactors. Here the first complete primary structure of a bacterial hydroxylase is provided. Sequence alignments of each of the NDH subunits show similarities to the sequences of eukaryotic xanthine dehydrogenase (XDH) but not to other known molybdenum-containing bacterial enzymes. Based on alignment with XDH it is inferred that the smallest subunit (NDHB) carries an iron-sulphur cluster, that the middle-sized subunit (NDHA) binds FAD, and that the largest NDH subunit (NDHC) corresponds to the molybdopterin-binding domain of XDH. Expression of both the ndh and the 6-hino genes required the presence of nicotine and molybdenum in the culture medium. Tungsten inhibited enzyme activity but not the synthesis of the enzyme protein. The enzyme was found in A. nicotinovorans cells in a soluble form and in a membrane-associated form. In the presence of tungsten the fraction of membrane-associated NDH increased.

Drosophila xanthine dehydrogenase variants re-visited

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Cloning of the cDNA encoding human xanthine dehydrogenase (oxidase): structural analysis of the protein and chromosomal location of the gene

The primary structure of human xanthine dehydrogenase (hXDH) was determined by cloning and sequence analysis of the cDNAs encoding the enzyme. The nucleotide (nt) sequence has an open reading frame of 3999 nt encoding a protein of 1333 amino acids (aa) with a calculated M(r) of 146,604. The deduced aa sequence of hXDH is homologous to the previously reported rat XDH (rXDH) and Drosophila melanogaster XDH sequences with identities of 90.2 and 52.0%, respectively. The aa residues involved in both the reversible and the irreversible conversion from the dehydrogenase type to the oxidase type of rXDH are completely conserved between the rat and the human enzymes. This implies that the molecular mechanisms of the conversion of hXDH from dehydrogenase to oxidase are common to those of the well-characterized rXDH. Five sequence variations were detected in the isolated cDNA clones. Spot blot hybridization using flow-sorted human chromosome revealed that the hXDH-encoding gene (hXDH) was located on chromosome 2.