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

Global adjustment of microbial physiology during free radical stress

Oxidation can damage all biological macromolecules, and the survival of a cell therefore depends on its ability to control the level of oxidants. Microbes possess an astonishing variety of antioxidant defences, ranging from small, oxidant-scavenging molecules to self-regulating, homeostatic gene networks. Most often these antioxidant defences are activated by exposure to specific classes of oxidants. Interestingly, the isolation of pleiotropic mutations that impair or exacerbate the expression of subsets of oxidant-responsive genes led to the identification of global regulators. In a few, well-characterized cases, these regulators can transduce oxidative damage into gene regulation. Recently, the application of genomic tools to study the antioxidant responses of E. coli has both confirmed previous observations and provided evidence for a wealth of putative new anti-oxidant functions. Here, we review the remarkable diversity of antioxidant defence mechanisms, with emphasis on signal transduction by global regulator proteins and the corresponding genetic networks that protect the microbial cell against oxidative stress.

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.

A potential role for periplasmic superoxide dismutase in blocking the penetration of external superoxide into the cytosol of Gram-negative bacteria

Superoxide is a key component of the antibacterial weaponry of phagocytes. Presumably, for this reason, strains of Salmonella typhimurium express a periplasmic superoxide dismutase (SOD) that is essential for full virulence. Because most anions cannot easily penetrate lipid membranes, it is thought that the phagosomal superoxide either damages an unknown target on the bacterial surface or reacts with nitric oxide to form peroxynitrite (HOONO), a toxic oxidant that can freely enter bacteria. However, in this study, we tested whether superoxide itself could penetrate membranes. Superoxide that was generated at high pH (>7.5) very slowly reduced cytochrome c that was encapsulated inside lipid vesicles. It did so much more quickly at lower pH (<7). Under the latter conditions, more superoxide was protonated and uncharged (HO2*), and the penetrance of superoxide was proportional to the concentration of this species. The permeability coefficient of HO2* was determined to be 9 x 10(-4) cm sec(-1), just slightly lower than that of water and far higher than the value of the anionic form (O2-, <10(-7) cm sec(-1). When Escherichia coli mutants that lack periplasmic SOD were exposed to super-oxide at pH 6.5, cytosolic fumarase B was damaged. Damage was minimal at higher pH or in strains that contained periplasmic SOD. Thus, in the acidic phagolysosome, superoxide may be able to penetrate and attack cytosolic targets of captive bacteria. This process may contribute to the potency of the oxidative burst. One role of periplasmic SOD may be to avert this damage. In contrast, periplasmic SOD was ineffective at lowering the extracellular super-oxide concentration and, therefore, may have little impact upon HOONO formation.

Cloning and sequencing the genomic encoding region of copper-zinc superoxide dismutase enzyme from several marine strains of the genus Debaryomyces (Lodder & Kreger-van Rij)

Copper-zinc superoxide dismutase (SODC) is a cytosolic enzyme which catalyses the dismutation of the superoxide radical. Due to its physiological importance, the encoding genes have been cloned from several species of higher eukaryotes. However, genes from moulds and yeast have not been studied extensively. In this paper, the encoding region of this gene (sod1) has been cloned from several strains of marine yeast belonging to the genus Debaryomyces (dvv sod1, dvy sod1 and dh sod1-61) through genomic DNA-PCR amplification. Fragments of 480-486 nucleotides were obtained, which contain information for products of 153-156 amino acids with calculated molecular masses of 15.8-16.6 kDa. The deduced amino acid sequence shows that D. vanrijiae enzymes present three additional amino acids not closely related to the active site conformation. In addition, in D. vanrijiae var. vanrijiae (strain 020), one histidine residue is apparently replaced by a proline; the incidence and function of other aromatic or heterocyclic amino acids is discussed. Homology and phylogenetic trees were constructed from amino-acid sequence multi-alignment analyses; the interrelationships among fungi are discussed. The sod-1 sequences reported in this paper were deposited in the public data library of the NCBI under Accession Nos AF301019, AF327449 and AF327448.

Characterization of the single superoxide dismutase of Staphylococcus xylosus

Staphylococcus xylosus is a facultative anaerobic bacterium used as a starter culture for fermented meat products. In an attempt to analyze the antioxidant capacities of this organism, the superoxide dismutase (SOD) was characterized. S. xylosus contains a single cytoplasmic SOD, which was not inhibited by H2O2. The SOD activity in crude extracts was completely lost upon metal depletion, but it could be recovered by manganese and very weakly by iron. It is therefore suggested that the S. xylosus SOD is a manganese-preferring enzyme. The corresponding gene, sod, was isolated from a genomic library of S. xylosus DNA and complemented the growth defect of an Escherichia coli SOD-deficient mutant. As deduced from the nucleotide sequence, sod encodes a protein of 199 amino acids with a molecular mass of 22.5 kDa. Two transcriptional start sites 25 and 120 bp upstream of the sod start codon were identified. A terminator-like structure downstream of the gene suggested a monocistronic sod mRNA. Regulation of sod expression was studied using fusions of the sod promoters to a genomic promoterless beta-galactosidase gene. The sod expression was not affected by manganese and increased slightly with paraquat. It was induced during stationary phase in a complex medium but not in a chemically defined medium. To investigate the physiological role of SOD, a mutant devoid of SOD activity was constructed. Growth experiments showed that sod is not essential for aerobic growth in complex medium. However, in chemically defined medium without leucine, isoleucine, and valine, the sod mutant hardly grew, in contrast to the wild-type strain. In addition, the mutant was sensitive to hyperbaric oxygen and to paraquat. Therefore, sod plays an important role in the protection of S. xylosus from oxidative stress.

Accumulation of manganese in Neisseria gonorrhoeae correlates with resistance to oxidative killing by superoxide anion and is independent of superoxide dismutase activity

As a facultative aerobe with a high iron requirement and a highly active aerobic respiratory chain, Neisseria gonorrhoeae requires defence systems to respond to toxic oxygen species such as superoxide. It has been shown that supplementation of media with 100 microM Mn(II) considerably enhanced the resistance of this bacterium to oxidative killing by superoxide. This protection was not associated with the superoxide dismutase enzymes of N. gonorrhoeae. In contrast to previous studies, which suggested that some strains of N. gonorrhoeae might not contain a superoxide dismutase, we identified a sodB gene by genome analysis and confirmed its presence in all strains examined by Southern blotting, but found no evidence for sodA or sodC. A sodB mutant showed very similar susceptibility to superoxide killing to that of wild-type cells, indicating that the Fe-dependent SOD B did not have a major role in resistance to oxidative killing under the conditions tested. The absence of a sodA gene indicated that the Mn-dependent protection against oxidative killing was independent of Mn-dependent SOD A. As a sodB mutant also showed Mn-dependent resistance to oxidative killing, then it is concluded that this resistance is independent of superoxide dismutase enzymes. Resistance to oxidative killing was correlated with accumulation of Mn(II) by the bacterium. We hypothesize that this bacterium uses Mn(II) as a chemical quenching agent in a similar way to the already established process in Lactobacillus plantarum. A search for putative Mn(II) uptake systems identified an ABC cassette-type system (MntABC) with a periplasmic-binding protein (MntC). An mntC mutant was shown to have lowered accumulation of Mn(II) and was also highly susceptible to oxidative killing, even in the presence of added Mn(II). Taken together, these data show that N. gonorrhoeae possesses a Mn(II) uptake system that is critical for resistance to oxidative stress.

A simple technique for the simultaneous determination of molecular weight and activity of superoxide dismutase using SDS-PAGE

A direct and rapid SDS-PAGE staining method for in situ identification of activity and molecular weight of superoxide dismutase following denaturing treatment has been developed. This technique was based on the removal of SDS after SDS-PAGE and two-step staining procedures of the SDS-polyacrylamide gel to present the achromatic activity-zones of the enzymes. We demonstrated that the detection sensitivity of SDS-PAGE staining method was the same as the traditional xanthine oxidase-NBT solution assay. Through the SDS-PAGE staining method, three classes of superoxide dismutases with distinct molecular sizes were identified in situ. Moreover, activity of copper and zinc containing superoxide dismutase in crude extracts of Escherichia coli and Actinobacillus pleuropneumoniae was significantly enhanced using the two-step staining procedure.

Variable assortment of prophages provides a transferable repertoire of pathogenic determinants in Salmonella

Gene transfer between separate lineages of a bacterial pathogen can promote recombinational divergence and the emergence of new pathogenic variants. Temperate bacteriophages, by virtue of their ability to carry foreign DNA, are potential key players in this process. Our previous work has shown that representative strains of Salmonella typhimurium (LT2, ATCC14028 and SL1344) are lysogenic for two temperate bacteriophages: Gifsy-1 and Gifsy-2. Several lines of evidence suggested that both elements carry genes that contribute to Salmonella virulence. One such gene, on the Gifsy-2 prophage, codes for the [Cu, Zn] superoxide dismutase SodCI. Other putative pathogenicity determinants were uncovered more recently. These include genes for known or presumptive type III-translocated proteins and a locus, duplicated on both prophages, showing sequence similarity to a gene involved in Salmonella enteropathogenesis (pipA). In addition to Gifsy-1 and Gifsy-2, each of the above strains was found to harbour a specific set of prophages also carrying putative pathogenicity determinants. A phage released from strain LT2 and identified as phage Fels-1 carries the nanH gene and a novel sodC gene, which was named sodCIII. Strain ATCC14028 releases a lambdoid phage, named Gifsy-3, which contains the phoP/phoQ-activated pagJ gene and the gene for the secreted leucine-rich repeat protein SspH1. Finally, a phage specifically released from strain SL1344 was identified as SopEPhi. Most phage-associated loci transferred efficiently between Salmonella strains of the same or different serovars. Overall, these results suggest that lysogenic conversion is a major mechanism driving the evolution of Salmonella bacteria.

Purification and characterization of iron superoxide dismutase and copper-zinc superoxide dismutase from Acanthamoeba castellanii

Two superoxide dismutases (SOD I and SOD II) were purified from Acanthamoeba castellanii and characterized for several biochemical properties. Analysis of the primary structure and inhibition studies revealed that SOD I is iron SOD (Fe-SOD), with a molecular mass of 50 kDa, and SOD II is copper-zinc SOD (Cu,Zn-SOD), with a molecular mass of 38 kDa. Both enzymes have a homodimeric structure consisting of 2 identical subunits, each with a molecular mass of 26 and 19 kDa for SOD I and SOD II, respectively. The isoelectric points of SOD I and SOD II were 6.4 and 3.5, respectively, and there were no isoenzyme forms detected. Both enzymes show a broad optimal pH of 7.0-11.0. Because no differences were observed in the apparent molecular weight of SOD I after addition of the reducing agent 2-mercaptoethanol, the subunits do not appear to be linked covalently by disulfide bonds. However, the subunits of SOD II were covalently linked by intra- and interdisulfide bonds. Western blot analyses showed that the 2 enzymes have different antigenicity. Both enzymes occur as cytoplasmic and detergent-extractable fractions. These enzymes may be potential virulence factors of A. castellanii by acting both as antioxidants and antiinflammatory agents. These enzymes may be attractive targets for chemotherapy and immunodiagnosis of acanthamoebiasis.

Expression and role of superoxide dismutases (SOD) in pathogenic bacteria

This review will be limited to the expression and roles of the family of metalloenzymes superoxide dismutases in pathogenic bacteria. Only animal pathogens will be described, with particular emphasis on those causing disease in man.

Regulation of Brucella abortus catalase

All aerobic organisms have mechanisms that protect against oxidative compounds. Catalase, peroxidase, superoxide dismutase, glutathione, and thioredoxin are widely distributed in many taxa and constitute elements of a nearly ubiquitous antioxidant metabolic strategy. Interestingly, the regulatory mechanisms that control these elements are rather different depending on the nature of the oxidative stress and the organism. Catalase is well documented to play an important role in protecting cells from oxidative stress. In particular, pathogenic bacteria seem to use this enzyme as a defensive tool against attack by the host. To investigate the significance of catalase in hostile environments, we made catalase deletion mutations in two different B. abortus strains and used two-dimensional gel analysis, survival tests, and adaptation experiments to explore the behavior and role of catalase under several oxidative stress conditions. These studies show that B. abortus strains that do not express catalase activity exhibit increased sensitivity to hydrogen peroxide. We also demonstrate that catalase expression is regulated in this species, and that preexposure to a sublethal concentration of hydrogen peroxide allows B. abortus to adapt so as to survive subsequent exposure to higher concentrations of hydrogen peroxide.

Identification of the copper-zinc superoxide dismutase activity in Mycoplasma hyopneumoniae

Copper-zinc superoxide dismutase (Cu/ZnSOD), a key enzyme in defense against toxic oxygen-free radicals, is widespread in eukaryotes and several species of gram-negative bacteria. The presence of this enzyme in Mycoplasma hyopneumoniae (M. hyopneumoniae), the primary pathogen of mycoplasmal pneumonia in pigs, was examined since the polyclonal antibody against bovine Cu/ZnSOD was dominantly cross-reactive with the M. hyopneumoniae Cu/ZnSOD from whole cellular proteins. In situ activity staining on SDS-PAGE showed that the molecular mass of M. hyopneumoniae Cu/ZnSOD in reducing form was approximately 17kDa. The presence of Cu and Zn ions at the active site of the enzyme was confirmed on the basis of inhibition by KCN and by H(2)O(2). The activity of M. hyopneumoniae Cu/ZnSOD on both SDS- and native-polyacrylamide gels was completely inhibited by 2mM KCN and the gels showed no iron-containing SOD (FeSOD) or manganese-containing SOD (MnSOD) in the crude extracts. The activity of M. hyopneumoniae Cu/ZnSOD in crude extract was 70units/mg protein and was 55% inhibited by 5mM KCN and 56% inactivated by 40mM H(2)O(2). This enzyme was growth-stage dependent and evidenced markedly higher production during the early log phase. Different expression levels of Cu/ZnSOD activity in field isolates were also detected. Taken together, the presence of Cu/ZnSOD in M. hyopneumoniae was identified for the first time.

Oxidative mutagenesis in Escherichia coli strains lacking ROS-scavenging enzymes and/or 8-oxoguanine defenses

Escherichia coli strains with different combinations of null mutations in the katG, katE, sodA, sodB, fpg, and mutY genes were constructed to compare their spontaneous mutation frequencies and sensitivities to various oxidants with those of bacteria solely deficient in catalase (katG katE) or cytosolic superoxide dismutase (sodA sodB) and the parental strain possessing a full complement of these enzymes. The MutY DNA glycosylase represented the major protection against the mutagenic consequences of processes associated with normal aerobic metabolism. Spontaneous mutagenesis in MutY-lacking bacteria was not influenced by the absence of (A)BC excinuclease or the presence of MucAB proteins, a result consistent with 8-oxoguanine being a principal premutational lesion. In contrast, catalase and SOD represented the major protection against the genotoxic consequences of bursts of oxidative stress caused by reactive-oxygen-generating compounds. Therefore, only bacteria simultaneously defective in both katG and katE or sodA and sodB genes were hypersensitive with respect to mutability by peroxide and superoxide, respectively. These data suggest that oxidative lesions other than 8-oxoguanine contribute to mutagenesis by hydrogen peroxide and redox-cycling chemicals.

Increased expression of periplasmic Cu,Zn superoxide dismutase enhances survival of Escherichia coli invasive strains within nonphagocytic cells

We have studied the influence of periplasmic Cu,Zn superoxide dismutase on the intracellular survival of Escherichia coli strains able to invade epithelial cells by the expression of the inv gene from Yersinia pseudotuberculosis but unable to multiply intracellularly. Intracellular viability assays, confirmed by electron microscopy observations, showed that invasive strains of E. coli engineered to increase Cu,Zn superoxide dismutase production are much more resistant to intracellular killing than strains containing only the chromosomal sodC copy. However, we have found only a slight difference in survival within HeLa cells between a sodC-null mutant and its isogenic wild-type strain. Such a small difference in survival correlates with the very low expression of this enzyme in the wild-type strain. We have also observed that acid- and oxidative stress-sensitive E. coli HB101(pRI203) is more rapidly killed in epithelial cells than E. coli GC4468(pRI203). The high mortality of E. coli HB101(pRI203), independent of the acidification of the endosome, is abolished by the overexpression of sodC. Our data suggest that oxyradicals are involved in the mechanisms of bacterial killing within epithelial cells and that high-level production of periplasmic Cu,Zn superoxide dismutase provides bacteria with an effective protection against oxidative damage. We propose that Cu,Zn superoxide dismutase could offer an important selective advantage in survival within host cells to bacteria expressing high levels of this enzyme.

Transcriptional regulation by iron of genes encoding iron- and manganese-superoxide dismutases from Pseudomonas putida

Genes from Pseudomonas putida (Pp), sodA, encoding manganese-superoxide dismutase (MnSOD) and, sodB, iron-superoxide dismutase (FeSOD) were cloned by hybridization with digoxigenin (dig)-labeled PCR products generated from Pp genomic DNA. The sodB gene had a 594 bp open reading frame (ORF), corresponding to 198 amino acids (aa), and a transcript of 880 bases. The sodA gene contained a 609 bp ORF encoding 203 aa and was transcribed as part of a polycistronic operon, consisting of orfY-fumC-orfX-sodA. Pp sodA or sodB genes both restored aerobic growth, growth on paraquat, and growth on minimal medium to an Escherichia coli (Ec) mutant deficient in SOD activity. Paraquat treatment did not enhance mRNA transcription of the sod genes or increase SOD activity in Pp. The Pp sodB gene was highly expressed throughout logarithmic-(log) growth phase and stationary-phase cells grown in medium supplemented with FeCl3, but was down-regulated in iron-deficient conditions, such as in stationary-phase or generated by 2,2'-dipyridyl (DP) treatment. This is the first evidence that iron regulates expression of the sodB gene at the transcriptional level. In contrast, iron-deficient conditions, or addition of MnCl2 to the growth medium, induced transcripts (2.4 kb and 1.2 kb) from the sodA operon. Our results reveal an intricate role of iron in the transcriptional regulation of both Pp sodA and sodB genes.

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.

Characterization of the major superoxide dismutase of Staphylococcus aureus and its role in starvation survival, stress resistance, and pathogenicity

A Staphylococcus aureus mutant (SPW1) which is unable to survive long-term starvation was shown to have a transposon insertion within a gene homologous to the sodA family of manganese-dependent superoxide dismutases (SOD). Whole-cell lysates of the parental 8325-4 strain demonstrated three zones of SOD activity by nondenaturing gel electrophoresis. The activities of two of these zones were dependent on manganese for activity and were absent in SPW1. The levels of SOD activity and sodA expression were growth-phase dependent, occurring most during postexponential phase. This response was also dependent on the level of aeration of the culture, with highest activity and expression occurring only under high aeration. Expression of sodA and, consequently, SOD activity could be induced by methyl viologen but only during the transition from exponential- to postexponential-phase growth. SPW1 was less able to survive amino acid limitation and acid stress but showed no alteration in pathogenicity in a mouse abscess model of infection compared to the parental strain 8325-4.

The regulation and role of the periplasmic copper, zinc superoxide dismutase of Escherichia coli

The discovery of superoxide dismutase (CuZnSOD) within the periplasms of several Gram-negative pathogens suggested that this enzyme evolved to protect cells from exogenous sources of superoxide, such as the oxidative burst of phagocytes. However, its presence in some non-pathogenic bacteria implies that there may be a role for this SOD during normal growth conditions. We found that sodC, the gene that encodes the periplasmic SOD of Escherichia coli, is repressed anaerobically by Fnr and is among the many antioxidant genes that are induced in stationary phase by RpoS. Surprisingly, the entry of wild-type E. coli into stationary phase is accompanied by a several-hour-long period of acute sensitivity to hydrogen peroxide. Induction of the RpoS regulon helps to diminish that sensitivity. While mutants of E. coli and Salmonella typhimurium that lacked CuZnSOD were not detectably sensitive to exogenous superoxide, both were killed more rapidly than their parent strains by exogenous hydrogen peroxide in early stationary phase. This sensitivity required prior growth in air. Evidently, periplasmic superoxide is generated during stationary phase by endogenous metabolism and, if it is not scavenged by CuZnSOD, it causes an unknown lesion that augments or accelerates the damage done by peroxide. The molecular details await elucidation.

In vivo formation of Cu,Zn superoxide dismutase disulfide bond in Escherichia coli

We have found that the in vivo folding of periplasmic Escherichia coli Cu,Zn superoxide dismutase is assisted by DsbA, which catalyzes the efficient formation of its single disulfide bond, whose integrity is essential to ensure full catalytic activity to the enzyme. In line with these findings, we also report that the production of recombinant Xenopus laevis Cu,Zn superoxide dismutase is enhanced when the enzyme is exported in the periplasmic space or is expressed in thioredoxin reductase mutant strains. Our data show that inefficient disulfide bond oxidation in the bacterial cytoplasm inhibits Cu,Zn superoxide dismutase folding in this cellular compartment.

Isolation and characterization of Staphylococcus aureus starvation-induced, stationary-phase mutants defective in survival or recovery

Ten Staphylococcus aureus mutants, defective in the starvation-induced stationary phase of growth were isolated from two independent Tn917-LTV1 transposon insertion libraries and were designated suv as they had apparent survival defects. Seven of these mutants were defective under amino-acid-limiting conditions alone. Two mutants (suv-3 and suv-20) demonstrated lower plating efficiency when starved for glucose, phosphate or amino acids and one mutant (suv-11) had reduced plating efficiency after amino acid or glucose starvation. All of the mutants tested were as resistant to hydrogen peroxide assault as the parent, but six were more sensitive to low pH conditions. All the mutants were physically mapped on the S. aureus chromosome using PFGE. Chromosomal DNA flanking the Tn917-LTV1 insertion sites was rescued by cloning into Escherichia coli. DNA sequence analysis resulted in the identification of a number of transposon-disrupted ORFs encoding putative components such as superoxide dismutase (suv-1), haem A synthase (suv-3), a component of the SOS response (suv-9) and hypoxanthine-guanine phosphoribosyltransferase (suv-20). The Tn917-LTV1 insertion created lacZ transcriptional fusions for some of the stationary-phase loci. Expression analysis indicated that suv-4 was induced at mid-exponential phase, whereas suv-3 and suv-11 were induced at the onset of stationary phase. The possible roles of these suv components in stationary-phase survival or recovery is discussed.

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

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

Molecular cloning and nucleotide sequence of the superoxide dismutase gene and characterization of its product from Bacillus subtilis

Bacillus subtilis was found to possess one detectable superoxide dismutase (Sod) in both vegetative cells and spores. The Sod activity in vegetative cells was maximal at stationary phase. Manganese was necessary to sustain Sod activity at stationary phase, but paraquat, a superoxide generator, did not induce the expression of Sod. The specific activity of purified Sod was approximately 2, 600 U/mg of protein, and the enzyme was a homodimer protein with a molecular mass of approximately 25,000 per monomer. The gene encoding Sod, designated sodA, was cloned by the combination of several PCR methods and the Southern hybridization method. DNA sequence analysis revealed the presence of one open reading frame consisting of 606 bp. Several putative promoter sites were located in the upstream region of sodA. The deduced amino acid sequence showed high homology with other bacterial manganese Sods. Conserved regions in bacterial manganese Sod could also be seen. The phenotype of double mutant Escherichia coli sodA sodB, which could not grow in minimal medium without supplemental amino acids, was complemented by the expression of B. subtilis sodA.

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.

Role of the dimeric structure in Cu,Zn superoxide dismutase. pH-dependent, reversible denaturation of the monomeric enzyme from Escherichia coli

To investigate the structural/functional role of the dimeric structure in Cu,Zn superoxide dismutases, we have studied the stability to a variety of agents of the Escherichia coli enzyme, the only monomeric variant of this class so far isolated. Differential scanning calorimetry of the native enzyme showed the presence of two well defined peaks identified as the metal free and holoprotein. Unlike dimeric Cu,Zn superoxide dismutases, the unfolding of the monomeric enzyme was found to be highly reversible, a behavior that may be explained by the absence of free cysteines and the highly polar nature of its molecular surface. The melting temperature of the E. coli enzyme was found to be pH-dependent with the holoenzyme transition centered at 66 degrees C at pH 7.8 and at 79.3 degrees C at pH 6.0. The active-site metals, which were easily displaced from the active site by EDTA, were found to enhance the thermal stability of the monomeric apoprotein but to a lower extent than in the dimeric enzymes from eukaryotic sources. Apo-superoxide dismutase from E. coli was shown to be nearly as stable as the bovine apoenzyme, whose holo form is much more stable and less sensitive to pH variations. The remarkable pH susceptibility of the E. coli enzyme structure was paralleled by the slow decrease in activity of the enzyme incubated at alkaline pH and by modification of the EPR spectrum at lower pH values than in the case of dimeric enzymes. Unlike eukaryotic Cu,Zn superoxide dismutases, the active-site structure of the E. coli enzyme was shown to be reversibly perturbed by urea. These observations suggest that the conformational stability of Cu,Zn superoxide dismutases is largely due to the intrinsic stability of the beta-barrel fold rather than to the dimeric structure and that pH sensitivity and weak metal binding of the E. coli enzyme are due to higher flexibility and accessibility to the solvent of its active-site region.

Overexpression of a hydrogen peroxide-resistant periplasmic Cu,Zn superoxide dismutase protects Escherichia coli from macrophage killing

We have studied the effect of H2O2 on the activity of the Escherichia coli Cu,ZnSOD showing that, unlike the bovine enzyme, this bacterial Cu,ZnSOD is highly resistant to inactivation by hydrogen peroxide. In view of the key role played by oxygen radicals in bacterial killing by phagocytes, we have tested the ability of E. coli strains expressing different amounts of Cu,ZnSOD in the periplasmic space to survive the phagocytic attack of activated macrophages. Overexpression of the enzyme effectively protected the bacterial cell from macrophage killing. The results obtained support the hypothesis that in pathogenic bacteria periplasmic Cu,ZnSOD may reduce the oxyradical damages induced by the respiratory burst and therefore be important in virulence.

Periplasmic superoxide dismutase in meningococcal pathogenicity

Meningococcal sodC encodes periplasmic copper- and zinc-cofactored superoxide dismutase (Cu,Zn SOD) which catalyzes the conversion of the superoxide radical anion to hydrogen peroxide, preventing a sequence of reactions leading to production of toxic hydroxyl free radicals. From its periplasmic location, Cu,Zn SOD was inferred to acquire its substrate from outside the bacterial cell and was speculated to play a role in preserving meningococci from the action of microbicidal oxygen free radicals produced in the context of host defense. A sodC mutant was constructed by allelic exchange and was used to investigate the role of Cu,Zn SOD in pathogenicity. Wild-type and mutant meningococci grew at comparable rates and survived equally long in aerobic liquid culture. The mutant showed no increased sensitivity to paraquat, which generates superoxide within the cytosol, but was approximately 1,000-fold more sensitive to the toxicity of superoxide generated in solution by the xanthine/xanthine oxidase system. These data support a role for meningococcal Cu,Zn SOD in protection against exogenous superoxide. In experiments to translate this into a role in pathogenicity, wild-type and mutant organisms were used in an intraperitoneal mouse infection model. The sodC mutant was significantly less virulent. We conclude that periplasmic Cu,Zn SOD contributes to the virulence of Neisseria meningitidis, most likely by reducing the effectiveness of toxic oxygen host defenses.

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.

Role of bacterial Mn-cofactored superoxide dismutase in oxidative stress responses, nasopharyngeal colonization, and sustained bacteremia caused by Haemophilus influenzae type b

Haemophilus influenzae type b, a causative agent of bacterial sepsis and meningitis in young children, contains a single superoxide dismutase (SOD), a cytoplasmic MnSOD. To study the role of this enzyme, a chromosomal sodA::lacZ mutant (M-2) was constructed. M-2 had an increased sensitivity towards oxygen and the redox-active agent paraquat. A 3.4-fold increase in sodA-lacZ expression was found in M-2 grown with oxygen supply rates between 3 and 36 mmol of O2/liter/h. In similar experiments with the wild type, assaying SodA activity, a 3.1-fold increase was found. Both the wild type and M-2 grew best at the lowest oxygen supply rate tested, consistent with the notion that H. influenzae prefers a more anaerobic environment. In the infant rat model of infection, the ability of M-2 to colonize the nasopharynx was found to be impaired, but its ability to cause invasive disease was unaffected. This suggests that after invasion, the growth disadvantage imposed by a SodA- phenotype is not limiting.

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).

Distribution, cloning, characterisation and mutagenesis of sodC, the gene encoding copper/zinc superoxide dismutase, a potential determinant of virulence, in Haemophilus ducreyi

The sodC gene encoding the periplasmic enzyme copper/zinc superoxide dismutase (CuZnSOD) has been cloned from Haemophilus ducreyi, the causative agent of the genital ulcer disease, chancroid. Examination of a collection of diverse strains indicates that it is present throughout the species. Cloned sodC has been expressed in E. coli and shown to encode active enzyme. Insertional mutagenesis was used to construct a non-functional version of the gene. This has been transferred into the chromosome of the parent H. ducreyi strain by electroporation and homologous recombination, in preparation for studies of the role of this enzyme in the interactive biology of the organism with its host, perhaps in protecting bacteria from superoxide radicals and their reactive progeny generated by neutrophils in the context of host defence.

The Cu,Zn superoxide dismutase from Escherichia coli retains monomeric structure at high protein concentration. Evidence for altered subunit interaction in all the bacteriocupreins

Gel-filtration chromatography experiments performed at high protein concentrations demonstrate that the Cu,Zn superoxide dismutase from Escherichia coli is monomeric irrespective of the buffer and of ionic strength. The catalytic activity of the recombinant enzyme is comparable with that of eukaryotic isoenzymes, indicating that the dimeric structure commonly found in Cu,Zn superoxide dismutases is not necessary to ensure efficient catalysis. The analysis of the amino acid sequences suggests that an altered interaction between subunits occurs in all bacterial Cu,Zn superoxide dismutases. The substitution of hydrophobic residues with charged ones at positions located at the dimer interface of all known Cu,Zn superoxide dismutases could be specifically responsible for the monomeric structure of the E. coli enzyme.

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.

The temperature-sensitive growth and survival phenotypes of Escherichia coli cydDC and cydAB strains are due to deficiencies in cytochrome bd and are corrected by exogenous catalase and reducing agents

The cydDC operon of Escherichia coli encodes an ATP-dependent transporter of unknown function that is required for cytochrome bd synthesis. Strains containing defects in either the cydD or cydC gene also demonstrate hypersensitivity to growth at high temperatures and the inability to exit the stationary phase at 37 degrees C. We wished to determine what is responsible for these hypersensitive phenotypes and whether they are due to a lack of the CydDC proteins or a defect of the cytochrome bd encoded by the cydAB genes. Using both K-12- and B-type strains of E. coli, we have compared the phenotypes of isogenic cydAB mutants and cydC mutants. In both K-12- and B-type backgrounds, the hypersensitive phenotypes are due to defects of cytochrome bd activity and not defects of the cydDC genes. We also found that the temperature-sensitive growth phenotypes can be suppressed by exogenous reducing agents, such as glutathione and cysteine. Strikingly, even the enzymes catalase and superoxide dismutase, when added exogenously, can correct the temperature-sensitive and stationary phase arrest phenotypes. We propose that the temperature sensitive growth phenotypes are due to a buildup of diffusible oxygen radicals brought on by the absence of cytochrome bd.