Bacteriocuprein superoxide dismutase of Photobacterium leiognathi. Isolation and sequence of the gene and evidence for a precursor form

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.

Cloning and high-level expression of monomeric human superoxide dismutase 1 (SOD1) and its interaction with pyrimidine analogs

Superoxide dismutase 1 (SOD1) is known to be involved in the pathogenesis of Amyotrophic Lateral Sclerosis (ALS) and is therefore considered to be an important ALS drug target. Identifying potential drug leads that bind to SOD1 and characterizing their interactions by nuclear magnetic resonance (NMR) spectroscopy is complicated by the fact that SOD1 is a homodimer. Creating a monomeric version of SOD1 could alleviate these issues. A specially designed monomeric form of human superoxide dismutase (T2M4SOD1) was cloned into E. coli and its expression significantly enhanced using a number of novel DNA sequence, leader peptide and growth condition optimizations. Uniformly ¹⁵N-labeled T2M4SOD1 was prepared from minimal media using ¹⁵NH₄Cl as the ¹⁵N source. The T2M4SOD1 monomer (both ¹⁵N labeled and unlabeled) was correctly folded as confirmed by ¹H-NMR spectroscopy and active as confirmed by an in-gel enzymatic assay. To demonstrate the utility of this new SOD1 expression system for NMR-based drug screening, eight pyrimidine compounds were tested for binding to T2M4SOD1 by monitoring changes in their ¹H NMR and/or ¹⁹F-NMR spectra. Weak binding to 5-fluorouridine (FUrd) was observed via line broadening, but very minimal spectral changes were seen with uridine, 5-bromouridine or trifluridine. On the other hand, ¹H-NMR spectra of T2M4SOD1 with uracil or three halogenated derivatives of uracil changed dramatically suggesting that the pyrimidine moiety is the crucial binding component of FUrd. Interestingly, no change in tryptophan 32 (Trp32), the putative receptor for FUrd, was detected in the ¹⁵N-NMR spectra of ¹⁵N-T2M4SOD1 when mixed with these uracil analogs. Molecular docking and molecular dynamic (MD) studies indicate that interaction with Trp32 of SOD1 is predicted to be weak and that there was hydrogen bonding with the nearby aspartate (Asp96), potentiating the Trp32-uracil interaction. These studies demonstrate that monomeric T2M4SOD1 can be readily used to explore small molecule interactions via NMR.

Immobilized transition metal ions stimulate contact activation and drive factor XII-mediated coagulation

BACKGROUND

Upon contact with an appropriate surface, factor XII (FXII) undergoes autoactivation or cleavage by kallikrein. Zn(2+) is known to facilitate binding of FXII and the cofactor, high molecular weight kininogen (HK), to anionic surfaces.

OBJECTIVES

To investigate whether transition metal ions immobilized on liposome surfaces can initiate coagulation via the contact pathway.

METHODS AND RESULTS

Liposomes containing a metal ion-chelating lipid, 1,2-dioleoyl-sn-glycero-3-{(N[5-amino-1-carboxypentyl]iminodiacetic acid)succinyl} ammonium salt (DOGS-NTA), were prepared by membrane extrusion (20% DOGS-NTA, 40% phosphatidylcholine, 10% phosphatidylserine, and 30% phosphatidylethanolamine). Ni(2+) immobilized on such liposomes accelerated clotting in normal plasma, but not factor XI (FXI)-deficient or FXII-deficient plasma. The results were similar to those obtained with a commercial activated partial thromboplastin time reagent. Charging such liposomes with other transition metal ions revealed differences in their procoagulant capacity, with Ni(2+) > Cu(2+) > Co(2+) and Zn(2+). Plasma could be depleted of FXI, FXII and HK by adsorption with Ni(2+) -containing beads, resulting in longer clot times. Consistent with this, FXI, FXII and HK bound to immobilized Ni(2+) or Cu(2+) with high affinity as determined by surface plasmon resonance. In the presence of Ni(2+) -bearing liposomes, K(m) and k(cat) values derived for autoactivation of FXII and prekallikrein, as well as for activation of FXII by kallikrein or prekallikrein by FXIIa, were similar to literature values obtained in the presence of dextran sulfate.

CONCLUSIONS

Immobilized Ni(2+) and Cu(2+) bind FXII, FXI and HK with high affinity and stimulate activation of the contact pathway, driving FXII-mediated coagulation. Activation of the contact system by immobilized transition metal ions may have implications during pathogenic infection or in individuals exposed to high levels of pollution.

The protein complex composed of nickel-binding SrnQ and DNA binding motif-bearing SrnR of Streptomyces griseus represses sodF transcription in the presence of nickel

Nickel-responsive transcriptional repression of sodF, which codes for iron- and zinc-containing superoxide dismutase of Streptomyces griseus, was mediated through an operator (-2 to +15) spanning over the 5' end (+1) of the transcript. Two open reading frames, SrnR (12,343 Da) and SrnQ (12,486 Da), with overlapping stop-start codons were identified downstream from sodF and found responsible for the repression of sodF. The deduced amino acid sequence of SrnR revealed a DNA binding motif and showed homology to the transcriptional regulators of ArsR family, whereas SrnQ did not show any similarity to any known proteins. When srnRQ DNA was maintained in trans in S. griseus on a multicopy plasmid, sodF transcription was highly repressed by nickel, but neither srnR nor srnQ alone showed the effect. Consistently, the sodF transcription of srnR-interrupted mutant was no longer repressed by nickel, which was complemented only with srnRQ DNA. Nickel-dependent binding of SrnR and SrnQ to the sodF operator DNA was observed only when the two proteins were provided together. The maximum protein-DNA interaction was shown when SrnR and SrnQ were present in one-to-one stoichiometric ratio. The two proteins appear to constitute an octamer composed of four subunits of each protein. SrnR directly interacted with SrnQ, and the protein interaction did not require nickel. The conformation of SrnQ was changed upon nickel binding, which was in the ratio of one Ni(2+) ion per protein molecule. A model is proposed in which SrnQ of the protein complex senses nickel and subsequently enhances the DNA binding activity of SrnR through the protein-protein interaction.

The role of two periplasmic copper- and zinc-cofactored superoxide dismutases in the virulence of Salmonella choleraesuis

Periplasmic copper- and zinc-cofactored superoxide dismutases ([Cu,Zn]-SODs, SodC) of several Gram-negative pathogens can protect against superoxide-radical-mediated host defences, and thus contribute to virulence. This role has been previously defined for one [Cu,Zn]-SOD in various Salmonella serovars. Following the recent discovery of a second periplasmic [Cu,Zn]-SOD in Salmonella, the effect of knockout mutations in one or both of the original sodC-1 and the new sodC-2 on the virulence of the porcine pathogen Salmonella choleraesuis is investigated here. In comparison to wild-type, while sodC mutants--whether single or double--showed no impairment in growth, they all showed equally enhanced sensitivity to superoxide and a dramatically increased sensitivity to the combination of superoxide and nitric oxide in vitro. This observation had its correlate in experimental infection both ex vivo and in vivo. Mutation of sodC significantly impaired survival of S. choleraesuis in interferon gamma-stimulated murine macrophages compared to wild-type organisms, and all S. choleraesuis sodC mutants persisted in significantly lower numbers than wild-type in BALB/c (Ity(s)) and C3H/HeN (Ity(r)) mice after experimental infection, but in no experimental system were sodC-1 sodC-2 double mutants more attenuated than either single mutant. These data suggest that both [Cu,Zn]-SODs are needed to protect bacterial periplasmic or membrane components. While SodC plays a role in S. choleraesuis virulence, the data presented here suggest that this is through overcoming a threshold effect, probably achieved by acquisition of sodC-1 on a bacteriophage. Loss of either sodC gene confers maximum vulnerability to superoxide on S. choleraesuis.

Structural and functional studies of monomeric mutant of Cu-Zn superoxide dismutase without Arg 143

Mutation of arginine (Arg) 143 with Ile in the monomeric mutant (Phe50Glu, Gly51Glu, Vall48Lys, Ile151Lys) of copper-zinc superoxide dismutase (R143I M4SOD, where M4SOD is the above mutant) leads to a protein with low copper content. Cobalt(II) binds the demetalized protein with a low and comparable affinity for the two metal sites, whereas it binds first and stochiometrically at the zinc site in the M4SOD protein and in the dimeric wild type SOD. However, a CuCo SOD derivative can be obtained whose NMR spectra indicate the structural changes induced by monomerization plus those induced by the Arg ---> Ile mutation. The electronic, circular dichroism, and EPR spectra provide structural information on the copper site. The low activity of the enzyme is accounted for on the basis of the structural properties of the active cavity.

Identification of copper-zinc superoxide dismutase gene from enteroaggregative Escherichia coli

We describe here the identification of sodC gene from enteroaggregative Escherichia coli (EAggEC). A 294 bp gene-specific fragment was amplified from the organism by DNA as well as RT-PCR using primers from bacterial sodC sequences. The metal co-factor present in the protein was confirmed by running samples in native gels and inhibiting with 2 mM potassium cyanide. However, the nonpathogenic E. coli possesses the gene but does not express it. Thus, the presence of copper-zinc superoxide dismutase encoded by sodC was demonstrated for the first time in EAggEC, which means it could be a novel candidate for a virulence marker.

Role of superoxide dismutase activity in the physiology of Porphyromonas gingivalis

Porphyromonas gingivalis is a gram-negative, obligate anaerobe strongly associated with chronic adult periodontitis. A previous study has demonstrated that this organism requires superoxide dismutase (SOD) for its modest aerotolerance. In this study, we have constructed a mutant deficient in SOD activity by insertional inactivation as well as a sod::lacZ reporter translational fusion construct to study the regulation of expression of this gene. We have confirmed that SOD is essential for tolerance to atmospheric oxygen but does not appear to be protective against hydrogen peroxide or exogenously generated reactive oxygen species. Furthermore, the sod mutant appeared to be no more sensitive to killing by neutrophils than the parental strain 381. SOD appears to be protective against oxygen-dependent DNA damage as measured by increased mutation to rifampin resistance by the sod mutant. Use of the sod::lacZ construct confirmed that SOD expression is maximal at mid-log phase and is influenced by oxygen, temperature, and pH. However, expression does not appear to be significantly affected by iron depletion, osmolarity, or nutrient depletion. The transcription start site of the sod gene was determined to be 315 bp upstream of the sod start codon and to be within an upstream open reading frame. Our studies demonstrate the essential role that SOD plays in aerotolerance of this organism as well as the selective induction of this enzyme by environmental stimuli.

Identification and subcellular localization of a novel Cu,Zn superoxide dismutase of Mycobacterium tuberculosis

Periplasmic copper, zinc superoxide dismutases (Cu,ZnSOD) of several Gram-negative pathogens have been shown to play an important role in protection against exogenous superoxide radicals and in determining virulence of the pathogens. Here we report the cloning and characterization of the sodC gene, encoding Cu,ZnSOD, from the Gram-positive Mycobacterium tuberculosis. The predicted protein sequence contains 240 amino acids with a putative signal peptide at the N-terminus and shows approximately 25% identity to other bacterial sodC. Recombinant proteins of a full-length sodC and a truncated form lacking the putative signal peptide were overexpressed in Escherichia coli and affinity purified. Renatured recombinant M. tuberculosis sodC protein possessed characteristics of a Cu,ZnSOD. Immunoblotting with an antiserum against the recombinant M. tuberculosis Cu,ZnSOD allowed detection of a single polypeptide in the lysate of M. tuberculosis. This polypeptide has a similar size as the recombinant protein without the putative signal peptide indicating that the endogenous Cu,ZnSOD in M. tuberculosis might be processed and secreted. Furthermore, immunogold electron microscopic image showed that Cu,ZnSOD is located in the periphery of M. tuberculosis. The enzymatic activity and subcellular localization of this novel Cu,ZnSOD suggest that it may play a role in determining virulence of M. tuberculosis.

Expression and regulation of the sodF gene encoding iron- and zinc-containing superoxide dismutase in Streptomyces coelicolor Müller

Streptomyces coelicolor Müller contains two superoxide dismutases (SODs), nickel-containing (NiSOD) and iron- and zinc-containing SOD (FeZnSOD). The sodF gene encoding FeZnSOD was isolated by using PCR primers corresponding to the N-terminal peptide sequence of the purified FeZnSOD and a C-terminal region conserved among known FeSODs and MnSODs. The deduced amino acid sequence exhibited highest similarity to Mn- and FeSODs from Propionibacterium shermanii and Mycobacterium spp. The transcription start site of the sodF gene was determined by primer extension. When the sodF gene was cloned in pIJ702 and introduced into Streptomyces lividans TK24, it produced at least 30 times more FeZnSOD than the control cells. We disrupted the sodF gene in S. lividans TK24 and found that the disruptant did not produce any FeZnSOD enzyme activity but produced more NiSOD. The expression of the cloned sodF gene in TK24 cells was repressed significantly by Ni, consistent with the regulation pattern in nonoverproducing cells. This finding suggests that the cloned sodF gene contains the cis-acting elements necessary for Ni regulation. When the sodF mRNA in S. coelicolor Muller cells was analyzed by S1 mapping of both 5' and 3' ends, we found that Ni caused a reduction in the level of monocistronic sodF transcripts. Ni did not affect the stability of sodF mRNA, indicating that it regulates transcription. S. lividans TK24 cells overproducing FeZnSOD became more resistant to oxidants such as menadione and lawsone than the control cells, suggesting the protective role of FeZnSOD. However, the sodF disruptant survived as well as the wild-type strain in the presence of these oxidants, suggesting the complementing role of NiSOD increased in the disruptant.

Periplasmic copper-zinc superoxide dismutase protects Haemophilus ducreyi from exogenous superoxide

Haemophilus ducreyi causes chancroid, a sexually transmitted genital ulcer disease implicated in increased heterosexual transmission of HIV. As part of an effort to identify H. ducreyi gene products involved in virulence and pathogenesis, we created random TnphoA insertion mutations in an H. ducreyi 35000 library cloned in Escherichia coli. Inserts encoding exported or secreted PhoA fusion proteins were characterized by DNA sequencing. One such clone encoded a Cu-Zn superoxide dismutase (SOD) enzyme. The Cu-Zn SOD was periplasmic in H. ducreyi and accounted for most of the detectable SOD activity in whole-cell lysates of H. ducreyi grown in vitro. To investigate the function of the Cu-Zn SOD, we created a Cu-Zn SOD-deficient H. ducreyi strain by inserting a cat cassette into the sodC gene. The wild-type and Cu-Zn SOD null mutant strains were equally resistant to excess cytoplasmic superoxide induced by paraquat, demonstrating that the Cu-Zn SOD did not function in the detoxification of cytoplasmic superoxide. However, the Cu-Zn SOD null strain was significantly more susceptible to killing by extracellular superoxide than the wild type. This result suggests that the H. ducreyi Cu-Zn SOD may play a role in bacterial defence against oxidative killing by host immune cells during infection.

Periplasmic superoxide dismutase protects Salmonella from products of phagocyte NADPH-oxidase and nitric oxide synthase

Superoxide dismutase (SOD) catalyzes the conversion of superoxide radical to hydrogen peroxide. Periplasmic localization of bacterial Cu,Zn-SOD has suggested a role of this enzyme in defense against extracellular phagocyte-derived reactive oxygen species. Sequence analysis of regions flanking the Salmonella typhimurium sodC gene encoding Cu,Zn-SOD demonstrates significant homology to lambda phage proteins, reflecting possible bacteriophage-mediated horizontal gene transfer of this determinant among pathogenic bacteria. Salmonella deficient in Cu,Zn-SOD has reduced survival in macrophages and attenuated virulence in mice, which can be restored by abrogation of either the phagocyte respiratory burst or inducible nitric oxide synthase. Moreover, a sodC mutant is extremely susceptible to the combination of superoxide and nitric oxide. These observations suggest that SOD protects periplasmic or inner membrane targets by diverting superoxide and limiting peroxynitrite formation, and they demonstrate the ability of the respiratory burst and nitric oxide synthase to synergistically kill microbial pathogens in vivo.

Spectroscopic characterization of recombinant Cu,Zn superoxide dismutase from Photobacterium leiognathi expressed in Escherichia coli: evidence for a novel catalytic copper binding site

Cu,Zn superoxide dismutase from Photobacterium leiognathi has been cloned and expressed in Escherichia coli. The circular dichroism spectrum in the UV region of the recombinant protein indicates an higher content of random coil structure with respect to the eukaryotic enzymes. Investigation of the active site by optical, CD, and EPR spectroscopy indicates a different coordination geometry around the catalytic copper site with respect to the eukaryotic enzymes. In particular a different orientation of the metal bridging histidine is suggested. The pH dependence of the copper EPR spectrum shows the presence of a single equilibrium which is at least one unit lower than the pK value observed for the bovine enzyme. Despite such structural differences the catalytic rate of this enzyme is identical to that observed for the eukaryotic Cu,Zn superoxide dismutase, suggesting that the overall electric field distribution is similar to that observed in the eukaryotic enzymes.

The copper- and zinc-containing superoxide dismutase from Escherichia coli: molecular weight and stability

The periplasmic Cu,Zn superoxide dismutase (Cu,ZnSOD) from Escherichia coli has been shown by sedimentation equilibrium to be a monomer with a molecular weight of approximately 17,000. The enzyme suffered a reversible inactivation when heated to 70 degrees C. This was minimized by added Cu(II) or Zn(II). Heat lability was greater in phosphate than in Tris buffer. The enzyme exhibited a time-dependent inactivation by Hg(II) and this too was greater in phosphate than in Tris. This behavior can be explained by a modest affinity of the enzyme for Cu(II) and Zn(II) which results in a dissociation/association equilibrium. Elevation of the temperature shifts this equilibrium toward dissociation and phosphate sequesters the released metals making them less available for reinsertion at the active site. Hg(II) competes for occupancy of the active site and there were more unoccupied sites in phosphate than in Tris. A parallel was drawn between the E. coli Cu,ZnSOD and FALS varients of human Cu,ZnSOD, which are also relatively unstable and exhibit low affinity for Cu(II).

Cloning and sequencing of the gene encoding the Cu,Zn-superoxide dismutase of Haemophilus ducreyi

The sodC gene of Haemophilus ducreyi was cloned and sequenced. The deduced amino acid sequence of this protein exhibited 71.6% identity and 81.8% similarity to the H. influenzae and H. parainfluenzae copper (Cu), zinc (Zn)-superoxide dismutase (SOD) enzymes. This gene was localized to a 2.2-kb H. ducreyi chromosomal DNA insert in plasmid pHdSOD. SOD activity was expressed in cell-free extracts of Escherichia coli containing the recombinant plasmid pHdSOD and was localized to the periplasmic space. The Cu,Zn-SOD produced by the H. ducreyi sodC gene did not complement the aerobic growth defect of an E. coli SOD-deficient mutant.

Cloning and molecular characterization of Cu,Zn superoxide dismutase from Actinobacillus pleuropneumoniae

Copper-zinc superoxide dismutases (Cu,Zn SODs), until recently considered very unusual in bacteria, are now being found in a wide range of gram-negative bacterial species. Here we report the cloning and characterization of sodC, encoding Cu,Zn SOD in Actinobacillus pleuropneumoniae, a major pathogen of pigs and the causative organism of porcine pleuropneumonia. sodC was shown to lie on a monocistronic operon, at the chromosomal locus between the genes asd (encoding aspartate semialdehyde dehydrogenase) and recF. The primary gene product was shown to have an N-terminal peptide extension functioning as a leader peptide, so that the mature Actinobacillus enzyme, like other bacterial examples, is directed to the periplasm, where it is appropriately located to dismutate exogenously generated superoxide. While the role of these secreted bacterial SODs is unknown, we speculate that in A. pleuropneumoniae the enzyme may confer survival advantage by accelerating dismutation of superoxide derived from neutrophils, a central host defense response in the course of porcine infection.

Novel dimeric interface and electrostatic recognition in bacterial Cu,Zn superoxide dismutase

Eukaryotic Cu,Zn superoxide dismutases (CuZnSODs) are antioxidant enzymes remarkable for their unusually stable beta-barrel fold and dimer assembly, diffusion-limited catalysis, and electrostatic guidance of their free radical substrate. Point mutations of CuZnSOD cause the fatal human neurodegenerative disease amyotrophic lateral sclerosis. We determined and analyzed the first crystallographic structure (to our knowledge) for CuZnSOD from a prokaryote, Photobacterium leiognathi, a luminescent symbiont of Leiognathid fish. This structure, exemplifying prokaryotic CuZnSODs, shares the active-site ligand geometry and the topology of the Greek key beta-barrel common to the eukaryotic CuZnSODs. However, the beta-barrel elements recruited to form the dimer interface, the strategy used to forge the channel for electrostatic recognition of superoxide radical, and the connectivity of the intrasubunit disulfide bond in P. leiognathi CuZnSOD are discrete and strikingly dissimilar from those highly conserved in eukaryotic CuZnSODs. This new CuZnSOD structure broadens our understanding of structural features necessary and sufficient for CuZnSOD activity, highlights a hitherto unrecognized adaptability of the Greek key beta-barrel building block in evolution, and reveals that prokaryotic and eukaryotic enzymes diverged from one primordial CuZnSOD and then converged to distinct dimeric enzymes with electrostatic substrate guidance.

Chromatographic and electrophoretic methods for analysis of superoxide dismutases

A brief overview of the family of superoxide dismutase (SOD) enzymes and their biomedical significance is presented. Methodology for the purification and electrophoretic analysis of superoxide dismutases is reviewed and discussed, with emphasis on the specific problems raised by the separation of individual superoxide dismutase isoenzymes. Purification methods and their performance, as reported in the literature are summarised in table form. Generally used methods for measuring SOD activity in vitro and SOD visualisation after electrophoresis are outlined, particularly those relevant to the monitoring of progress of SOD purification.

Occurrence of [copper, zinc]-cofactored superoxide dismutase in Pasteurella haemolytica and its serotype distribution

Fifty-two ovine strains of Pasteurella haemolytica and P. trehalosi representing serotypes 1-16 were examined for the presence of [copper, zinc]superoxide dismutase DNA sequences. This was done using a combination of polymerase chain reaction with degenerate primers based on the sequence of the [Cu,Zn]superoxide dismutase gene (sodC) in related species and Southern hybridization using a fragment of sodC from P. haemolytica A2 serotype as a probe. Both detection methods identified a fragment of the sodC gene in 9/9 strains of P. haemolytica serotype 2 examined and in 5/8 strains of serotype 7. No evidence of this gene was found in any other serotype of P. haemolytica or in any P. trehalosi serotype. Comparison of DNA sequence showed near identity between sodC from the A2 and A7 serotypes of P. haemolytica and substantial similarity (70%) to sodC previously sequenced in P. multocida, Haemophilus parainfluenzae and H. influenzae. Analysis by gel electrophoresis of the superoxide dismutase activity present in cell lysates showed that one or more superoxide dismutase is present in all serotypes. However, cyanide-inhibitable activity, corresponding to [Cu,Zn]superoxide dismutase, was detected only in those strains of serotypes A2 and A7 which showed evidence of the sodC gene fragment.

Cloning and analysis of sodC, encoding the copper-zinc superoxide dismutase of Escherichia coli

Benov and Fridovich recently reported the existence of a copper- and zinc-containing superoxide dismutase (CuZnSOD) in Escherichia coli (L. T. Benov and I. Fridovich, J. Biol. Chem. 269:25310-25314,1994). We have used the N-terminal protein sequence to isolate the gene encoding this enzyme. The gene, denoted sodC, is located at 37.1 min on the chromosome, adjacent to lhr and sodB. A monocistronic transcript of sodC accumulates only in stationary phase. The presence of a conventional leader sequence is consistent with physical data indicating that the E. coli enzyme, like other bacterial CuZnSODs, is secreted into the periplasm. Because superoxide cannot cross membranes, this localization indicates that the enzyme has evolved to defend periplasmic biomolecules against an extracytoplasmic superoxide source. Neither the source nor the target of the superoxide is known. Although once considered an exclusively eukaryotic enzyme, CuZnSOD has now been found in species that span three subdivisions of the purple bacteria. The bacterial CuZnSODs are more homologous to one another than to the eukaryotic enzymes, but active-site residues and structural motifs are clearly shared by both families of enzymes. The use of copper and an invariant disulfide bond suggest that the ancestral gene of present-day CuZnSODs evolved in an aerobic environment, long after the evolutionary split between the eukaryotes and the eubacteria. If so, a CuZnSOD gene must have been transferred laterally between members of these domains. The eukaryotic SODs most closely resemble that of Caulobacter crescentus, a relatively close descendant of the mitochondrial ancestor, suggesting that sodC may have entered the eukaryotes during the establishment of mitochondria.

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.

Identification of sodC encoding periplasmic [Cu,Zn]-superoxide dismutase in Salmonella

sodC, encoding [Cu,Zn]-cofactored superoxide dismutase, once thought to be virtually confined to eukaryotes, has now been described in many Gram-negative pathogens that have their primary niche of colonization in the upper respiratory tract. Their role in host-parasite interactive biology is unknown. We here show that members of the major human and animal enteric pathogenic species Salmonella harbour a version of sodC most closely resembling that found in Brucella abortus. The enzyme it encodes is a novel candidate determinant of virulence in Salmonella, an intracellular pathogen potentially exposed to toxic oxygen free radicals within its intracellular niche.

Synthesis and characterization of a monomeric mutant Cu/Zn superoxide dismutase with partially reconstituted enzymic activity

A monomeric analog of human Cu/Zn superoxide dismutase (F50E/G51E SOD), previously characterized and found to have reduced enzymic activity, was here further modified by replacing Glu133 with Gln. This substitution does not dramatically affect the coordination geometry at the active site, but enhances enzymic activity, and also increases the affinity for anions at the active site. This behavior parallels earlier published results in which this point mutation was made in the dimeric wild-type enzyme. The analog described here has afforded for the first time a monomeric superoxide dismutase with substantial activity. This point mutation does not significantly influence the protein structure but interactions with anions, including superoxide, are altered with respect to the monomeric form. The present monomeric Glu133Gln mutant has partially restored enzymic activity. The diminished activity of the monomeric analogs is discussed in the light of possible minor structural changes and some of their characteristics are compared with those of naturally occurring mutants associated with various neurological diseases.

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.

The role of arginine 143 in the electrostatics and mechanism of Cu,Zn superoxide dismutase: computational and experimental evaluation by mutational analysis

Cu,Zn superoxide dismutase protects cells from oxidative damage by removing superoxide radicals in one of the fastest enzyme reactions known. The redox reaction at the active-site Cu ion is rate-limited by diffusion and enhanced by electrostatic guidance. To quantitatively define the electrostatic and mechanistic contributions of sequence-invariant Arg-143 in human Cu,Zn superoxide dismutase, single-site mutants at this position were investigated experimentally and computationally. Rate constants for several Arg-143 mutants were determined at different pH and ionic strength conditions using pulse radiolytic methods and compared to results from Brownian dynamics simulations. At physiological pH, substitution of Arg-143 by Lys caused a 2-fold drop in rate, neutral substitutions (Ile, Ala) reduced the rate about 10-fold, while charge-reversing substitutions (Asp, Glu) caused a 100-fold decrease. Position 143 mutants showed pH dependencies not seen in other mutants. At low pH, the acidic residue mutations exhibited protonation/deprotonation effects. At high pH, all enzymes showed typical decreases in rate except the Lys mutant in which the rate dropped off at an unusually low pH. Increasing ionic strength at acidic pH decreased the rates of the wild-type enzyme and Lys mutant, while the rate of the Glu mutant was unaffected. Increasing ionic strength at higher pH (> 10) increased the rates of the Lys and Glu mutants while the rate of the wild-type enzyme was unaffected. Reaction simulations with Brownian dynamics incorporating electrostatic effects tested computational predictability of ionic strength dependencies of the wild-type enzyme and the Lys, Ile, and Glu mutants. The calculated and experimental ionic strength profiles gave similar slopes in all but the Glu mutant, indicating that the electrostatic attraction of the substrate is accurately modeled. Differences between the calculated and experimental rates for the Glu and Lys mutants reflect the mechanistic contribution of Arg-143. Results from this joint analysis establish that, aside from the Cu ligands, Arg-143 is the single most important residue in Cu,Zn superoxide dismutase both electrostatically and mechanistically, and provide an explanation for the evolutionary selection of arginine at position 143.

Extracellular and cytoplasmic CuZn superoxide dismutases from Brugia lymphatic filarial nematode parasites

We have isolated full-length cDNAs encoding two distinct types of CuZn superoxide dismutases (SODs) from the filarial nematode parasite Brugia pahangi. The derived amino acid sequences suggested that one class of cDNAs represented a cytoplasmic form of SOD and the second class represented an extracellular (EC) variant. The predicted proteins were highly homologous to each other, but the sequence of the latter contained an additional 43 residues at the N terminus, the first 16 of which were markedly hydrophobic, and four potential sites for N-linked glycosylation. Western blotting (immunoblotting) with an antiserum to a partial SOD expressed in Escherichia coli revealed two proteins with estimated molecular masses of 19 and 29 kDa. Digestion with N-glycanase indicated that the latter protein corresponded to the EC form, as it possessed N-linked oligosaccharide chains at three sites, leaving a peptide backbone with an estimated molecular mass of 22 kDa, which was consistent with the additional 27 amino acids predicted from the cDNA sequence. Gel filtration indicated that both enzymes were dimeric in their native forms, in contrast to the human EC-SOD, which is tetrameric. Comparison of the primary structure of the parasite EC-SOD with that of the human EC enzyme revealed two major differences: the N-terminal extension of the parasite enzyme was shorter by 25 residues, and it also lacked the C-terminal charged extension which mediates binding to cell surface sulfated proteoglycans. Lavage of Mongolian jirds infected intraperitoneally with Brugia malayi resulted in the recovery of filarial CuZn SODs, principally the EC form, indicating that this form of SOD is secreted in vivo. This EC enzyme may contribute to parasite persistence by neutralizing superoxide generated by activated leukocytes, thus acting as both an antioxidant and an anti-inflammatory factor.

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.

Construction of Cu-Zn superoxide dismutase deletion mutants of Brucella abortus: analysis of survival in vitro in epithelial and phagocytic cells and in vivo in mice

Cu-Zn superoxide dismutase (SOD) deletion mutants of Brucella abortus S2308, a virulent strain, and S19, a vaccine strain, were generated by gene replacement. A deletion plasmid, pBA delta sodknr, was constructed by excising the Cu-Zn SOD gene (Cu-Zn sod) from a 2.3-kb B. abortus DNA fragment of plasmid pBA20-1527 and inserting a 1.4-kb DNA fragment encoding kanamycin resistance into the Cu-Zn sod excision site. The deletion plasmid was introduced into B. abortus by electroporation, and Southern blot analysis confirmed that the antibiotic resistance fragment had replaced Cu-Zn sod in kanamycin-resistant colonies. The survival and growth of Cu-Zn SOD mutant strains were compared with that of the parental strains in HeLa cells and in the mouse macrophagelike cell line J774. The survival and growth of the Cu-Zn SOD mutant strains were similar to those of their respective parental strains in HeLa and J774 cell lines. The kinetics of infection with these strains were examined in BALB/c mice. The splenic levels of the S19 Cu-Zn SOD mutant recovered from intraperitoneally infected BALB/c mice were approximately 10-fold lower than those of the parental strain through 26 days postinfection. Thereafter, infection sharply declined in both groups, and by 105 days postinfection, no organisms were detected. The splenic levels of the S2308 Cu-Zn SOD mutant were lower than those of wild-type S2308-infected mice. The spleen weights of mice infected with the S2308 Cu-Zn SOD mutant were consistently lower than those of wild-type S2308-infected mice. These results suggest that the antioxidant enzyme Cu-Zn SOD plays a role in the survival and pathogenicity of B. abortus in vivo.

Faster superoxide dismutase mutants designed by enhancing electrostatic guidance

The enzyme Cu, Zn superoxide dismutase (SOD) protects against oxidative damage by dismuting the superoxide radical O2-. to molecular oxygen and hydrogen peroxide at the active-site Cu ion in a reaction that is rate-limited by diffusion and enhanced by electrostatic guidance. SOD has evolved to be one of the fastest enzymes known (V(max) approximately 2 x 10(9) M-1 s-1). The new crystal structures of human SOD show that amino-acid site chains that are implicated in electrostatic guidance (Glu 132, Glu 133 and Lys 136) form a hydrogen-bonding network. Here we show that site-specific mutants that increase local positive charge while maintaining this orienting network (Glu----Gln) have faster reaction rates and increased ionic-strength dependence, matching brownian dynamics simulations incorporating electrostatic terms. Increased positive charge alone is insufficient: one charge reversal (Glu----Lys) mutant is slower than the equivalent charge neutralization (Glu----Gln) mutant, showing that the newly introduced positive charge disrupts the orienting network. Thus, electrostatically facilitated diffusion rates can be increased by design, provided the detailed structural integrity of the active-site electrostatic network is maintained.

Copper-zinc superoxide dismutase of Haemophilus influenzae and H. parainfluenzae

Copper-zinc superoxide dismutase ([Cu,Zn]-SOD) is widely found in eukaryotes but has only rarely been identified in bacteria. Here we describe sodC, encoding [Cu,Zn]-SOD in Haemophilus influenzae and H. parainfluenzae, frequent colonists and pathogens of the human respiratory tract. In capsulate H. influenzae, sodC was found in only one division of the bacterial population, and although the protein it encoded was clearly [Cu,Zn]-SOD from its deduced sequence, it lacked enzymatic activity. In H. parainfluenzae, in contrast, active enzyme was synthesized which appeared to be secreted beyond the cytoplasm when the gene was expressed in Escherichia coli minicells. The origin of gene transcription differed between the Haemophilus species, but protein synthesis from cloned genes in vitro was comparable. A C-T transition was found in the H. influenzae sequence compared with the H. parainfluenzae sequence, leading to a histidine, known to be crucial in eukaryotic [Cu,Zn]-SOD for copper ion coordination and so for enzymatic activity, to be changed to tyrosine. This is speculated to be the cause of inactivity of the H. influenzae enzyme. Secreted SODs have only been described in a few bacterial species, and this is the first identification of [Cu,Zn]-SOD in a common human upper respiratory tract colonist. The role of secreted bacterial SODs is unknown, and we speculate that in Haemophilus species the enzyme may confer survival advantage by accelerating dismutation of superoxide of environmental origin to hydrogen peroxide, disruptive to the normal mucociliary clearance process in the host.

Superoxide dismutases of virulent and avirulent strains of Brucella abortus

Extracts of Brucella abortus strains 2308,RB51,45/20 and ST 19 had no significant differences in superoxide dismutase (SOD) activity as measured by the epinephrine assay. These B. abortus strains represent smooth, intermediate and rough colony forms. SOD activity was inhibited 60 to 75% by 2 mM KCN and suggests the presence of Cu/Zn SOD. The SOD activities were similar when the strains were grown in trypticase soy broth containing either 0.5% glucose or erythritol. There were two distinct SOD activity bands in native polyacrylamide gel electrophoresis with identical mobilities for each of the strains. When the native gel was stained for SOD activities in the presence of 2 mM KCN, the SOD band that co-migrated with the bovine erythrocyte Cu/Zn SOD activity disappeared. The band of SOD activity that migrated similar to E. coli iron SOD activity was unaffected by KCN. There were no significant differences in either the total SOD or Cu/Zn SOD activities among the strains. As the Brucella strains represent ranges of virulence, it is difficult to associate any primary role for SOD as a virulence factor.

Evolutionary aspects of superoxide dismutase: the copper/zinc enzyme

Copper/zinc superoxide dismutase is typically an enzyme of eukaryotes. The presence of the enzyme in the ponyfish symbiont Photobacterium leiognathi and some free living bacteria does not have an immediate explanation. Amino acid sequence alignment of 19 Cu/Zn superoxide dismutases shows 21 invariant residues in key positions related to maintenance of the beta-barrel fold, the active site structure including the electrostatic channel loop, and dimer contacts. Nineteen other residues are invariant in 18 of the 19 sequences. Thirteen of these nearly invariant residues show substitutions in Photobacterium Cu/Zn superoxide dismutase. Copper/zinc superoxide dismutase from the trematode Schistosoma mansoni shows an N-terminal sub-domain with a hydrophobic leader peptide, as in human extracellular superoxide dismutase which is a Cu/Zn enzyme. The latter also has a C-terminal sub-domain with preponderance of hydrophilic and positively charged residues. The amino acid sequence of this superoxide dismutase between the N-terminal and C-terminal regions shares many features of cytosolic Cu/Zn superoxide dismutase, including 20 of the 21 invariant residues found in 19 Cu/Zn enzymes, suggesting a similar type of beta-barrel fold and active site structure for the extracellular enzyme.

Probing the structural basis for enzyme-substrate recognition in Cu,Zn superoxide dismutase

A full understanding of enzyme-substrate interactions requires a detailed knowledge of their structural basis at atomic resolution. Crystallographic and biochemical data have been analyzed with coupled computational and computer graphic approaches to characterize the molecular basis for recognition of the superoxide anion substrate by Cu,Zn superoxide dismutase (SOD). Detailed analysis of the bovine SOD structure aligned with SOD sequences from 15 species provides new results concerning the significance and molecular basis for sequence conservation. Specific roles have been assigned for all 23 invariant residues and additional residues exhibiting functional equivalence. Sequence invariance is dominated by 15 residues that form the active site stereochemistry, supporting a primary biological function of superoxide dismutation. Using data from crystallographic structures and site-directed mutants, we are testing the role of individual residues in the active site channel, including (in human SOD) Glu 132, Glu 133, Lys 136, Thr 137, and Arg 143. Electrostatic calculations incorporating molecular flexibility suggest that the region of positive electrostatic potential in and over the active site channel above the Cu ion sweeps through space during molecular motion to enhance the facilitated diffusion responsible for the enzyme's rapid catalytic rate.

Potential effects of UV-B on the chemical environment of marine organisms: a review

An increase in ultraviolet-B (UV-B) due to depletion of stratospheric ozone may affect growth of marine phytoplankton by altering the chemistry of their environment. Production of bioactive free radicals, photodecomposition of organic matter, and availability of trace metals are likely to be altered by increased UV-B flux. Such changes to the chemical environment may be both deleterious and beneficial to marine phytoplankton. Extracellular free radicals such as OH, Br(2)(-), and CO(3)(-) are predicted to have a negligible impact, but superoxide and its decomposition product hydrogen peroxide may react rapidly with cell surfaces and destroy membrane function and integrity. Increased UV-B will enhance the bioavailability of the redox active trace metals Fe and Cu. Thus, in the Fe-limited high latitude ocean, increased Fe availability may promote phytoplankton production, while in other parts of the ocean increased Cu availability may be toxic. Overall, the interdependent direct and indirect effects of UV-B on phytoplankton may compensate for each other and account for the ability of marine ecosystems to be subjected to widely variable UV-B flux without apparent damage.

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.

Crystallographic characterization of a Cu,Zn superoxide dismutase from Photobacterium leiognathi

Crystals of a copper-zinc superoxide dismutase from Photobacterium leiognathi, a luminescent marine bacterium that is the species-specific symbiont of the ponyfish, have been obtained from 2-methyl-2,4-pentanediol solutions. The space group was determined using screenless small-angle precession photographs, and was confirmed by analyzing area detector diffraction data with the XENGEN programs for indexing and refinement. The crystals are monoclinic, space group C2 (a = 126.4 A, b = 87.0 A, c = 44.4 A, beta = 92.8 A), and have two 32,000 Mr dimers per asymmetric unit. The crystals diffract to at least 2.7 A resolution, are resistant to radiation damage, and are suitable for determination of the structure by X-ray diffraction.

Phylogenetic distribution of superoxide dismutase supports an endosymbiotic origin for chloroplasts and mitochondria

Three isozymes of superoxide dismutase (SOD) have been identified and characterized. The iron and manganese isozymes (Fe-SOD and Mn-SOD, respectively) show extensive primary sequence and structural homology, suggesting a common evolutionary ancestor. In contrast, the copper/zinc isozyme (CuZn-SOD) shows no homology with Fe-SOD or Mn-SOD, suggesting an independent origin for this enzyme. The three isozymes are unequally distributed throughout the biological kingdoms and are located in different subcellular compartments. Obligate anaerobes and aerobic diazotrophs contain Fe-SOD exclusively. Facultative aerobes contain either Fe-SOD or Mn-SOD or both. Fe-SOD is found in the cytosol of cyanobacteria while the thylakoid membranes of these organisms contain a tightly bound Mn-SOD. Similarly, most eukaryotic algae contain Fe-SOD in the chloroplast stroma and Mn-SOD bound to the thylakoids. Most higher plants contain a cytosol-specific and a chloroplast-specific CuZn-SOD, and possibly a thylakoid-bound Mn-SOD as well. Plants also contain Mn-SOD in their mitochondria. Likewise, animals and fungi contain a cytosolic CuZn-SOD and a mitochondrial Mn-SOD. The Mn-SOD found in the mitochondria of eukaryotes shows strong homology to the prokaryotic form of the enzyme. Taken together, the phylogenetic distribution and subcellular localization of the SOD isozymes provide strong support for the hypothesis that the chloroplasts and mitochondria of eukaryotic cells arose from prokaryotic endosymbionts.

Evolution of CuZn superoxide dismutase and the Greek key beta-barrel structural motif

Detailed analysis of the CuZn superoxide dismutase (SOD) structure provides new results concerning the significance and molecular basis for sequence conservation, intron-exon boundary locations, gene duplication, and Greek key beta-barrel evolution. Using 15 aligned sequences, including a new mouse sequence, specific roles have been assigned to all 23 invariant residues and additional residues exhibiting functional equivalence. Sequence invariance is dominated by 15 residues that form the active site stereochemistry, supporting a primary biological function of superoxide dismutation. The beta-strands have no sequence insertions and deletions, whereas insertions occur within the loops connecting the beta-strands and at both termini. Thus, the beta-barrel with only four invariant residues is apparently over-determined, but dependent on multiple cooperative side chain interactions. The regions encoded by exon I, a proposed nucleation site for protein folding, and exon III, the Zn loop involved in stability and catalysis, are the major structural subdomains not included in the internal twofold axis of symmetry passing near the catalytic Cu ion. This provides strong confirmatory evidence for gene evolution by duplication and fusion followed by the addition of these two exons. The proposed evolutionary pathway explains the structural versatility of the Greek key beta-barrel through functional specialization and subdomain insertions in new loop connections, and provides a rationale for the size of the present day enzyme.

Molecular genetics of superoxide dismutases

Molecular genetics of SOD has been recently developed primarily due to the new biotechnologies. Different types of isoenzymes have now been cloned and sequenced from several species ranging from bacteria to human and plants. Knowledge of the nucleotide sequences permitted refinement of structural models and provided information on subcellular locations. Cloned genes allowed the production of large amounts of SOD. They have been used for physiological and regulation studies, structural and enzymatic analyses, and are vital tools for the isolation of mutants. Isolation of mutants is generally essential to the understanding of the biological function of the gene in question. Indeed, SOD deficient mutants have now been isolated in bacteria and yeast. Their properties support, at numerous levels, a major role of SOD in cellular defense against oxygen toxicity. Few data are presently available on the molecular basis of mechanisms that regulate the expression of SOD.