Periplasmic copper-zinc superoxide dismutase of Legionella pneumophila: role in stationary-phase survival.

Copper-zinc superoxide dismutases (CuZnSODs) are infrequently found in bacteria although widespread in eukaryotes. Legionella pneumophila, the causative organism of Legionnaires' disease, is one of a small number of bacterial species that contain a CuZnSOD, residing in the periplasm, in addition to an iron SOD (FeSOD) in their cytoplasm. To investigate CuZnSOD function, we purified the enzyme from wild-type L. pneumophila, obtained amino acid sequence data from isolated peptides, cloned and sequenced the gene from a L. pneumophila library, and then constructed and characterized a CuZnSOD null mutant. In contrast to the cytoplasmic FeSOD, the CuZnSOD of L. pneumophila is not essential for viability. However, CuZnSOD is critical for survival during the stationary phase of growth. The CuZnSOD null mutant survived 10(4)- to 10(6)-fold less than wild-type L. pneumophila. In wild-type L. pneumophila, the specific activity of CuZnSOD increased during the transition from exponential to stationary-phase growth while the FeSOD activity was constant. These data support a role of periplasmic CuZnSOD in survival of L. pneumophila during stationary phase. Since L. pneumophila survives extensive periods of dormancy between growth within hosts. CuZnSOD may contribute to the ability of this bacterium to be a pathogen. In exponential phase, wild-type and CuZnSOD null strains grew with comparable doubling times. In cultured HL-60 and THP-1 macrophage-like cell lines and in primary cultures of human monocytes, multiplication of the CuZnSOD null mutant was comparable to that of wild type. This indicated that CuZnSOD is not essential for intracellular growth within macrophages or for killing of macrophages in those systems.     

Function and stationary-phase induction of periplasmic copper-zinc superoxide dismutase and catalase/peroxidase in Caulobacter crescentus.

Although cytosolic superoxide dismutases (SODs) are widely distributed among bacteria, only a small number of species contain a periplasmic SOD. One of these is Caulobacter crescentus, which has a copper-zinc SOD (CuZnSOD) in the periplasm and an iron SOD (FeSOD) in the cytosol. The function of periplasmic CuZnSOD was studied by characterizing a mutant of C. crescentus with an insertionally inactivated CuZnSOD gene. Wild-type and mutant strains showed identical tolerance to intracellular superoxide. However, in response to extracellular superoxide, the presence of periplasmic CuZnSOD increased survival by as much as 20-fold. This is the first demonstration that periplasmic SOD defends against external superoxide of environmental origin. This result has implications for those bacterial pathogens that contain a CuZnSOD. C. crescentus was shown to contain a single catalase/peroxidase which, like Escherichia coli KatG catalase/peroxidase, is present in both the periplasmic and cytoplasmic fractions. The growth stage dependence of C. crescentus catalase/peroxidase and SOD activity was studied. Although FeSOD activity was identical in exponential- and stationary-phase cultures, CuZnSOD was induced nearly 4-fold in stationary phase and the catalase/peroxidase was induced nearly 100-fold. Induction of antioxidant enzymes in the periplasm of C. crescentus appears to be an important attribute of the stationary-phase response and may be a useful tool for studying its regulation.     

Bacterial Evolutionary Precursors of Eukaryotic Copper–Zinc Superoxide Dismutases

Internalization of a bacteria by an archaeal cell expedited eukaryotic evolution. An important feature of the species that diversified into the great variety of eukaryotic life visible today was the ability to combat oxidative stress with a copper–zinc superoxide dismutase (CuZnSOD) enzyme activated by a specific, high-affinity copper chaperone. Adoption of a single protein interface that facilitates homodimerization and heterodimerization was essential; however, its evolution has been difficult to rationalize given the structural differences between bacterial and eukaryotic enzymes. In contrast, no consistent strategy for the maturation of periplasmic bacterial CuZnSODs has emerged. Here, 34 CuZnSODs are described that closely resemble the eukaryotic form but originate predominantly from aquatic bacteria. Crystal structures of a Bacteroidetes bacterium CuZnSOD portray both prokaryotic and eukaryotic characteristics and propose a mechanism for self-catalyzed disulfide maturation. Unification of a bacterial but eukaryotic-like CuZnSOD along with a ferredoxin-fold MXCXXC copper-binding domain within a single polypeptide created the advanced copper delivery system for CuZnSODs exemplified by the human copper chaperone for superoxide dismutase-1. The development of this system facilitated evolution of large and compartmentalized cells following endosymbiotic eukaryogenesis.     

Characterization of the iron superoxide dismutase gene of Azotobacter vinelandii: sodB may be essential for viability

Azotobacter vinelandii contains two superoxide dismutases (SODs), a cytoplasmic iron-containing enzyme (FeSOD), and a periplasmic copper/zinc-containing enzyme (CuZnSOD). In this study, the FeSOD was found to be constitutive, while the activity of CuZnSOD increased as the culture entered the stationary phase. Total SOD (units/mg protein) in stationary phase cells grown under nitrogen-fixing conditions was not significantly different from those grown under non-nitrogen-fixing conditions. The gene encoding FeSOD (sodB) was isolated from an A. vinelandii cosmid library. A 1-kb fragment containing the coding region and 400 base pairs of upstream sequence was cloned and sequenced. The nucleotide sequence and the deduced amino acid sequence had a high degree of homology with other bacterial FeSODs, particularly with P. aeruginosa. Attempts to construct a sodB mutant by recombination of a sodB::kan insertion mutation into the multicopy chromosome of A. vinelandii were unsuccessful even in the presence of SOD mimics or nutritional supplements. These results suggest that FeSOD may be essential for the growth and survival of A. vinelandii, and that the periplasmic CuZnSOD cannot replace the function of FeSOD.     

Function of periplasmic copper-zinc superoxide dismutase in Caulobacter crescentus.

Caulobacter crescentus is one of a small number of bacterial species that contain a periplasmic copper-zinc superoxide dismutase (CuZnSOD). A C. crescentus mutant, with the CuZnSOD gene interrupted by a promoterless cat gene, was constructed and characterized to analyze CuZnSOD function. Periplasmic SOD does not protect against oxyradical damage in the cytosol or play a major role in maintaining the integrity of the cell envelope. Studies of the effect of sodium citrate on plating efficiency suggest that CuZnSOD protects a periplasmic or membrane function(s) requiring magnesium or calcium.     

LSD1 regulates salicylic acid induction of copper zinc superoxide dismutase in Arabidopsis thaliana.

We characterized the accumulation patterns of Arabidopsis thaliana proteins, two CuZnSODs, FeSOD, MnSOD, PR1, PR5, and GST1, in response to various pathogen-associated treatments. These treatments included inoculation with virulent and avirulent Pseudomonas syringae strains, spontaneous lesion formation in the lsd1 mutant, and treatment with the salicylic acid (SA) analogs INA (2,6-dichloroisonicotinic acid) and BTH (benzothiadiazole). The PR1, PR5, and GST1 proteins were inducible by all treatments tested, as expected from previous mRNA blot analysis. The two CuZnSOD proteins were induced by SA analogs and in conjunction with lsd1-mediated spreading cell death. Additionally, LSD1 is a part of a signaling pathway for the induction of the CuZnSOD proteins in response to SA but not in lsd1-mediated cell death. We suggest that the spreading lesion phenotype of lsd1 results from a lack of up-regulation of a CuZnSOD responsible for detoxification of accumulating superoxide before the reactive oxygen species can trigger a cell death cascade.     

Two Fe-superoxide dismutase families respond differently to stress and senescence in legumes

Three main families of SODs in plants may be distinguished according to the metal in the active center: CuZnSODs, MnSOD, and FeSOD. CuZnSODs have two sub-families localized either in plant cell cytosol or in plastids, the MnSOD family is essentially restricted to mitochondria, and the FeSOD enzyme family has been typically localized into the plastid. Here, we describe, based on a phylogenetic tree and experimental data, the existence of two FeSOD sub-families: a plastidial localized sub-family that is universal to plants, and a cytosolic localized FeSOD sub-family observed in determinate-forming nodule legumes. Anti-cytosolic FeSOD (cyt_FeSOD) antibodies were employed, together with a novel antibody raised against plastidial FeSOD (p_FeSOD). Stress conditions, such as nitrate excess or drought, markedly increased cyt_FeSOD contents in soybean tissues. Also, cyt_FeSOD content and activity increased with age in both soybean and cowpea plants, while the cyt_CuZnSOD isozyme was predominant during early stages. p_FeSOD in leaves decreased with most of the stresses applied, but this isozyme markedly increased with abscisic acid in roots. The great differences observed for p_FeSOD and cyt_FeSOD contents in response to stress and aging in plant tissues reveal distinct functionality and confirm the existence of two immunologically differentiated FeSOD sub-families. The in-gel FeSOD activity patterns showed a good correlation to cyt_FeSOD contents but not to those of p_FeSOD. This indicates that cyt_FeSOD is the main active FeSOD in soybean and cowpea tissues. The diversity of functions associated with the complexity of FeSOD isoenzymes depending of the location is discussed.     

THE ROLE AND EVOLUTION OF SUPEROXIDE DISMUTASES IN ALGAE1

Superoxide dismutases (SOD) catalyze the disproportionation of the potentially destructive superoxide anion radical (O₂^??, a byproduct of aerobic metabolism) to molecular oxygen and hydrogen peroxide: 2O₂^??+2H⁺→H₂O₂+O₂. Based on metal cofactors, four known metalloforms of SOD enzymes have been identified: they contain either Fe, Mn, Cu and Zn, or Ni. Orthologs of all metalloforms are present in oxygenic photoautotrophs. The expression of SOD is highly regulated, with specific metalloforms playing an inducible protective role for specific cellular compartments. The various metalloforms of SOD are not distributed equally within either cyanobacteria or eukaryotic algae. Typically, cyanobacteria contain either an NiSOD alone or combinations of Mn and Ni or Fe and Mn metalloforms (CuZn is rare among the cyanobacteria). The bacillariophytes and rhodophytes retain an active MnSOD, whereas the chlorophytes, haptophytes, and embryophytes have either FeSOD or multiple combinations of Fe, Mn, and CuZnSODs. The NiSOD is a relatively novel SOD and has been generally excluded from evolutionary analyses. In both cyanobacteria and chlorophyte algae, the FeSOD metalloform appears to be associated with PSI, where its primary role is most likely to deactivate reactive oxygen produced by the Mehler reaction. The CuZnSOD also appears to be associated with the plastid but is phylogenetically more restricted in its distribution. In eukaryotic algae, SODs are all nuclear encoded and, based on nucleotide sequence, protein structures, and phylogenetic distributions, appear to have unique evolutionary histories arising from the lateral gene transfer of three distinct genes to the nucleus after the endosymbiotic acquisition of mitochondria and plastids. The varied phylogenetic histories and subcellular localizations suggest significantly different selection on these SOD metalloforms after the endosymbiont organelle‐to‐host gene transfer.     

Semi-quantitative evaluation of genotoxic activity of chemical substances and evidence for a biological threshold of genotoxic activity

Oxidative DNA damage caused by a cysteine metal-catalyzed oxidation system (Cys-MCO) comprised of Fe(3+), O(2), and a cysteine as an electron donor was enhanced by copper, zinc superoxide dismutase (CuZnSOD) in a concentration-dependent manner, as reflected by the formation of 8-hydroxy-2'-deoxyguanosine (8-OH-dG) and strand breaks. Unlike CuZnSOD, manganese SOD (MnSOD) as well as iron SOD (FeSOD) did not enhance DNA damage. The capacity of CuZnSOD to enhance damage to DNA was inhibited by a spin-trapping agent, 5, 5-dimethyl-1-pyrroline N-oxide (DMPO) and a metal chelator, diethylenetriaminepentaacetic acid (DETAPAC). The deoxyribose assay showed that hydroxyl free radicals were generated in the reaction of CuZnSOD with Cys-MCO. We found that the Cys-MCO system caused the release of free copper from CuZnSOD. CuZnSOD also caused the two-fold enhancement of a mutation in the pUC18 lacZ' gene in the presence of Cys-MCO when measured as a loss of alpha-complementation. Based on these results, we interpret the effects of CuZnSOD on Cys-MCO-induced DNA damage and mutation as due to reactive oxygen species, probably hydroxyl free radicals, formed by the reaction of free Cu(2+), released from oxidatively damaged CuZnSOD, and H(2)O(2) produced by the Cys-MCO system.     

The CcrM DNA methyltransferase is widespread in the alpha subdivision of proteobacteria, and its essential functions are conserved in Rhizobium meliloti and Caulobacter crescentus.

The Caulobacter crescentus DNA methyltransferase CcrM (M.CcrMI) methylates the adenine residue in the sequence GANTC. The CcrM DNA methyltransferase is essential for viability, but it does not appear to be part of a DNA restriction-modification system. CcrM homologs are widespread in the alpha subdivision of gram-negative bacteria. We have amplified and sequenced a 258-bp region of the cerM gene from several of these bacteria, including Rhizobium meliloti, Brucella abortus, Agrobacterium tumefaciens, and Rhodobacter capsulatus. Alignment of the deduced amino acid sequences revealed that these proteins constitute a highly conserved DNA methyltransferase family. Isolation of the full-length ccrM genes from the aquatic bacterium C. crescentus, the soil bacterium R. meliloti, and the intracellular pathogen B. abortus showed that this sequence conservation extends over the entire protein. In at least two alpha subdivision bacteria, R. meliloti and C. crescentus, CcrM-mediated methylation has important cellular functions. In both organisms, CcrM is essential for viability. Overexpression of CcrM in either bacterium results in defects in cell division and cell morphology and in the initiation of DNA replication. Finally, the C. crescentus and R. meliloti ccrM genes are functionally interchangeable, as the complemented strains are viable and the chromosomes are methylated. Thus, in both R. meliloti and C. crescentus, CcrM methylation is an integral component of the cell cycle. We speculate that CcrM-mediated DNA methylation is likely to have similar roles among alpha subdivision bacteria.     

The virulence of Vibrio vulnificus is affected by the cellular level of superoxide dismutase activity

The virulence of superoxide dismutase (SOD) mutants of Vibrio vulnificus, as tested by intraperitoneal injection into mice, decreases in the order of sodC mutant, sodA mutant, and sodB mutant lacking CuZnSOD, MnSOD, and FeSOD, respectively. The survival of SOD mutants under superoxide stress also decreases in the same order. The virulence of soxR mutant, which is unable to induce MnSOD in response to superoxide, is similar to that of the sodA mutant, as the survival of the soxR mutant under superoxide stress is similar to that of the sodA mutant. Consistently, the lowered survival of the soxR mutant is complemented not only with soxR but also with sodA. Thus, the virulence of V. vulnificus is significantly affected by the cellular level of SOD activity, and an increase in SOD level through MnSOD induction by SoxR under superoxide stress is essential for virulence.     

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.     

Loss of in vitro metal ion binding specificity in mutant copper-zinc superoxide dismutases associated with familial amyotrophic lateral sclerosis

The presence of the copper ion at the active site of human wild type copper-zinc superoxide dismutase (CuZnSOD) is essential to its ability to catalyze the disproportionation of superoxide into dioxygen and hydrogen peroxide. Wild type CuZnSOD and several of the mutants associated with familial amyotrophic lateral sclerosis (FALS) (Ala(4) --> Val, Gly(93) --> Ala, and Leu(38) --> Val) were expressed in Saccharomyces cerevisiae. Purified metal-free (apoproteins) and various remetallated derivatives were analyzed by metal titrations monitored by UV-visible spectroscopy, histidine modification studies using diethylpyrocarbonate, and enzymatic activity measurements using pulse radiolysis. From these studies it was concluded that the FALS mutant CuZnSOD apoproteins, in direct contrast to the human wild type apoprotein, have lost their ability to partition and bind copper and zinc ions in their proper locations in vitro. Similar studies of the wild type and FALS mutant CuZnSOD holoenzymes in the "as isolated" metallation state showed abnormally low copper-to-zinc ratios, although all of the copper acquired was located at the native copper binding sites. Thus, the copper ions are properly directed to their native binding sites in vivo, presumably as a result of the action of the yeast copper chaperone Lys7p (yeast CCS). The loss of metal ion binding specificity of FALS mutant CuZnSODs in vitro may be related to their role in ALS.     

Expression and localization of a Rhizobium-derived cambialistic superoxide dismutase in pea (Pisum sativum) nodules subjected to oxidative stress

Two phylogenetically unrelated superoxide dismutase (SOD) families, i.e., CuZnSOD (copper and zinc SOD) and FeMn-CamSOD (iron, manganese, or cambialistic SOD), eliminate superoxide radicals in different locations within the plant cell. CuZnSOD are located within the cytosol and plastids, while the second family of SOD, which are considered to be of bacterial origin, are usually located within organelles, such as mitochondria. We have used the reactive oxygen species-producer methylviologen (MV) to study SOD isozymes in the indeterminate nodules on pea (Pisum sativum). MV caused severe effects on nodule physiology and structure and also resulted in an increase in SOD activity. Purification and N-terminal analysis identified CamSOD from the Rhizobium leguminosarum endosymbiont as one of the most active SOD in response to the oxidative stress. Fractionation of cell extracts and immunogold labeling confirmed that the CamSOD was present in both the bacteroids and the cytosol (including the nuclei, plastids, and mitochondria) of the N-fixing cells, and also within the uninfected cortical and interstitial cells. These findings, together with previous reports of the occurrence of FeSOD in determinate nodules, indicate that FeMnCamSOD have specific functions in legumes, some of which may be related to signaling between plant and bacterial symbionts, but the occurrence of one or more particular isozymes depends upon the nodule type.     

Electrochemistry of immobilized CuZnSOD and FeSOD and their interaction with superoxide radicals

Copper, zinc superoxide dismutase (CuZnSOD) from bovine erythrocytes and iron superoxide dismutase from Escherichia coli (FeSOD) were immobilized on 3-mercaptopropionic acid (MPA)-modified gold electrodes, respectively. The characterization of the SOD electrodes showed a quasi-reversible, electrochemical redox behavior with a formal potential of 47+/-4 mV and -154+/-5 mV (vs. Ag/AgCl, 1 M KCl) for surface adsorbed CuZnSOD and FeSOD, respectively. The heterogeneous electron transfer rate constants were determined to be about 65 and 35/s, respectively. Covalent fixation of both SODs was also feasible with only slight changes in the formal potential. The interaction of superoxide radicals (O(2)(-)) with the SOD electrode was investigated. No catalytic current could be observed. However, due to the fast cyclic redox reaction of SOD with superoxide, the communication of the protein with the electrode was strongly influenced. The amperometric detection of superoxide radicals is discussed.     

In vivo peroxidative activity of FALS-mutant human CuZnSODs expressed in yeast.

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disorder leading to loss of motor neurons. We previously characterized the enhanced peroxidative activity of the human familial ALS (FALS) mutants of copper-zinc superoxide dismutase (CuZnSOD) A4V and G93A in vitro. Here, a similar activity is demonstrated for human FALS CuZnSOD mutants in an in vivo model system, the yeast Saccharomyces cerevisiae. Spin trap adducts of alpha-(pyridyl-4-N-oxide)-N-tert-butylnitrone (POBN) have been measured by electron paramagnetic resonance (EPR) in yeast expressing mutant (A4V, L38V, G93A, and G93C) and wild type CuZnSOD upon addition of hydrogen peroxide to the culture. The trapped radical is a hydroxyethyl adduct of POBN, identified by spectral parameters. Mutant CuZnSODs produced greater concentrations of the trapped adduct compared to the wild type enzyme. This observation provides evidence for an oxidative radical mechanism, whereby the mutants of CuZnSOD catalyze the formation of reactive oxygen species that may be related to the development or progression of FALS. This study also presents an in vivo model system to study free radical production in FALS-associated CuZnSOD mutations.     

The amino acid sequences of the copper/zinc superoxide dismutases from swordfish and Photobacter leiognathi confirm the predictions made from the compositions

Recent suggestions that the amino acid sequence of the copper/zinc superoxide dismutases of swordfish and Photobacter leiognathi do not support the theory that the bacterium obtained the gene for the enzyme by transfer from its eucaryotic symbiont [Rocha, H. A., Bannister, W. H. and Bannister, J. V. (1984) Eur. J. Biochem. 145, 477-484] are examined. The amount of difference between the sequence is in good agreement with expectation from the amino acid compositions. Moreover, the gene-transfer hypothesis cannot be discarded without postulating an enormous increase in the rate at which the superoxide dismutase gene has accumulated amino acid substitutions since the divergence of the swordfish and cattle lineages.     

Characterization of the iron-containing superoxide dismutase and its response to stress in cyanobacterium Spirulina (Arthrospira) platensis

Superoxide dismutase (SOD) is considered a primary antioxidant which defends against reactive oxygen species that are induced by environmental stress. In this study, we examined changes in SOD activity and expression in the cyanobacterium Spirulina (Arthrospira) platensis under iron and salinity stress; we characterized its induction under these stress conditions and we overexpressed the enzyme in a bacterial host for preliminary characterization. Analysis of SOD isoforms concludes that S. platensis was found to regulate only the iron-containing SOD isoform (FeSOD) in response to two types of stress that were tested. The FeSOD expression (on the level of both mRNA and enzyme activity) was induced by the stress conditions of salinity and iron levels. The FeSOD from S. platensis was overexpressed in Escherichia coli BL21. The recombinant FeSOD protein (about 23 kDa) was purified for characterization. It showed high specific activity and pH stability at 6.0–9.0, and it is relatively thermostable, retaining 45 % of its activity after 30 min at 90 °C. Phylogenetic analysis reveals that S. platensis FeSOD is grouped with the FeSODs from other cyanobacterial species and separated from those of the eukaryotic Chlorophyta, suggesting that the FeSOD gene may be used as a molecular marker in physiological, phylogenetic, and taxonomic studies. This study also suggests that the increased activity and expression of SOD may play a role in algal survival under stress conditions.