Glutathione reductase from Escherichia coli: cloning and sequence analysis of the gene and relationship to other flavoprotein disulfide oxidoreductases

Screening of genes involved in phage-resistance of Escherichia coli and effects of substances interacting with primosomal protein A on the resistant bacteria

AIMS

The study was to identify the genes involved in phage resistance and to develop an effective biocontrol method to improve the lytic activity of phages against foodborne pathogens.

METHODS AND RESULTS

A total of 3,909 single gene-deletion mutants of Escherichia coli BW25113 from the Keio collection were individually screened for genes involved in phage resistance. Phage S127BCL3 isolated from chicken liver, infecting both E. coli BW25113 and O157: H7, was characterized and used for screening. The 10 gene-deletion mutants showed increased susceptibility to phage S127BCL3. Among them, priA gene-deletion mutant strain showed significant susceptibility to the phages S127BCL3 and T7. Furthermore, we investigated the substances that have been reported to inhibit the function of primosomal protein A (PriA) and were used to confirm increased phage susceptibility in E. coli BW25113 (Parent strain) and O157: H7.

CONCLUSION

PriA inhibitors at a low concentration showed combined effects with phage against E. coli O157: H7 and delayed the regrowth rate of phage-resistant cells.

Gene knockout of glutathione reductase results in increased sensitivity to heavy metals in Acidithiobacillus caldus

Acidithiobacillus caldus plays an important role in bioleaching of low-grade metal ore. It can promote the release of heavy metals in mining-associated habitats and survive in high concentrations of heavy metals. Functions of glutathione reductase (GR) in cell defense against reactive oxygen species caused by heavy metals have been elucidated in some eukaryotic cells and bacteria; however, no information is available in A. caldus. In this research, the methods of bioinformatics, gene expression, GR activity assays were used to detect and characterize the glutathione reductase gene from the A. caldus MTH-04 strain. Then, A. caldus gr knockout mutant and gr overexpression strain were constructed, and the heavy metal tolerant properties and transcriptional levels of ROS related genes of them were compared to study the function of GR. The results showed that, a putative gr gene F0726_RS04210 was detected in the genome of A. caldus MTH-04. The purified recombinant protein of F0726_RS04210 showed remarkable GR activity at optimal pH 7.0 and 30°C using in vitro assay. The evolutionary relationship of GR from A. caldus MTH-04 was close to that from Escherichia coli K12. Gene knockout or overexpression of gr in A. caldus did not affect the growth rate on S0 medium, suggesting that GR did not play a key role in the activation of sulfur. Deletion of gr resulted in increased sensitivity to heavy metals (Cu2+ and Zn2+) in A. caldus, and the gr overexpression strain showed enhanced tolerance to heavy metals. Furthermore, transcription analysis also revealed strong correlations between GR and the antioxidant pathway. The above results suggest that GR can play an important role in heavy metal tolerance in A. caldus.

The role of glutathione in periplasmic redox homeostasis and oxidative protein folding in Escherichia coli

The thiol redox balance in the periplasm of E. coli depends on the DsbA/B pair for oxidative power and the DsbC/D system as its complement for isomerization of non-native disulfides. While the standard redox potentials of those systems are known, the in vivo "steady state" redox potential imposed onto protein thiol disulfide pairs in the periplasm remains unknown. Here, we used genetically encoded redox probes (roGFP2 and roGFP-iL), targeted to the periplasm, to directly probe the thiol redox homeostasis in this compartment. These probes contain two cysteine residues that are virtually completely reduced in the cytoplasm, but once exported into the periplasm, can form a disulfide bond, a process that can be monitored by fluorescence spectroscopy. Even in the absence of DsbA, roGFP2, exported to the periplasm, was almost fully oxidized, suggesting the presence of an alternative system for the introduction of disulfide bonds into exported proteins. However, the absence of DsbA shifted the steady state periplasmic thiol-redox potential from -228 mV to a more reducing -243 mV and the capacity to re-oxidize periplasmic roGFP2 after a reductive pulse was significantly decreased. Re-oxidation in a DsbA strain could be fully restored by exogenous oxidized glutathione (GSSG), while reduced GSH accelerated re-oxidation of roGFP2 in the WT. In line, a strain devoid of endogenous glutathione showed a more reducing periplasm, and was significantly worse in oxidatively folding PhoA, a native periplasmic protein and substrate of the oxidative folding machinery. PhoA oxidative folding could be enhanced by the addition of exogenous GSSG in the WT and fully restored in a ΔdsbA mutant. Taken together this suggests the presence of an auxiliary, glutathione-dependent thiol-oxidation system in the bacterial periplasm.

Multiple-pathway remediation of mercury contamination by a versatile selenite-reducing bacterium

Mercury contamination is a global concern because of its high toxicity, persistence, bioaccumulative nature, long distance transport and wide distribution in the environment. In this study, the efficiency and multiple-pathway remediation mechanisms of Hg²⁺ by a selenite reducing Escherichia coli was assessed. E. coli can reduce Hg²⁺ to Hg⁺ and Hg⁰ and selenite to selenide at the same time. This makes a multiple-pathway mechanisms for removal of Hg²⁺ from water in addition to biosorption. It was found that when the original Hg²⁺ concentration was 40μgL^-1, 93.2±2.8% of Hg²⁺ was removed from solution by E. coli. Of the total Hg removed, it was found that 3.3±0.1% was adsorbed to the bacterium, 2.0±0.5% was bioaccumulated, and 7.3±0.6% was volatilized into the ambient environment, and most (80.6±5.7%) Hg was removed as HgSe and HgCl precipitates and Hg⁰. On one hand, selenite is reduced to selenide and the latter further reacts with Hg²⁺ to form HgSe precipitates. On the other hand Hg²⁺ is successively reduced to Hg⁺, which forms solid HgCl, and Hg⁰. This is the report on bacterially transformation of Hg²⁺ to HgSe, HgCl and Hg⁰ via multiple pathways. It is suggested that E. coli or other selenite reducing microorganisms are promising candidates for mercury bioremediation of contaminated wastewaters, as well as simultaneous removal of Hg²⁺ and selenite.

Impacts of silver nanoparticle coating on the nitrification potential of Nitrosomonas europaea

Silver nanoparticles (AgNPs) are increasingly used as bacteriostatic agents to prevent microbial growth. AgNPs are manufactured with a variety of coatings, and their potential impacts on wastewater treatment in general are poorly understood. In the present study, Nitrosomonas europaea, a model ammonia oxidizing bacterium, was exposed to AgNPs with citrate, gum arabic (GA), and polyvinylpyrrolidone (PVP). GA and citrate AgNPs inhibited nitrification most strongly (67.9 ± 3.6% and 91.4 ± 0.2%, respectively at 2 ppm). Our data indicate that Ag(+) dissolution and colloid stability of AgNPs were the main factors in AgNP toxicity. In general, low amounts of dissolved Ag initially caused a post-transcriptional interruption of membrane-bound nitrifying enzyme function, reducing nitrification by 10% or more. A further increase in dissolved Ag resulted in heavy metal stress response (e.g., merA up-regulation) and ultimately led to membrane disruption. The highest effect on membrane disruption was observed for citrate AgNPs (64 ± 11% membranes compromised at 2 ppm), which had high colloidal stability. This study demonstrates that coating plays a very important role in determining Ag dissolution and ultimately toxicity to nitrifiers. More research is needed to characterize these parameters in complex growth media such as wastewater.

Molecular cloning and expression analysis of glutathione reductase gene in Chlamydomonas sp. ICE-L from Antarctica

A cDNA (GenBank ID: GU395492) encoding cytosolic glutathione reductase (named ICE-LGR) in Antarctic microalgae Chlamydomonas sp. ICE-L was successfully cloned by RT-PCR and rapid amplification of cDNA ends technique (RACE). The expression patterns of ICE-LGR under different salinity stresses were determined by real-time PCR. ICE-LGR cDNA has 1913 bp nucleotides with an open reading frame (ORF) of 1458 bp, encoding 485 amino acid residues. The deduced amino acid sequence shows 79% homology with glutathione reductase (GR) of Chlamydomonas reinhardtii. Activity assessment and mRNA expression analysis results showed that activity and expression level of GR in ICE-L cells were up-regulated under either high or low salinity. Together, our results revealed that ICE-LGR might play an important role in Antarctic ice algae Chlamydomonas sp. ICE-L acclimatizing to polar high salinity environment as well as low salinity. These results provide us valuable information on further investigating the molecular mechanism of ICE-LGR.

Expression of a glutathione reductase from Brassica rapa subsp. pekinensis enhanced cellular redox homeostasis by modulating antioxidant proteins in Escherichia coli

Glutathione reductase (GR) is an enzyme that recycles a key cellular antioxidant molecule glutathione (GSH) from its oxidized form (GSSG) thus maintaining cellular redox homeostasis. A recombinant plasmid to overexpress a GR of Brassica rapa subsp. pekinensis (BrGR) in E. coli BL21 (DE3) was constructed using an expression vector pKM260. Expression of the introduced gene was confirmed by semiquantitative RT-PCR, immunoblotting and enzyme assays. Purification of the BrGR protein was performed by IMAC method and indicated that the BrGR was a dimmer. The BrGR required NADPH as a cofactor and specific activity was approximately 458 U. The BrGR-expressing E. coli cells showed increased GR activity and tolerance to H(2)O(2), menadione, and heavy metal (CdCl(2), ZnCl(2) and AlCl(2))-mediated growth inhibition. The ectopic expression of BrGR provoked the co-regulation of a variety of antioxidant enzymes including catalase, superoxide dismutase, glutathione peroxidase, and glucose-6-phosphate dehydrogenase. Consequently, the transformed cells showed decreased hydroperoxide levels when exposed to stressful conditions. A proteomic analysis demonstrated the higher level of induction of proteins involved in glycolysis, detoxification/oxidative stress response, protein folding, transport/binding proteins, cell envelope/porins, and protein translation and modification when exposed to H(2)O(2) stress. Taken together, these results indicate that the plant GR protein is functional in a cooperative way in the E. coli system to protect cells against oxidative stress.

Plastidial glutathione reductase from Haynaldia villosa is an enhancer of powdery mildew resistance in wheat (Triticum aestivum)

A full-length cDNA (Hv-GR) whose transcript accumulation increased in response to infection by Blumeria graminis DC.f.sp. tritici (Bgt) was isolated from Haynaldia villosa. Southern analysis revealed a single copy of Hv-GR present in H. villosa. This gene encodes a glutathione reductase (GR) with high similarity to chloroplastic GRs from other plant species. Chloroplastic localization of Hv-GR was confirmed by targeting of the green fluorescent protein (GFP)-Hv-GR fusion protein to chloroplasts of epidermal guard cells. Following inoculation with Bgt, transcript accumulation of Hv-GR increased in a resistant line of wheat, but no significant change was observed in a susceptible line. In vivo function of Hv-GR in converting oxidized glutathione (GSSG) to the reduced form (GSH) was verified through heterologous expression of Hv-GR in a yeast GR-deficient mutant. As expected, overexpression of this gene resulted in increased resistance of the mutant to H(2)O(2), indicating a critical role for Hv-GR in protecting cells against oxidative stress. Moreover, overexpression of Hv-GR in a susceptible wheat variety, Triticum aestivum cv. Yangmai 158, enhanced resistance to powdery mildew and induced transcript accumulation of other pathogenesis-related genes, PR-1a and PR-5, through increasing the foliar GSH/GSSG ratio. Therefore, we concluded that a high ratio of GSH to GSSG is required for wheat defense against Bgt, and that chloroplastic GR enzymes might serve as a redox mediator for NPR1 activation.

Expression and enzyme activity of glutathione reductase is upregulated by Fe-deficiency in graminaceous plants

Glutathione reductase (GR) plays an important role in the response to biotic and abiotic stresses in plants. We studied the expression patterns and enzyme activities of GR in graminaceous plants under Fe-sufficient and Fe-deficient conditions by isolating cDNA clones for chloroplastic GR (HvGR1) and cytosolic GR (HvGR2) from barley. We found that the sequences of GR1 and GR2 were highly conserved in graminaceous plants. Based on their nucleotide sequences, HvGR1 and HvGR2 were predicted to encode polypeptides of 550 and 497 amino acids, respectively. Both proteins showed in vitro GR activity, and the specific activity for HvGR1 was 3-fold that of HvGR2. Northern blot analyses were performed to examine the expression patterns of GR1 and GR2 in rice (Os), wheat (Ta), barley (Hv), and maize (Zm). HvGR1, HvGR2, and TaGR2 were upregulated in response to Fe-deficiency. Moreover, HvGR1 and TaGR1 were mainly expressed in shoot tissues, whereas HvGR2 and TaGR2 were primarily observed in root tissues. The GR activity increased in roots of barley, wheat, and maize and shoot tissues of rice, barley, and maize in response to Fe-deficiency. Furthermore, it appeared that GR was not post-transcriptionally regulated, at least in rice, wheat, and barley. These results suggest that GR may play a role in the Fe-deficiency response in graminaceous plants.

Molecular basis of glutathione reductase deficiency in human blood cells

Hereditary glutathione reductase (GR) deficiency was found in only 2 cases when testing more than 15 000 blood samples. We have investigated the blood cells of 2 patients (1a and 1b) in a previously described family suffering from favism and cataract and of a novel patient (2) presenting with severe neonatal jaundice. Red blood cells and leukocytes of the patients in family 1 did not contain any GR activity, and the GR protein was undetectable by Western blotting. Owing to a 2246-bp deletion in the patients' DNA, translated GR is expected to lack almost the complete dimerization domain, which results in unstable and inactive enzyme. The red blood cells from patient 2 did not exhibit GR activity either, but the patient's leukocytes contained some residual activity that correlated with a weak protein expression. Patient 2 was found to be a compound heterozygote, with a premature stop codon on one allele and a substitution of glycine 330, a highly conserved residue in the superfamily of NAD(P)H-dependent disulfide reductases, into alanine on the other allele. Studies on recombinant GR G330A revealed a drastically impaired thermostability of the protein. This is the first identification of mutations in the GR gene causing clinical GR deficiency.

Glutathione in bacteria

Glutathione metabolism and its role in vital functions of bacterial cells are considered, as well as common features and differences between the functions of glutathione in prokaryotic and eukaryotic cells. Particular attention is given to the recent data for the role of glutathione in bacterial redox-regulation and adaptation to stresses.

Evidence for a novel mechanism of time-resolved flavin fluorescence depolarization in glutathione reductase

Time-resolved flavin fluorescence anisotropy studies on glutathione reductase (GR) have revealed a remarkable new phenomenon: wild-type GR displays a rapid process of fluorescence depolarization, that is absent in mutant enzymes lacking a nearby tyrosine residue that blocks the NADPH-binding cleft. Fluorescence lifetime data, however, have shown a more rigid active-site structure for wild-type GR than for the tyrosine mutants. These results suggest that the rapid depolarization in wild-type GR originates from an interaction with the flavin-shielding tyrosine, and not from restricted reorientational motion of the flavin. A novel mechanism of fluorescence depolarization is proposed that involves a transient charge-transfer complex between the tyrosine and the light-excited flavin, with a concomitant change in the direction of the emission dipole moment of the flavin. This interaction is likely to result from side-chain relaxation of the tyrosine in the minor fraction of enzyme molecules in which this residue is in an unsuitable position for immediate fluorescence quenching at the moment of excitation. Support for this mechanism is provided by binding studies with NADP+ and 2'P-5'ADP-ribose that can intercalate between the flavin and tyrosine and/or block the latter. Fluorescence depolarization analyses as a function of temperature and viscosity confirm the dynamic nature of the process. A comparison with fluorescence depolarization effects in a related flavoenzyme indicates that this mechanism of flavin fluorescence depolarization is more generally applicable.

Reduction of Fe(III) ions complexed to physiological ligands by lipoyl dehydrogenase and other flavoenzymes in vitro: implications for an enzymatic reduction of Fe(III) ions of the labile iron pool

Enzymatic reduction of physiological Fe(III) complexes of the "labile iron pool" has not been studied so far. By use of spectrophotometric assays based on the oxidation of NAD(P)H and formation of [Fe(II) (1,10-phenanthroline)3]2+ as well as by utilizing electron paramagnetic resonance spectrometry, it was demonstrated that the NAD(P)H-dependent flavoenzyme lipoyl dehydrogenase (diaphorase, EC 1.8.1.4) effectively catalyzes the one-electron reduction of Fe(III) complexes of citrate, ATP, and ADP at the expense of the co-enzymes NAD(P)H. Deactivated or inhibited lipoyl dehydrogenase did not reduce the Fe(III) complexes. Likewise, in the absence of NAD(P)H or in the presence of NAD(P)+, Fe(III) reduction could not be detected. The fact that reduction also occurred in the absence of molecular oxygen as well as in the presence of superoxide dismutase proved that the Fe(III) reduction was directly linked to the enzymatic activity of lipoyl dehydrogenase and not mediated by O2. Kinetic studies revealed different affinities of lipoyl dehydrogenase for the reduction of the low molecular weight Fe(III) complexes in the relative order Fe(III)-citrate > Fe(III)-ATP > Fe(III)-ADP (half-maximal velocities at 346-485 microm). These Fe(III) complexes were enzymatically reduced also by other flavoenzymes, namely glutathione reductase (EC 1.6.4.2), cytochrome c reductase (EC 1.6.99.3), and cytochrome P450 reductase (EC 1.6.2.4) with somewhat lower efficacy. The present data suggest a (patho)physiological role for lipoyl dehydrogenase and other flavoenzymes in intracellular iron metabolism.

Electrostatic control of the isoalloxazine environment in the two-electron reduced states of yeast glutathione reductase

The resonance Raman spectra of the oxidized and two-electron reduced forms of yeast glutathione reductase are reported. The spectra of the oxidized enzyme indicate a low electron density for the isoalloxazine ring. As far as the two-electron reduced species are concerned, the spectral comparison of the NADPH-reduced enzyme with the glutathione- or dithiothreitol-reduced enzyme shows significant frequency differences for the flavin bands II, III, and VII. The shift of band VII was correlated with a change in steric or electronic interaction of the hydroxyl group of a conserved Tyr with the N(10)-C(10a) portion of the isoalloxazine ring. Upward shifts of bands II and III observed for the glutathione- or dithiothreitol-reduced enzyme indicate both a slight change in isoalloxazine conformation and a hydrogen bond strengthening at the N(1) and/or N(5) site(s). The formation of a mixed disulfide intermediate tends to slightly decrease the frequency of bands II, III, X, XI, and XIV. To account for the different spectral features observed for the NADPH- and glutathione-reduced species, several possibilities have been examined. In particular, we propose a hydrogen bonding modulation at the N(5) site of FAD through a variable conformation of an ammonium group of a conserved Lys residue. Changes in N(5)(flavin)-protein interaction in the two-electron reduced forms of glutathione reductase are discussed in relation to a plausible mechanism of the regulation of the enzyme activity via a variable redox potential of FAD.

Characterization of glutathione amide reductase from Chromatium gracile. Identification of a novel thiol peroxidase (Prx/Grx) fueled by glutathione amide redox cycling

Among the Chromatiaceae, the glutathione derivative gamma-l-glutamyl-l-cysteinylglycine amide, or glutathione amide, was reported to be present in facultative aerobic as well as in strictly anaerobic species. The gene (garB) encoding the central enzyme in glutathione amide cycling, glutathione amide reductase (GAR), has been isolated from Chromatium gracile, and its genomic organization has been examined. The garB gene is immediately preceded by an open reading frame encoding a novel 27.5-kDa chimeric enzyme composed of one N-terminal peroxiredoxin-like domain followed by a glutaredoxin-like C terminus. The 27.5-kDa enzyme was established in vitro to be a glutathione amide-dependent peroxidase, being the first example of a prokaryotic low molecular mass thiol-dependent peroxidase. Amino acid sequence alignment of GAR with the functionally homologous glutathione and trypanothione reductases emphasizes the conservation of the catalytically important redox-active disulfide and of regions involved in binding the FAD prosthetic group and the substrates glutathione amide disulfide and NADH. By establishing Michaelis constants of 97 and 13.2 microm for glutathione amide disulfide and NADH, respectively (in contrast to K(m) values of 6.9 mm for glutathione disulfide and 1.98 mm for NADPH), the exclusive substrate specificities of GAR have been documented. Specificity for the amidated disulfide cofactor partly can be explained by the substitution of Arg-37, shown by x-ray crystallographic data of the human glutathione reductase to hydrogen-bond one of the glutathione glycyl carboxylates, by the negatively charged Glu-21. On the other hand, the preference for the unusual electron donor, to some extent, has to rely on the substitution of the basic residues Arg-218, His-219, and Arg-224, which have been shown to interact in the human enzyme with the NADPH 2'-phosphate group, by Leu-197, Glu-198, and Phe-203. We suggest GAR to be the newest member of the class I flavoprotein disulfide reductase family of oxidoreductases.

Heterogeneity of AmpC cephalosporinases of Hafnia alvei clinical isolates expressing inducible or constitutive ceftazidime resistance phenotypes

Ten unrelated Hafnia alvei clinical isolates were grouped according to either their low-level and inducible cephalosporinase production or their high-level and constitutive cephalosporinase production phenotype. Their AmpC sequences shared 85 to 100% amino acid identity. The immediate genetic environment of ampC genes was conserved in H. alvei isolates but was different from that found in other ampC-possessing enterobacterial species.

Shifting the NAD/NADP preference in class 3 aldehyde dehydrogenase

Among pyridine-nucleotide-dependent oxidoreductases, the class 3 family of aldehyde dehydrogenases (ALDHs) is unusual in its ability to function with either NAD or NADP. This is all the more surprising because an acidic residue, Glu140, coordinates the adenine ribose 2' hydroxyl. In many NAD-dependent dehydrogenases a similarly placed carboxylate is thought to be responsible for exclusion of NADP. The corresponding residue in most (approximately 71%) sequences in the ALDH extended family is also Glu, and most of these are NAD-specific enzymes. Site-directed mutagenesis was performed on this residue in rat class 3 ALDH. Our results indicate that this residue contributes to tighter binding of NAD in the native enzyme, but suggest that additional factors must contribute to the ability to utilize NADP. Mutagenesis of an adjacent basic residue (Lys137) indicates that it is even more essential for binding both coenzymes, consistent with its conservation in nearly all ALDHs (> 98%).

Biochemical-genetic characterization and regulation of expression of an ACC-1-like chromosome-borne cephalosporinase from Hafnia alvei

A naturally occurring AmpC beta-lactamase (cephalosporinase) gene was cloned from the Hafnia alvei 1 clinical isolate and expressed in Escherichia coli. The deduced AmpC beta-lactamase (ACC-2) had a pI of 8 and a relative molecular mass of 37 kDa and showed 50 and 47% amino acid identity with the chromosome-encoded AmpCs from Serratia marcescens and Providentia stuartii, respectively. It had 94% amino acid identity with the recently described plasmid-borne cephalosporinase ACC-1 from Klebsiella pneumoniae, suggesting the chromosomal origin of ACC-1. The hydrolysis constants (k(cat) and K(m)) showed that ACC-2 was a peculiar cephalosporinase, since it significantly hydrolyzed cefpirome. Once its gene was cloned and expressed in E. coli (pDEL-1), ACC-2 conferred resistance to ceftazidime and cefotaxime but also an uncommon reduced susceptibility to cefpirome. A divergently transcribed ampR gene with an overlapping promoter compared with ampC (bla(ACC-2)) was identified in H. alvei 1, encoding an AmpR protein that shared 64% amino acid identity with the closest AmpR protein from P. stuartii. beta-Lactamase induction experiments showed that the ampC gene was repressed in the absence of ampR and was activated when cefoxitin or imipenem was added as an inducer. From H. alvei 1 cultures that expressed an inducible-cephalosporinase phenotype, several ceftazidime- and cefpirome-cross-resistant H. alvei 1 mutants were obtained upon selection on cefpirome- or ceftazidime-containing plates, and H. alvei 1 DER, a ceftazidime-resistant mutant, stably overproduced cephalosporinase. Transformation of H. alvei 1 DER or E. coli JRG582 (ampDE mutant) harboring ampC and ampR from H. alvei 1 with a recombinant plasmid containing ampD from E. coli resulted in a decrease in the MIC of beta-lactam and recovery of an inducible phenotype for H. alvei 1 DER. Thus, AmpR and AmpD proteins may regulate biosynthesis of the H. alvei cephalosporinase similarly to other enterobacterial cephalosporinases.

Lysine 219 participates in NADPH specificity in a flavin-containing monooxygenase from Saccharomyces cerevisiae

The flavin-containing monooxygenase from Saccharomyces cerevisiae (yFMO) uses NADPH and O(2) to oxidize thiol containing substrates such as GSH and thereby generates the oxidizing potential for the ER. The enzyme uses NADPH 12 times more efficiently than NADH. Amino acid sequence analysis suggests that Lys 219 and/or Lys 227 may act as counterions to the 2' phosphate of NADPH and to help determine the preference for pyridine nucleotides. Site directed mutations show that Lys 219 makes the greater contribution to cosubstrate recognition. Conversion of Lys 219 to Ala reduces NADPH dependent activity 90-fold, but has no effect on NADH-dependent activity. Conversion of Lys 227 to Ala reduces NADPH-dependent activity fivefold and NADH-dependent activity threefold. Dissociation constants for NADP(+) to oxidized yFMO were measured spectroscopically. K(d) is 12 microM for the wild-type enzyme and 243 microM for the K219A mutant, consistent with the role of Lys 219 in pyridine nucleotide binding.

Evidence for the co-existence of glutathione reductase and trypanothione reductase in the non-trypanosomatid Euglenozoa: Euglena gracilis Z

Two NADPH-dependent disulfide reductases, glutathione reductase and trypanothione reductase, were shown to be present in Euglena gracilis, purified to homogeneity and characterized. The glutathione reductase (Mr 50 kDa) displays a high specificity towards glutathione disulfide with a KM of 54 microM. The amino acid sequences of two peptides derived from the trypanothione reductase (Mr 54 kDa) show a high level of identity (81% and 64%) with sequences of trypanothione reductases from trypanosomatids. The trypanothione reductase is able to efficiently reduce trypanothione disulfide (KM 30.5 microM) and glutathionylspermidine disulfide (KM 90.6 microM) but not glutathione disulfide, nor Escherichia coli thioredoxin disulfide, nor 5,5'-dithiobis(2-nitrobenzoate) (DTNB). These results demonstrate for the first time (i) the existence of trypanothione reductase in a non-trypanosomatid organism and (ii) the coexistence of trypanothione reductase and glutathione reductase in E. gracilis.

Enterococcus faecalis glutathione reductase: purification, characterization and expression under normal and hyperbaric O2 conditions

Glutathione reductase is found ubiquitously in eukaryotes and Gram-negative bacteria, and plays a significant role in bacterial defense against oxidative stress. Glutathione reductase from the Gram-positive bacterium Enterococcus faecalis was purified to homogeneity using anion exchange, hydrophobic interaction, and affinity chromatography. The homogeneous 49-kDa enzyme contained 1 mol bound FAD per subunit. The determined N-terminal amino acid sequence of the E. faecalis enzyme displays significant identity with glutathione reductases from other Gram-negative and Gram-positive bacteria, as well as yeast and human erythrocyte reductases. The kinetic mechanism is ping-pong, and the determined kinetic parameters exhibited by the E. faecalis glutathione reductase are similar to those found for glutathione reductases from yeast, Escherichia coli, and human erythrocyte. A two-fold increased expression of glutathione reductase activity and a three-fold induction of glutathione peroxidase activity were observed under hyperbaric O2 growth conditions without a corresponding change in the total glutathione and soluble thiol content. The difference in the expression of the enzyme, and its cognate substrate's intracellular concentration, under these conditions suggest that the gene encoding glutathione reductase is responsive to oxygen concentration, but that the genes encoding the glutathione synthesizing enzymes are not linked to an oxygen-sensitive promoter.

Linkage map of Escherichia coli K-12, edition 10: the traditional map

This map is an update of the edition 9 map by Berlyn et al. (M. K. B. Berlyn, K. B. Low, and K. E. Rudd, p. 1715-1902, in F. C. Neidhardt et al., ed., Escherichia coli and Salmonella: cellular and molecular biology, 2nd ed., vol. 2, 1996). It uses coordinates established by the completed sequence, expressed as 100 minutes for the entire circular map, and adds new genes discovered and established since 1996 and eliminates those shown to correspond to other known genes. The latter are included as synonyms. An alphabetical list of genes showing map location, synonyms, the protein or RNA product of the gene, phenotypes of mutants, and reference citations is provided. In addition to genes known to correspond to gene sequences, other genes, often older, that are described by phenotype and older mapping techniques and that have not been correlated with sequences are included.

Flavin fluorescence dynamics and photoinduced electron transfer in Escherichia coli glutathione reductase

Time-resolved polarized flavin fluorescence was used to study the active site dynamics of Escherichia coli glutathione reductase (GR). Special consideration was given to the role of Tyr177, which blocks the access to the NADPH binding-site in the crystal structure of the enzyme. By comparing wild-type GR with the mutant enzymes Y177F and Y177G, a fluorescence lifetime of 7 ps that accounts for approximately 90% of the fluorescence decay could be attributed to quenching by Y177. Based on the temperature invariance for this lifetime, and the very high quenching rate, electron transfer from Y177 to the light-excited isoalloxazine part of flavin adenine dinucleotide (FAD) is proposed as the mechanism of flavin fluorescence quenching. Contrary to the mutant enzymes, wild-type GR shows a rapid fluorescence depolarization. This depolarization process is likely to originate from a transient charge transfer interaction between Y177 and the light-excited FAD, and not from internal mobility of the flavin, as has previously been proposed. Based on the fluorescence lifetime distributions, the mutants Y177F and Y177G have a more flexible protein structure than wild-type GR: in the range of 223 K to 277 K in 80% glycerol, both tyrosine mutants mimic the closely related enzyme dihydrolipoyl dehydrogenase. The fluorescence intensity decays of the GR enzymes can only be explained by the existence of multiple quenching sites in the protein. Although structural fluctuations are likely to contribute to the nonexponential decay and the probability of quenching by a specific site, the concept of conformational substates need not be invoked to explain the heterogeneous fluorescence dynamics.

Recombinant expression and biochemical characterization of an NADPH:flavin oxidoreductase from Entamoeba histolytica

The gene encoding a putative NADPH:flavin oxidoreductase of the protozoan parasite Entamoeba histolytica (Eh34) was recombinantly expressed in Escherichia coli. The purified recombinant protein (recEh34) has a molecular mass of about 35 kDa upon SDS/PAGE analysis, exhibits a flavoprotein-like absorption spectrum and contains 1 mol of non-covalently bound FMN per mol of protein. RecEh34 reveals two different enzymic activities. It catalyses the NADPH-dependent reduction of oxygen to hydrogen peroxide (H2O2), as well as of disulphides such as 5,5'-dithiobis-(2-nitrobenzoic acid) (DTNB) and cystine. The disulphide reductase but not the H2O2-forming NADPH oxidase activity is inhibitable by sulphydryl-active compounds, indicating that a thiol component is part of the active site for the disulphide reductase activity, whereas for the H2O2-forming NADPH oxidase activity only the flavin is required. Compared with the recombinant protein, similar activities are present in amoebic extracts. Native Eh34 is active in a monomeric as well as in a dimeric state. In contrast to recEh34, no flavin was associated with the native protein. However, both NADPH oxidase as well as DTNB reductase activity were found to be dependent on the addition of FAD or FMN.

Molecular cloning and characterization of the gene encoding glutathione reductase in Brassica campestris

We have isolated the Brassica campestris cDNA encoding glutathione reductase of 502 amino acid residues with molecular mass of 54.5 kDa. The deduced amino acid sequences were 92.2%, and 79.5% identical to those of Arabidopsis thaliana, and pea, respectively. As expected, it exhibited a high degree of conservation within the region responsible for the redox reaction and for the binding of GSSG or NADPH. The gene was highly inducible by ozone fumigation or by paraquat treatment.

Missense mutation in flavin-containing mono-oxygenase 3 gene, FMO3, underlies fish-odour syndrome

Individuals with primary trimethylaminuria exhibit a body odour reminiscent of rotting fish, due to excessive excretion of trimethylamine (TMA; refs 1-3). The disorder, colloquially known as fish-odour syndrome, is inherited recessively as a defect in hepatic N-oxidation of dietary-derived TMA and cannot be considered benign, as sufferers may display a variety of psychosocial reactions, ranging from social isolation of clinical depression and attempted suicide. TMA oxidation is catalyzed by flavin-containing mono-oxygenase (FMO; refs 7,8), and tissue localization and functional studies have established FMO3 as the form most likely to be defective in fish-odour syndrome. Direct sequencing of the coding exons of FMO3 amplified from a patient with fish-odour syndrome identified two missense mutations. Although one of these represented a common polymorphism, the other, a C-->T transition in exon 4, was found only in an affected pedigree, in which it segregated with the disorder. The latter mutation predicts a proline-->leucine substitution at residue 153 and abolishes FMO3 catalytic activity. Our results indicate that defects in FMO3 underlie fish-odour syndrome and that the Pro 153-->Leu 153 mutation described here is a cause of this distressing condition.

Involvement of conserved glycine residues, 229 and 234, of Vibrio harveyi aldehyde dehydrogenase in activity and nucleotide binding

The involvement of two conserved glycine residues (Gly229 and Gly234) in activity and nucleotide binding in Vibrio harveyi aldehyde dehydrogenase (ALDH) have been investigated. Each of the glycine residues has been mutated to alanine and the mutant ALDHs have been expressed in Escherichia coli and specifically labelled with [35S]methionine. The G229A mutant was inactive with either NADP+ or NAD+ as coenzyme and did not bind to 2',5'-ADP Sepharose, indicating a complete loss of nucleotide affinity. In contrast, the G234A mutant showed a high affinity for 2',5'-ADP Sepharose. Purified G234A mutant showed similar kinetic properties to the native enzyme including a pre-steady-state burst of NADPH; however, the Michaelis constants for NAD+ and NADP+ were increased by 3- to 9-fold, showing that the mutation had an effect on saturation of the enzyme with NAD(P)+. These data are consistent with the structure for the nucleotide binding domain of Vh.ALDH being similar to that of class 3 or class 2 mammalian ALDHs which differ from the classical nucleotide binding domain found in most dehydrogenases.

Isolation, expression, and regulation of the pgr1(+) gene encoding glutathione reductase absolutely required for the growth of Schizosaccharomyces pombe

The pgr1(+) gene encoding glutathione reductase (GR, EC 1.6.4.2) was isolated from Schizosaccharomyces pombe using a polymerase chain reaction fragment as a probe. The gene consists of two exons and an intron of 55 nucleotides, encoding a polypeptide of 465 amino acids (50,238 Da) with conserved residues characteristic of GR. The transcriptional start site was localized at 239 nucleotides upstream from the ATG initiation codon. The level of transcript as well as the GR enzyme activity increased more than 11-fold when the cloned pgr1(+) gene was expressed on a multicopy plasmid. This overexpression conferred on S. pombe cells more resistance against menadione, a redox cycling agent, but not against H2O2. The level of pgr1(+) transcripts increased by treatment with oxidants such as menadione, cumene hydroperoxide, and diamide. It also increased by treatment with high osmolarity, heat shock, or at the stationary growth phase. The deletion of the pap1(+) gene encoding an AP-1 homolog in S. pombe caused reduction in the pgr1(+) gene expression. Furthermore, Deltapap1 cells lost the inducibility of pgr1(+) gene expression by the above stresses, implying that Pap1 is involved in general stress-inducible gene expression. When the pgr1(+) gene was disrupted, the haploid spores were not viable. Repression of nmt1 promoter-driven pgr1(+) expression by thiamine caused cessation of growth, which was rescued by the episomal pgr1(+) gene. These results indicate that GR activity, which efficiently reduces GSSG, is essentially required for the growth of S. pombe, unlike in Saccharomyces cerevisiae or Escherichia coli.

Secretion of authentic 20-kDa human growth hormone (20K hGH) in Escherichia coli and properties of the purified product

Using Bacillus amyloliquefaciens neutral protease gene (npr), we have constructed a secretion system of 20-kDA human growth hormone (20K hGH) in E. coli. The secretion-signal region from npr was modified inserting a fragment coding a 2Lys-5Leu cluster. In this system we found that co-expression of glutathione reductase remarkably increased accumulation level of 20K hGH in periplasm and confirmed that secreted 20K hGH was correctly processed. The recombinant 20K hGH was highly purified and subjected to analyses of physicochemical properties and biological activities which are still unclear and controversial due to difficulty in preparing the sample with authentic structure. The secreted recombinant product had authentic disulfide linkages and showed molecular weight of 20,270.5 +/- 3.7 (theoretical value, 20,269.9). The results suggest that the recombinant 20K hGH is a full agonist on rat growth promotion and lipolysis stimulation in isolated rat adipose tissues. In particular, the lipolysis-stimulating activity of 20K hGH was distinct as compared with that of 22K hGH under physiological concentration. Cell proliferation activity via prolactin-receptor in Nb-2 lymphoma was obviously low as compared with that of 22K hGH.

Cloning, sequence, and properties of the soluble pyridine nucleotide transhydrogenase of Pseudomonas fluorescens

The gene encoding the soluble pyridine nucleotide transhydrogenase (STH) of Pseudomonas fluorescens was cloned and expressed in Escherichia coli. STH is related to the flavoprotein disulfide oxidoreductases but lacks one of the conserved redox-active cysteine residues. The gene is highly similar to an E. coli gene of unknown function.

Molecular organization of the glutathione reductase gene in Drosophila melanogaster

Glutathione reductase catalyzes the conversion of the oxidized form of glutathione to regenerate reduced glutathione, which acts as a versatile intracellular reductant. The present study provides initial characterization of the glutathione reductase gene in Drosophila melanogaster and its response to experimentally induced oxidative stress. Drosophila cDNA clones were isolated, based on cross-hybridization to the Musca domestica glutathione reductase cDNA. Genomic clones were isolated by cross-hybridization with the Drosophila cDNA as hybridization probe. Northern analysis of adult Drosophila poly(A)+ RNA, utilizing the Drosophila cDNA probe, revealed a hybridization signal in the 2-kb range. The entire sequence of one cDNA was determined. In addition to a coding domain of 1431 bases, the sequence included 206 bases upstream of a putative start codon and 355 bases downstream of a putative stop codon. Based on the cDNA sequence, the 476 amino acid sequence of the Drosophila glutathione reductase gene was deduced and was found to have extensive similarities with the glutathione reductase gene from other species. Gene mapping of a 13-kb genomic fragment revealed that the glutathione reductase gene consists of at least two exons spanning approximately 5 kb. A first exon contains sequence for only the first 5 amino acids and the first base of the sixth and appears to be separated by a ca. 2.5-kb intron from the remainder of the coding region, which is confined to <2 kb. The Drosophila glutathione reductase is single copy and its cytogenetic position, as determined by in situ hybridization, is 7D-E on the X chromosome. mRNA levels of glutathione reductase, measured by RT-PCR, increased in response to exposure to 100% ambient oxygen by almost twofold and administration of paraquat by greater than threefold. Exposure of flies to hyperoxia also induced a 60% increase in the activity of glutathione reductase and augmented the concentration of total glutathione by ca. 40% following an initial drop. The present study, besides providing an initial molecular characterization of the glutathione reductase gene in Drosophila, demonstrates its dynamic involvement in response to experimentally induced oxidative stress.

A thioredoxin reductase-class of disulphide reductase in the protozoan parasite Giardia duodenalis

We describe the purification and characterisation of a thioredoxin reductase-like disulphide reductase from the ancient protozoan parasite, Giardia duodenalis. This dimeric flavoprotein contains 1 mol FAD per subunit and had an apparent subunit molecular mass of 35 kDa. The purified enzyme catalysed the NADPH-dependent (Km = 8 microM) reduction of 5,5'-dithio-bis(2-nitrobenzoic acid) to thionitrobenzoate and was unable to utilise NADH as an electron donor. The sulphydryl-active compounds, N-ethylmaleimide, sodium arsenite and Zn2+ ions, strongly inhibited the enzyme suggesting that a thiol component forms part of the active site. Purified enzyme was able to utilise a variety of substrates, including cystine and oxidised glutathione, which suggests that it is a broad-range disulphide reductase, probably accounting for the majority of thiol cycling activity in this organism. While the G. duodenalis enzyme does not require an intermediate electron transport protein, analogous to thioredoxin, for activity, we have identified a candidate carrier protein which enhances DTNB turnover six fold, therefore implying that Giardia contains a thioredoxin-like system. Physical, enzymatic and spectral properties of the G. duodenalis disulphide reductase are also consistent with it being a member of the thioredoxin reductase-class of disulphide reductases. Furthermore, the internal amino acid sequence of a tryptic peptide generated from the purified protein was highly homologous with thioredoxin reductases from other sources. This is the first report of a disulphide reductase to be purified from the anaerobic protozoa and explains the so called "glutathione-induced thiol-reductase activity' previously observed in G. duodenalis.

Hypochlorous acid stress in Escherichia coli: resistance, DNA damage, and comparison with hydrogen peroxide stress

We have investigated the mechanisms of killing of Escherichia coli by HOCl by identifying protective functions. HOCl challenges were performed on cultures arrested in stationary phase and in exponential phase. Resistance to HOCl in both cases was largely mediated by genes involved in resistance to hydrogen peroxide (H2O2). In stationary phase, a mutation in rpoS, which controls the expression of starvation genes including those which protect against oxidative stress, renders the cells hypersensitive to killing by HOCl. RpoS-regulated genes responsible for this sensitivity were dps, which encodes a DNA-binding protein, and, to a lesser extent, katE and katG, encoding catalases; all three are involved in resistance to H2O2. In exponential phase, induction of the oxyR regulon, an adaptive response to H2O2, protected against HOCl exposure, and the oxyR2 constitutive mutant is more resistant than the wild-type strain. The genes involved in this oxyR-dependent resistance have not yet been identified, but they differ from those primarily involved in resistance to H2O2, including katG, ahp, and dps. Pretreatment with HOCl conferred resistance to H2O2 in an OxyR-independent manner, suggesting a specific adaptive response to HOCl. fur mutants, which have an intracellular iron overload, were more sensitive to HOCl, supporting the generation of hydroxyl radicals upon HOCl exposure via a Fenton-type reaction. Mutations in recombinational repair genes (recA or recB) increased sensitivity to HOCl, indicative of DNA strand breaks. Sensitivity was visible in the wild type only at concentrations above 0.6 mg/liter, but it was observed at much lower concentrations in dps recA mutants.

A H2O-producing NADH oxidase from the protozoan parasite Giardia duodenalis

We describe the purification of a H2O-producing NADH oxidase from the protozoan parasite Giardia duodenalis. The enzyme is a monomeric flavoprotein containing flavin adenine dinucleotide in a 1:1 molar ratio with the polypeptide. The NADH oxidase has an apparent molecular mass of 46 kDa and was homogenous as determined by denaturing gel electrophoresis and N-terminal amino acid sequencing. NADPH could substitute for NADH as an electron donor with a K(m) value of 4.2 microM for NADH and 16 microM for NADPH (pH 7.8 at room temperature). With oxygen as the primary electron acceptor under aerobic conditions, the pure enzyme did not produce O.-2 nor H2O2 as stoichiometric products of oxygen reduction, implicating H2O as the end product and obviating the need for superoxide dismutase. The ability to utilise oxygen explains the apparent respiration of the amitochondrial fermentative metabolism of Giardia. Mercurials, flavoantagonists and heavy metals (Cu2+ and Zn2+) inhibited this activity. Under anaerobic conditions the enzyme catalysed electron transfer at lower efficiencies to other electron acceptors including nitroblue tetrazolium, potassium ferricyanide, FAD and FMN, using either NADH or NADPH as electron donors. NADPH, however, was a more efficient electron donor. Cytochrome c was not reduced under any assay conditions used. The enzyme reduced the nitrofuran drugs, furazolidone (an antigiardial) and nitrofurantoin, to their toxic radical forms as determined by EPR. Metronidazole, a nitroimidazole, was not reduced. Pure NADH oxidase did not demonstrate ferredoxin:NAD(P)1 oxidoreductase activity since it could not accept electrons from reduced ferredoxin to regenerate NAD(P)H. The G. duodenalis NADH oxidase may, therefore, function as a terminal oxidase, similar to the mitochondrial cytochrome oxidase, and in the maintenance of an optimum intracellular redox ratio. This report of a flavoenzyme from Giardia places Giardia close to the anaerobic bacteria in evolutionary terms.

Inactivation of yeast glutathione reductase by O-phthalaldehyde

Yeast glutathione reductase was inactivated by the bifunctional reagent, o-phthalaldehyde. The initial rate of inactivation followed pseudo-first order kinetics. Fluorescence spectral properties of modified enzyme indicated the formation of an isoindole derivative from cysteine and lyaine residues present in close proximity as shown by typical fluorescence emission and excitation maximum at 410 nm and 337 nm, respectively. The fluorescence spectral studies with o-phthalaldehyde in the presence and absence of N-ethylmaleimide indicated that both the inhibitors react with the same cysteine residue, which is non-essential for enzyme activity. The coenzyme NADPH did not protect the enzyme against the o-phthalaldehyde reaction while oxidised glutathione prevented o-phthalaldehyde inactivation. This could be due to reaction of the amino group of glutathione with o-phthalaldehyde. Stoichiometry of the reaction showed that the formation of approximately 2 isoindole derivatives per subunit of glutathione reductase is accompanied by 75% loss of activity. The results suggest that o-phthalaldehyde binds to non-essential cysteine and lysine residues present in close proximity which results in conformational changes leading to enzyme inactivation.

Glutathionylspermidine metabolism in Escherichia coli

Intracellular levels of glutathione and glutathionylspermidine conjugates have been measured throughout the growth phases of Escherichia coli. Glutathionylspermidine was present in mid-log-phase cells, and under stationary and anaerobic growth conditions accounted for 80% of the total glutathione content. N1,N8-bis(glutathionyl)spermidine (trypanothione) was undetectable under all growth conditions. The catalytic constant kcat/Km of recombinant E. coli glutathione reductase for glutathionylspermidine disulphide was approx. 11,000-fold lower than that for glutathione disulphide. The much higher catalytic constant for the mixed disulphide of glutathione and glutathionylspermidine (11% that of GSSG), suggests a possible explanation for the low turnover of trypanothione disulphide by E. coli glutathione reductase, given the apparent lack of a specific glutathionylspermidine disulphide reductase in E. coli.

Altering kinetic mechanism and enzyme stability by mutagenesis of the dimer interface of glutathione reductase

In wild-type glutathione reductase from Escherichia coli residues Val421 and Ala422 are located in an alpha-helix in a densely packed and hydrophobic region of the dimer interface, with their side chains packed against those of residues Ala422' and Val421' in the second subunit. A series of mutant glutathione reductases was constructed in which the identities of the residues at positions 421 and 422 were changed. Mutations were designed so as to present like charges (mutants Val421-->Glu:Ala422-->Glu and Val421-->Lys:Ala422-->Lys) or opposite charges (mutant Val421-->Lys:Ala422-->Glu) across the dimer interface to assess the role of electrostatic interactions in dimer stability. A fourth mutant (Val421-->His:Ala422-->His) was also constructed to investigate the effects of introducing a potentially protonatable bulky side chain into a crowded region of the dimer interface. In all cases, an active dimeric enzyme was found to be assembled but each mutant protein was thermally destabilized. A detailed steady-state kinetic analysis indicated that each mutant enzyme no longer displayed the Ping Pong kinetic behaviour associated with the wild-type enzyme but exhibited what was best described as a random bireactant ternary complex mechanism. This leads, depending on the chosen substrate concentration, to apparent sigmoidal, hyperbolic or complex kinetic behaviour. These experiments, together with others reported previously, indicate that simple mutagenic changes in regions distant from the active site can lead to dramatic switches in steady-state kinetic mechanism.

Cloning and sequencing of a human thioredoxin reductase

The DNA sequence encoding human placental thioredoxin reductase has been determined. Of the 3826 base pairs sequenced, 1650 base pairs were in an open reading frame encoding a mature protein with 495 amino acids and a calculated molecular mass of 54,171. Sequence analysis showed strong similarity to glutathione reductases and other NADPH-dependent reductases. Human thioredoxin reductase contains the redox-active cysteines in the putative FAD binding domain and has a dimer interface domain not previously seen with prokaryote and lower eukaryote thioredoxin reductases.

Plasmodium falciparum glutathione reductase exhibits sequence similarities with the human host enzyme in the core structure but differs at the ligand-binding sites

The homodimeric flavoenzyme glutathione reductase (GR) which catalyzes the reduction of glutathione disulfide is a cornerstone of the malaria parasite antioxidant defense and repair mechanisms. Here we report on the identification of the GR gene from Plasmodium falciparum. A 1.4-kb fragment of the gene was amplified by polymerase chain reaction (PCR). Using this PCR fragment as a probe a full length cDNA clone (2085 bp) was isolated from a P. falciparum gametocyte library. The deduced amino acid sequence of 541 residues shows an overall identity of 35% when compared to the human enzyme. Most amino acids of known function are identical. However, notable differences between human and parasite protein occur in the glutathione-binding pocket (for instance, Glu374 instead of the expected basic residue) and at the intersubunit contact area. These regions are of particular interest since they represent binding sites of known GR inhibitors. Consequently, parasite GR can serve as a target structure for the design of antimalarial drugs.

Cloning, sequencing, and regulation of the glutathione reductase gene from the cyanobacterium Anabaena PCC 7120

Glutathione reductase (GR) was purified from the cyanobacterium Anabaena PCC 7120. A 3-kilobase genomic DNA fragment containing the coding sequence for the GR gene (gor) was identified and cloned by polymerase chain reaction based on sequences of selected peptides isolated from proteolyzed GR. The coding sequence encompassing 458 amino acid residues, as well as 360 base pairs of the 5'-flanking region and 430 base pairs of the 3'-flanking region, were determined. Genomic Southern analysis indicates that gor is a single-copy gene. A gor antisense RNA probe hybridized with a 1.4-kilobase transcript, suggesting that the gene is not part of an operon including additional genes. The deduced GR amino acid sequence shows 41 to 48% identity with those of human, Escherichia coli, Pseudomonas aeruginosa, pea, and Arabidopsis thaliana GR. The coding sequence of GR was overexpressed in a GR-deficient E. coli strain, SG5, and the recombinant protein was purified. Anabaena GR is NADPH-linked, but a Lys residue replaces an Arg residue involved in NADPH binding in GR from other species. In addition, Anabaena GR carries the GXGXXG "fingerprint" motif which otherwise characterizes NAD(H)-dependent enzymes. These differences may contribute to the lack of affinity for 2',5'-ADP-Sepharose 4B of Anabaena GR. Three E. coli-type promoter sequences and a BifA/NtcA binding motif were found upstream of the open reading frame. The middle and the proximal promoters were shown to be active. However, the use of the middle promoter was dependent on the nitrogen source in the culture medium. Both GR activity and GR protein concentration increased in ammonium grown cultures in which both the middle and proximal promoters were used for transcriptional initiation. The BifA/NtcA-binding site overlaps the middle promoter sequence and may thus be involved in regulation of differential transcription.

Bacterial steroid monooxygenase catalyzing the Baeyer-Villiger oxidation of C21-ketosteroids from Rhodococcus rhodochrous: the isolation and characterization

Steroid monooxygenase from Rhodococcus rhodochrous, isolated in homogeneity with a high yield, catalyzes Baeyer-Villiger oxidation of progesterone to produce testosterone acetate with the stoichiometric consumptions of NADPH and molecular oxygen. It is a flavoenzyme with the molecular size of 60 kDa in the monomeric form and the isoelectric point of 4.9. The absorption spectrum has the maxima at 278, 376, and 439 nm and the shoulders at 360 and 465 nm, indicating a strong hypsochromic shift (blue-shift) of the absorption peak in the visible wavelength region. The prosthetic group of the enzyme was identified to be FAD, and the Kd value was estimated to be 0.95 microM. The enzyme catalyzed only the oxidative esterification of progesterone, 11 alpha- and 11 beta-hydroxyprogesterone and not the oxidative lactonization of androstenedione. Km for progesterone was 100 microM, for NADPH was 3.3 microM, and the turnover number was 185 min-1. Kd values for progesterone, 11 alpha-hydroxyprogesterone, deoxycorticosterone, and androstenedione were 110, 130, 2000, and 450 microM, respectively. The optimum pH of the reaction was about 8.5. The reaction was inhibited competitively by 17 alpha-hydroxyprogesterone and androstenedione. Amino terminal sequences of the enzymes from the bacterium and also from fungus, Cylindrocarpon radiocicola were considerably different, and the potential flavin-binding site could be detected on the amino-terminal region of the fungus enzyme but not on that of the bacterial enzyme. Western blotting analyses of the two steroid monooxygenases resulted that mouse antiserum raised for each enzyme reacted only with the antigenic enzyme protein but did not show the cross-reactions. It is clarified that bacterial steroid monooxygenase is distinctly different from the fungal enzyme in the molecular and enzymic properties.

Characterization of the gor gene of the lactic acid bacterium Streptococcus thermophilus CNRZ368

Cloning and characterization of the gor gene of the lactic acid bacterium Streptococcus thermophilus, encoding glutathione reductase, are described in this paper. This enzyme is a part of the enzymatic defences against oxidative stress in eukaryotic cells and in Gram-negative bacteria, but was never found in Gram-positive bacteria before this study. The amino acid sequence shares extensive similarities with glutathione reductases from other organisms, e.g. 62% amino acid identity with Escherichia coli protein. Northern blot analysis and glutathione reductase enzyme assays gave evidence that the gene is expressed in aerobically growing cells.

An L40C mutation converts the cysteine-sulfenic acid redox center in enterococcal NADH peroxidase to a disulfide

Multiple sequence alignments including the enterococcal NADH peroxidase and NADH oxidase indicate that residues Ser38 and Cys42 align with the two cysteines of the redox-active disulfides found in glutathione reductase (GR), lipoamide dehydrogenase, mercuric reductase, and trypanothione reductase. In order to evaluate those structural determinants involved in the selection of the cysteine-sulfenic acid (Cys-SOH) redox centers found in the two peroxide reductases and the redox-active disulfides present in the GR class of disulfide reductases, NADH peroxidase residues Ser38, Phe39, Leu40, and Ser41 have been individually replaced with Cys. Both the F39C and L40C mutant peroxidases yield active-site disulfides involving the new Cys and the native Cys42; formation of the Cys39-Cys42 disulfide, however, precludes binding of the FAD coenzyme. In contrast, the L40C mutant contains tightly-bound FAD and has been analyzed by both kinetic and spectroscopic approaches. In addition, the L40C and S41C mutant structures have been determined at 2.1 and 2.0 A resolution, respectively, by X-ray crystallography. Formation of the Cys40-Cys42 disulfide bond requires a movement of Cys42-SG to a new position 5.9 A from the flavin-C(4a) position; this is consistent with the inability of the new disulfide to function as a redox center in concert with the flavin. Stereochemical constraints prohibit formation of the Cys41-Cys42 disulfide in the latter mutant.

Isolation, characterization and overexpression of the yeast gene, GLR1, encoding glutathione reductase

Using degenerate oligodeoxyribonucleotides based on the N-terminal amino acid (aa) sequence of a yeast glutathione reductase (GR) CNBr-generated peptide fragment and a conserved C-terminal region of known GR aa sequences, the yeast gene encoding GR, GLR1, was isolated using PCR followed by screening of a yeast genomic DNA plasmid library. GLR1 encodes a 467-aa protein with a deduced M(r) of 51,545. Comparison with Escherichia coli and human GR sequences reveals 49.8% aa identity. Yeast cells transformed with a multicopy plasmid containing the genomic clone overproduced GR activity sixfold. GLR1 was found not to be an essential gene.

An Escherichia coli chromosomal ars operon homolog is functional in arsenic detoxification and is conserved in gram-negative bacteria

Arsenic is a known toxic metalloid, whose trivalent and pentavalent ions can inhibit many biochemical processes. Operons which encode arsenic resistance have been found in multicopy plasmids from both gram-positive and gram-negative bacteria. The resistance mechanism is encoded from a single operon which typically consists of an arsenite ion-inducible repressor that regulates expression of an arsenate reductase and inner membrane-associated arsenite export system. Using a lacZ transcriptional gene fusion library, we have identified an Escherichia coli operon whose expression is induced by cellular exposure to sodium arsenite at concentrations as low as 5 micrograms/liter. This chromosomal operon was cloned, sequenced, and found to consist of three cistrons which we named arsR, arsB, and arsC because of their strong homology to plasmid-borne ars operons. Mutants in the chromosomal ars operon were found to be approximately 10- to 100-fold more sensitive to sodium arsenate and arsenite exposure than wild-type E. coli, while wild-type E. coli that contained the operon cloned on a ColE1-based plasmid was found to be at least 2- to 10-fold more resistant to sodium arsenate and arsenite. Moreover, Southern blotting and high-stringency hybridization of this operon with chromosomal DNAs from a number of bacterial species showed homologous sequences among members of the family Enterobacteriaceae, and hybridization was detectable even in Pseudomonas aeruginosa. These results suggest that the chromosomal ars operon may be the evolutionary precursor of the plasmid-borne operon, as a multicopy plasmid location would allow the operon to be amplified and its products to confer increased resistance to this toxic metalloid.