Expression studies of superoxide dismutases in nodules and leaves of transgenic alfalfa reveal abundance of iron-containing isozymes, posttranslational regulation, and compensation of isozyme activities

The composition of antioxidant enzymes, especially superoxide dismutase (SOD), was studied in one nontransgenic and three transgenic lines of nodulated alfalfa plants. Transgenic lines overproduced MnSOD in the mitochondria of nodules and leaves (line 1-10), MnSOD in the chloroplasts (line 4-6), and FeSOD in the chloroplasts (line 10-7). In nodules of line 10-7, the absence of transgene-encoded FeSOD activity was due to a lack of mRNA, whereas in nodules of line 4-6 the absence of transgene-encoded MnSOD activity was due to enzyme inactivation or degradation. Transgenic alfalfa showed a novel compensatory effect in the activities of MnSOD (mitochondrial) and FeSOD (plastidic) in the leaves, which was not caused by changes in the mRNA levels. These findings imply that SOD activity in plant tissues and organelles is regulated, at least partially, at the posttranslational level. All four lines had low CuZnSOD activities and an abundant FeSOD isozyme, especially in nodules, indicating that FeSOD performs important antioxidant functions other than the scavenging of superoxide radicals generated in photosynthesis. This was confirmed by the detection of FeSOD cDNAs and proteins in nodules of other legumes such as cowpea, pea, and soybean. The cDNA encoding alfalfa nodule FeSOD was characterized and the deduced protein found to contain a plastid transit peptide. A comparison of sequences and other properties reveals that there are two types of FeSODs in nodules.     

Superoxide Dismutase in Arabidopsis: An Eclectic Enzyme Family with Disparate Regulation and Protein Localization

A number of environmental stresses can lead to enhanced production of superoxide within plant tissues, and plants are believed to rely on the enzyme superoxide dismutase (SOD) to detoxify this reactive oxygen species. We have identified seven cDNAs and genes for SOD in Arabidopsis. These consist of three CuZnSODs (CSD1, CSD2, and CSD3), three FeSODs (FSD1, FSD2, and FSD3), and one MnSOD (MSD1). The chromosomal location of these seven SOD genes has been established. To study this enzyme family, antibodies were generated against five proteins: CSD1, CSD2, CSD3, FSD1, and MSD1. Using these antisera and nondenaturing-polyacrylamide gel electrophoresis enzyme assays, we identified protein and activity for two CuZnSODs and for FeSOD and MnSOD in Arabidopsis rosette tissue. Additionally, subcellular fractionation studies revealed the presence of CSD2 and FeSOD protein within Arabidopsis chloroplasts. The seven SOD mRNAs and the four proteins identified were differentially regulated in response to various light regimes, ozone fumigation, and ultraviolet-B irradiation. To our knowledge, this is the first report of a large-scale analysis of the regulation of multiple SOD proteins in a plant species.     

Enhancement of oxidative stress tolerance in transgenic tobacco plants overproducing Fe-superoxide dismutase in chloroplasts.

A chimeric gene consisting of the coding sequence for chloroplastic Fe superoxide dismutase (FeSOD) from Arabidopsis thaliana, coupled to the chloroplast targeting sequence from the pea ribulose-1,5-bisphosphate carboxylase/oxygenase small subunit, was expressed in Nicotiana tabacum cv Petit Havana SR1. Expression of the transgenic FeSOD protected both the plasmalemma and photosystem II against superoxide generated during illumination of leaf discs impregnated with methyl viologen. By contrast, overproduction of a mitochondrial MnSOD from Nicotiana plumbaginifolia in the chloroplasts of cv SR1 protected only the plasmalemma, but not photosystem II, against methyl viologen (L. Slooten, K. Capiau, W. Van Camp, M. Van Montagu, C. Sybesma, D. Inzé [1995] Plant Physiol 107: 737-750). The difference in effectiveness correlates with different membrane affinities of the transgenic FeSOD and MnSOD. Overproduction of FeSOD does not confer tolerance to H2O2, singlet oxygen, chilling-induced photoinhibition in leaf disc assays, or to salt stress at the whole plant level. In nontransgenic plants, salt stress led to a 2- to 3-fold increase in activity, on a protein basis, of FeSOD, cytosolic and chloroplastic Cu/ZnSOD, ascorbate peroxidase, dehydroascorbate reductase, and glutathione reductase. In FeSOD-overproducing plants under salt stress, the induction of cytosolic and chloroplastic Cu/ZnSOD was suppressed, whereas induction of a water-soluble chloroplastic ascorbate peroxidase isozyme was promoted.     

The crystal structure of an eukaryotic iron superoxide dismutase suggests intersubunit cooperation during catalysis

Superoxide dismutases (SODs) are a family of metalloenzymes that catalyze the dismutation of superoxide anion radicals into molecular oxygen and hydrogen peroxide. Iron superoxide dismutases (FeSODs) are only expressed in some prokaryotes and plants. A new and highly active FeSOD with an unusual subcellular localization has recently been isolated from the plant Vigna unguiculata (cowpea). This protein functions as a homodimer and, in contrast to the other members of the SOD family, is localized to the cytosol. The crystal structure of the recombinant enzyme has been solved and the model refined to 1.97 A resolution. The superoxide anion binding site is located in a cleft close to the dimer interface. The coordination geometry of the Fe site is a distorted trigonal bipyramidal arrangement, whose axial ligands are His43 and a solvent molecule, and whose in-plane ligands are His95, Asp195, and His199. A comparison of the structural features of cowpea FeSOD with those of homologous SODs reveals subtle differences in regard to the metal-protein interactions, and confirms the existence of two regions that may control the traffic of substrate and product: one located near the Fe binding site, and another in the dimer interface. The evolutionary conservation of reciprocal interactions of both monomers in neighboring active sites suggests possible subunit cooperation during catalysis.     

Thermostability of manganese- and iron-superoxide dismutases from Escherichia coli is determined by the characteristic position of a glutamine residue.

The structurally homologous mononuclear iron and manganese superoxide dismutases (FeSOD and MnSOD, respectively) contain a highly conserved glutamine residue in the active site which projects toward the active-site metal centre and participates in an extensive hydrogen bonding network. The position of this residue is different for each SOD isoenzyme (Q69 in FeSOD and Q146 in MnSOD of Escherichia coli). Although site-directed mutant enzymes lacking this glutamine residue (FeSOD[Q69G] and MnSOD[Q146A]) demonstrated a higher degree of selectivity for their respective metal, they showed little or no activity compared with wild types. FeSOD double mutants (FeSOD[Q69G/A141Q]), which mimic the glutamine position in MnSOD, elicited 25% the activity of wild-type FeSOD while the activity of the corresponding MnSOD double mutant (MnSOD[G77Q/Q146A]) increased to 150% (relative to wild-type MnSOD). Both double mutants showed reduced selectivity toward their metal. Differences exhibited in the thermostability of SOD activity was most obvious in the mutants that contained two glutamine residues (FeSOD[A141Q] and MnSOD[G77Q]), where the MnSOD mutant was thermostable and the FeSOD mutant was thermolabile. Significantly, the MnSOD double mutant exhibited a thermal-inactivation profile similar to that of wild-type FeSOD while that of the FeSOD double mutant was similar to wild-type MnSOD. We conclude therefore that the position of this glutamine residue contributes to metal selectivity and is responsible for some of the different physicochemical properties of these SODs, and in particular their characteristic thermostability.     

Copper-zinc superoxide dismutase of Caulobacter crescentus: cloning, sequencing, and mapping of the gene and periplasmic location of the enzyme.

Although widely found in the cytoplasm of eucaryotes, the copper-zinc form of superoxide dismutase (CuZnSOD) has been identified in only a small number of bacterial species. One species is the freshwater bacterium Caulobacter crescentus, which also contains an SOD with iron as the metal cofactor (FeSOD). To investigate the function of this CuZnSOD and its structural relationship to the eucaryotic CuZnSODs, the gene encoding CuZnSOD (sodC) of C. crescentus CB15 was cloned and sequenced. By hybridization to pulsed-field electrophoresis gels, sodC was mapped near cysE in the C. crescentus chromosome. Through analysis of spheroplasts, the two SODs of C. crescentus were shown to be differently localized, CuZnSOD in the periplasm and FeSOD in the cytoplasm. In its natural habitat, C. crescentus is frequently associated with blue-green algae (cyanobacteria). The oxygen evolved by these photosynthetic algae may create an extracellular oxidative stress against which the periplasmic CuZnSOD may defend more effectively than the cytoplasmic FeSOD. Amino acid sequence alignments of C. crescentus CuZnSOD with eucaryotic CuZnSODs and with CuZnSOD of Photobacterium leiognathi (the only other bacterium from which CuZnSOD has been isolated and sequenced) suggest similar supersecondary structures for bacterial and eucaryotic CuZnSODs but reveal four novel substitutions in C. crescentus CuZnSOD: a phenylalanine critical to intrasubunit hydrophobic bonding replaced by alanine, a histidine ligand of zinc replaced by aspartate, and substitutions of two other previously invariant residues that stabilize zinc or both copper and zinc. These amino acid substitutions in C. crescentus CuZnSOD may have implications for its catalysis and stability.     

Induction of superoxide dismutases in Escherichia coli B by metal chelators

The effects of metal salts, chelating agents, and paraquat on the superoxide dismutases (SODs) of Escherichia coli B were explored. Mn(II) increased manganese-containing SOD (MnSOD), whereas Fe(II) increased iron-containing SOD (FeSOD). Chelating agents induced MnSOD but decreased FeSOD and markedly increased the degree of induction seen with Mn(II). Paraquat also exerted a synergistic effect with Mn(II). High levels of MnSOD were achieved in the combined presence of Mn(II), chelating agent, and paraquat. All of these effects were dependent on the presence of oxygen. MnSOD, not ordinarily present in anaerobically grown E. coli cells, was present when the cells were grown anaerobically in the presence of chelating agents. These results are accommodated by a scheme which incorporates autogenous repression by the apoSODs and competition between Fe(II) and Mn(II) for the metal-binding sites of the apoSODs. It is further supposed that oxygenation and intracellular O2- production favor MnSOD production because O2- oxidizes Mn(II) to Mn(III), which competes favorably with Fe(II) for the apoSODs.     

Increased Tolerance to Mn Deficiency in Transgenic Tobacco Overproducing Superoxide Dismutase

The effect of Mn deficiency on plant growth and activities ofsuperoxide dismutase (SOD) was studied in hydroponically-grownseedlings of transgenic tobacco (Nicotiana tabacum L.) engineeredto overexpress FeSOD in chloroplasts or MnSOD in chloroplastsor mitochondria. In comparison to the non-transgenic parentalline, the activity of MnSOD in the lines overproducing MnSODwas 1.6-fold greater, and the activity of FeSOD in the FeSOD-overproducinglines was 3.2-fold greater, regardless of the Mn treatment (deficientor sufficient). The MnSOD activities decreased due to Mn deficiency,while activities of FeSOD and Cu/ZnSOD remained unaffected 25d after transplanting (DAT). With an increased duration of theMn deficiency stress (45 DAT), FeSOD activity decreased, andthat of MnSOD continued to decrease, while Cu/ZnSOD activitysimultaneously increased. Under Mn sufficiency, non-transgenicparental plants had greater shoot biomass than the transgenics;however, when subjected to Mn deficiency stress, non-transgenicparents suffered a proportionally greater growth reduction thantransgenic lines. Thus, overproduction of MnSOD in chloroplastsmay provide protection from oxidative stress caused by Mn deficiency.Copyright 1999 Annals of Botany Company Manganese deficiency, Nicotiana tabacum, superoxide dismutase (SOD), transgenic tobacco.     

Improved procedure for detection of superoxide dismutase isoforms in potato,Solanum tuberosum L.

Superoxide dismutase (SOD) in-gel activity assay with selective inhibitors (KCN and H₂O₂) is one of the most commonly used methods for identification of SOD isoform types, i.e., FeSOD, MnSOD or Cu/ZnSOD, and evaluation of oxidative stress response in plants. However, there are potential pitfalls that surround this assay, such as problem to detect isoforms with low activity, comigration of SOD isoforms or application of inappropriate inhibitor concentration. We propose an improved method based on the combination of in-gel analysis of SOD activity and native-PAGE immunoblotting for identification of isoforms and determination of SOD isoenzyme activity pattern in potato. Depending on cultivar and growing conditions, one MnSOD, 3 FeSOD and 5–6 Cu/ZnSOD isoforms were identified in potato leaves. The most important qualitative difference between ex vitro- and in vitro-grown plants was the presence of additional FeSOD and Cu/ZnSOD isoforms in plantlets grown in vitro. Compared with results of in-gel activity assay with selective inhibitors, new method allowed accurate identification of comigrating FeSOD and Cu/ZnSOD isoforms and two protein bands of ambiguous identities. Potato SODs were also characterized by SDS-PAGE immunoblotting and single MnSOD (23.6 kDa), three Cu/ZnSOD polypeptides (17.9, 17 and 16.3 kDa) and single FeSOD (25.1 kDa) polypeptide were detected in leaves of four examined cultivars. The difference in the number of FeSOD and Cu/ZnSOD isoforms/polypeptides between native-PAGE and SDS-PAGE immunoblots suggests that SOD proteins may have undergone post-translational modifications affecting protein mobility or existence of isoforms that differ from each other in total protein charge, but not in molecular weight.     

Characterization of genomic clones and expression analysis of the three types of superoxide dismutases during nodule development in Lotus japonicus

Superoxide dismutases (SODs) are metalloenzymes that play a primary role in the protection against oxidative stress in plants and other organisms. We have characterized four SOD genes in Lotus japonicus and have analyzed their expression in roots and four developmental stages of nodules. The expression of cytosolic CuZnSOD, at the mRNA, protein, and enzyme activity levels, decreases with nodule age, and the protein is localized in the dividing cells and infection threads of emergent nodules and in the infected cells of young nodules. The mitochondrial MnSOD was downregulated, whereas the bacteroidal MnSOD displayed maximal protein and enzyme activity levels in older nodules. Two additional genes, encoding plastidic (FeSOD1) and cytosolic (FeSOD2) FeSOD isoforms, were identified and mapped. The genes are located in different chromosomes and show differential expression. The FeSOD1 mRNA level did not change during nodule development, whereas FeSOD2 was upregulated. The distinct expression patterns of the SOD genes may reflect different regulatory mechanisms of the enzyme activities during nodule ontogeny. In particular, at the mRNA and activity levels, the virtual loss of cytosolic CuZnSOD in mature and old nodules, concomitant with the induction of FeSOD2, suggests that the two enzymes may functionally compensate each other in the cytosol at the late stages of nodule development.     

Proton-coupled electron transfer in Fe-superoxide dismutase and Mn-superoxide dismutase

Fe-containing superoxide dismutase (FeSOD) and MnSOD are widely assumed to employ the same catalytic mechanism. However this has not been completely tested. In 1985, Bull and Fee showed that FeSOD took up a proton upon reduction [J. Am. Chem. Soc. 107 (1985) 3295]. We now demonstrate that MnSOD incorporates the same crucial coupling between electron transfer and proton transfer. The redox-coupled H(+) acceptor has been presumed to be the coordinated solvent molecule, in both FeSOD and MnSOD, however this is very difficult to test experimentally. We have now examined the most plausible alternative: that Tyr34 accepts a proton upon SOD reduction. We report specific incorporation of 13C in the C(zeta) positions of Tyr residues, assignment of the C(zeta) signal of Tyr34 in each of oxidized FeSOD and MnSOD, and direct NMR observations showing that in both cases, Tyr34 is in the neutral protonated state. Thus Tyr34 cannot accept a proton upon SOD reduction, and coordinated solvent is concluded to be the redox-coupled H(+) acceptor instead, in both FeSOD and MnSOD. We have also confirmed by direct 13C observation that the pK of 8.5 of reduced FeSOD corresponds to deprotonation of Tyr34. This work thus provides experimental proof of important commonalities between the detailed mechanisms of FeSOD and MnSOD.     

Importance of anaerobic superoxide dismutase synthesis in facilitating outgrowth of Escherichia coli upon entry into an aerobic habitat.

The manganese-containing isozyme of superoxide dismutase (MnSOD) is synthesized by Escherichia coli only during aerobiosis, in accordance with the fact that superoxide can be formed only in aerobic environments. In contrast, E. coli continues to synthesize the iron-containing isozyme (FeSOD) even in the absence of oxygen. A strain devoid of FeSOD exhibited no deficits during either anaerobic or continuously aerobic growth, but its growth lagged for 2 h during the transition from anaerobiosis to aerobiosis. Complementation of this defect with heterologous SODs established that anaerobic SOD synthesis per se is necessary to permit a smooth transition to aerobiosis. The growth deficit was eliminated by supplementation of the medium with branched-chain amino acids, indicating that the growth interruption was due to the established sensitivity of dihydroxyacid dehydratase to endogenous superoxide. Components of the anaerobic respiratory chain rapidly generated superoxide when exposed to oxygen in vitro, suggesting that this transition may be a period of acute oxidative stress. These results show that facultative bacteria must preemptively synthesize SOD during anaerobiosis in preparation for reaeration. The data suggest that evolution has chosen FeSOD for this function because of the relative availability of iron, in comparison to manganese, during anaerobiosis.     

Cloned prokaryotic iron superoxide dismutase protects yeast cells against oxidative stress depending on mitochondrial location

Superoxide dismutase (SOD) is considered to be the first line of defense against oxygen toxicity. It exists as a family of three metalloproteins with copper,zinc (Cu,ZnSOD), manganese (MnSOD), and iron (FeSOD) forms. In this work, we have targeted Escherichia coli FeSOD to the mitochondrial intermembrane space (IMS) of yeast cells deficient in mitochondrial MnSOD. Our results show that FeSOD in the IMS increases the growth rate of the cells growing in minimal medium in air but does not protect the MnSOD-deficient yeast cells when exposed to induced oxidative stress. Cloned FeSOD must be targeted to the mitochondrial matrix to protect the cells from both physiological and induced oxidative stress. This confirms that the superoxide radical is mainly generated on the matrix side of the inner mitochondrial membrane of yeast cells, without excluding its potential appearance in the mitochondrial IMS where its elimination by SOD is beneficial to the cells.     

In vivo competition between iron and manganese for occupancy of the active site region of the manganese-superoxide dismutase of Escherichia coli

Three forms of the dimeric manganese superoxide dismutase (MnSOD) were isolated from aerobically grown Escherichia coli which contained 2 Mn, 1 Mn and 1 Fe, or 2 Fe, respectively. These are designated Mn2-MnSOD, Mn,Fe-MnSOD, and Fe2-MnSOD. Substitution of iron in place of manganese, eliminated catalytic activity, decreased the isoelectric point, and increased the native electrophoretic anodic mobility, although circular dichroism, high performance liquid chromatography gel exclusion chromatography, and sedimentation equilibrium revealed no gross changes in conformation. Moreover, replacement of iron by manganese restored enzymatic activity. Fe2-MnSOD and the iron-superoxide (FeSOD) of E. coli exhibit distinct optical absorption spectra. These data indicate that the active site environments of E. coli MnSOD and FeSOD must differ. They also indicate that competition between iron and manganese for nascent MnSOD polypeptide chains occurs in vivo, and copurification of these variably substituted MnSODs can explain the substoichiometric manganese contents and the variable specific activities which have been reported for this enzyme.     

Effects of water stress on antioxidant enzymes of leaves and nodules of transgenic alfalfa overexpressing superoxide dismutases

The antioxidant composition and relative water stress tolerance of nodulated alfalfa plants ( Medicago sativa L. ×  Sinorhizobium meliloti 102F78) of the elite genotype N4 and three derived transgenic lines have been studied in detail. These transgenic lines overproduced, respectively, Mn-containing superoxide dismutase (SOD) in the mitochondria of leaves and nodules, MnSOD in the chloroplasts, and FeSOD in the chloroplasts. In general for all lines, water stress caused moderate decreases in MnSOD and FeSOD activities in both leaves and nodules, but had distinct tissue-dependent effects on the activities of the peroxide-scavenging enzymes. During water stress, with a few exceptions, ascorbate peroxidase and catalase activities increased moderately in leaves but decreased in nodules. At mild water stress, transgenic lines showed, on average, 20% higher photosynthetic activity than the parental line, which suggests a superior tolerance of transgenic plants under these conditions. However, the untransformed and the transgenic plants performed similarly during moderate and severe water stress and recovery with respect to important markers of metabolic activity and of oxidative stress in leaves and nodules. We conclude that the base genotype used for transformation and the background SOD isozymic composition may have a profound effect on the relative tolerance of the transgenic lines to abiotic stress.     

The iron superoxide dismutase from the filamentous cyanobacterium Nostoc PCC 7120. Localization, overexpression, and biochemical characterization

The nitrogen-fixing filamentous cyanobacterium Nostoc PCC 7120 (formerly named Anabaena PCC 7120) possesses two genes for superoxide dismutase, a unique membrane-associated manganese superoxide dismutase (MnSOD) and a soluble iron superoxide dismutase (FeSOD). A phylogenetic analysis of FeSODs shows that cyanobacterial enzymes form a well separated cluster with filamentous species found in one subcluster and unicellular species in the other. Activity staining, inhibition patterns, and immunogold labeling show that FeSOD is localized in the cytosol of vegetative cells and heterocysts (nitrogenase containing specialized cells formed during nitrogen-limiting conditions). The recombinant Nostoc FeSOD is a homodimeric, acidic enzyme exhibiting the characteristic iron peak at 350 nm in its ferric state, an almost 100% occupancy of iron per subunit, a specific activity using the ferricytochrome assay of (2040 +/- 90) units mg(-1) at pH 7.8, and a dissociation constant Kd of the azide-FeSOD complex of 2.1 mM. Using stopped flow spectroscopy it was shown that the decay of superoxide in the presence of various FeSOD concentrations is first-order in enzyme concentration allowing the calculation of the catalytic rate constants, which increase with decreasing pH: 5.3 x 10(9) M(-1) s(-1) (pH 7) to 4.8 x 10(6) M(-1) s(-1) (pH 10). FeSOD and MnSOD complement each other to keep the superoxide level low in Nostoc PCC 7120, which is discussed with respect to the fact that Nostoc PCC 7120 exhibits oxygenic photosynthesis and oxygen-dependent respiration within a single prokaryotic cell and also has the ability to form differentiated cells under nitrogen-limiting conditions.     

Induction of new isoforms of superoxide dismutase and catalase enzymes in the flag leaf of wheat during monocarpic senescence.

Leaf senescence is a programmed cell death phenomenon and involves oxidative stress. Superoxide dismutase (SOD, EC 1.15.1.1) and catalase (CAT EC 1.11.1.6) activities were studied in the flag leaf of Triticum aestivum cv. Kundan at different stages of grain development. Both SOD and CAT activities showed a decline during monocarpic senescence. Three SOD isozymes were observed in the cytosol, of which one isozyme was observed in the chloroplasts as well. Mitochondria showed the presence of three low abundant SOD isoforms. Inhibitor studies revealed the cytosolic and chloroplastic isoforms to be Cu/Zn SODs. In mitochondria however, two isozymes were MnSOD and one of them appeared to be FeSOD. These isoforms present in the mitochondria increased in activity as senescence progressed. Three isoforms of CAT were observed in peroxisomes which responded differentially during monocarpic senescence. The changes in the kind and pattern of the antioxidant enzymes supported the ordered sequence of events during leaf senescence. This is the first report showing an increase in mitochondrial FeSOD activity during leaf senescence.     

Factors Affecting the Enhancement of Oxidative Stress Tolerance in Transgenic Tobacco Overexpressing Manganese Superoxide Dismutase in the Chloroplasts

Two varieties of tobacco (Nicotiana tabacum var PBD6 and var SR1) were used to generate transgenic lines overexpressing Mn-superoxide dismutase (MnSOD) in the chloroplasts. The overexpressed MnSOD suppresses the activity of those SODs (endogenous MnSOD and chloroplastic and cytosolic Cu/ZnSOD) that are prominent in young leaves but disappear largely or completely during aging of the leaves. The transgenic and control plants were grown at different light intensities and were then assayed for oxygen radical stress tolerance in leaf disc assays and for abundance of antioxidant enzymes and substrates in leaves. Transgenic plants had an enhanced resistance to methylviologen (MV), compared with control plants, only after growth at high light intensities. In both varieties the activities of FeSOD, ascorbate peroxidase, dehydroascorbate reductase, and monodehydroascorbate reductase and the concentrations of glutathione and ascorbate (all expressed on a chlorophyll basis) increased with increasing light intensity during growth. Most of these components were correlated with MV tolerance. It is argued that SOD overexpression leads to enhancement of the tolerance to MV-dependent oxidative stress only if one or more of these components is also present at high levels. Furthermore, the results suggest that in var SR1 the overexpressed MnSOD enhances primarily the stromal antioxidant system.