Roles of manganese and iron in the regulation of the biosynthesis of manganese-superoxide dismutase in Escherichia coli

Virulence gene mutations as a differentiator of clinical phenotypes: insights from community-acquired uropathogenic Escherichia coli

Uropathogenic Escherichia coli (UPEC) remains an important cause of urinary tract infection during pregnancy. Multiple molecular virulence determinants and antibiotic resistant genes facilitate its pathogenesis and virulence phenotype. Hence it is hypothesized that there will be considerable variation in genes among the isolates from symptomatic as well as asymptomatic bacteriuria (ABU) during pregnancy. The aim of this study was to decipher the genetic variation among the two phenotypes. Six different UPEC isolates collected from urine specimens of consecutive pregnant females (five, symptomatic bacteriuria and one, ABU) were tested for their growth kinetics, and biofilm formation. A total of 87 virulence determinants and 56 antibiotic resistance genes were investigated using whole-genome sequencing, to identify putative drives of virulence phenotype. In this analysis, we identified eight different types of fully functional toxin antitoxin (TA) systems [HipAB, YefM-YoeB, YeeU-YeeV (CbtA), YhaV-PrlF, ChpBS, HigAB, YgiUT and HicAB] in the isolates from symptomatic bacteriuria; whereas partially functional TA system with mutations were observed in the asymptomatic one. Isolates of both the groups showed equivalent growth characteristics and biofilm-formation ability. Genes for an iron transport system (Efe UOB system, Fhu system except FhuA) were observed functional among all symptomatic and asymptomatic isolates, however functional mutations were observed in the latter group. Gene YidE was observed predominantly associated with the biofilm formation along with few other genes (BssR, BssS, YjgK, etc.). This study outlines putative critical relevance of specific variations in the genes for the TA system, biofilm formation, cell adhesion and colonization among UPEC isolates from symptomatic and asymptomatic bacteriuria among pregnant women. Further functional genomic study in the same cohort is warranted to establish the pathogenic role of these genes.

Insights into the antibacterial mechanism of action of chelating agents by selective deprivation of iron, manganese and zinc

Bacterial growth and proliferation can be restricted by limiting the availability of metal ions in their environment. Humans sequester iron, manganese, and zinc to help prevent infection by pathogens, a system termed nutritional immunity. Commercially used chelants have high binding affinities with a variety of metal ions, which may lead to antibacterial properties that mimic these innate immune processes. However, the modes of action of many of these chelating agents in bacterial growth inhibition and their selectivity in metal deprivation in cellulo remain ill-defined. We address this shortcoming by examining the effect of 11 chelators on Escherichia coli growth and their impact on the cellular concentration of five metals. The following four distinct effects were uncovered: (i) no apparent alteration in metal composition, (ii) depletion of manganese alongside reductions in iron and zinc levels, (iii) reduced zinc levels with a modest reduction in manganese, and (iv) reduced iron levels coupled with elevated manganese. These effects do not correlate with the absolute known chelant metal ion affinities in solution; however, for at least five chelators for which key data are available, they can be explained by differences in the relative affinity of chelants for each metal ion. The results reveal significant insights into the mechanism of growth inhibition by chelants, highlighting their potential as antibacterials and as tools to probe how bacteria tolerate selective metal deprivation.

IMPORTANCE Chelating agents are widely used in industry and consumer goods to control metal availability, with bacterial growth restriction as a secondary benefit for preservation. However, the antibacterial mechanism of action of chelants is largely unknown, particularly with respect to the impact on cellular metal concentrations. The work presented here uncovers distinct metal starvation effects imposed by different chelants on the model Gram-negative bacterium Escherichia coli. The chelators were studied both individually and in pairs, with the majority producing synergistic effects in combinations that maximize antibacterial hostility. The judicious selection of chelants based on contrasting cellular effects should enable reductions in the quantities of chelant required in numerous commercial products and presents opportunities to replace problematic chemistries with biodegradable alternatives.

Characterisation of the manganese superoxide dismutase of Enterococcus faecium

The manganese superoxide dismutase (SodA) of E. faecium strain AUS0004 has been characterised. It is most closely related to Enterococcus hirae, Enterococcus durans, Enterococcus villorium, and Enterococcus mundtii with 100%, 91,55%, 90,85%, and 90,58% homology, respectively, but more distant from SodA of E. faecalis (81.68%). A sodA deletion mutant has been constructed. Compared to the parental strain, the ΔsodA mutant was affected in aerobic growth and more sensitive to hydrogen peroxide (H₂O₂), cumene hydroperoxide (CuOOH), and the superoxide anion (O₂^•-) generator menadione. The E. faecium strain AUS0004 is part of those bacteria accumulating H₂O₂ to high concentrations (around 5 mM) starting from late exponential growth phase. Accumulation of the peroxide was around 25% less in the mutant suggesting that this part of H₂O₂ is due to the dismutation of O₂^•- by SodA. The sodA gene of E. faecium AUS0004 was induced by oxygen, peroxides and menadione but the corresponding regulator remains hitherto unknown. Finally, we showed that SodA activity is important for virulence in the Galleria mellonella model.

Characterization of an Antibacterial Agent Targeting Ferrous Iron Transport Protein FeoB against Staphylococcus aureus and Gram-Positive Bacteria

The emergence of multidrug-resistant Staphylococcus aureus strains has become a serious clinical problem. Iron is absolutely required for the bacterial growth, virulence associated with colonization, and survival from the host immune system. The FeoB protein is a major iron permease in bacterial ferrous iron transport systems (Feo) that has been shown to play a crucial role in virulence of some pathogenic bacteria. However, FeoB is still uncharacterized in Gram-positive pathogens, and its effects on S. aureus pathogenesis are unknown. In this study, we identified a novel inhibitor, GW3965·HCl, that targets FeoB in S. aureus. The molecule effectively inhibited FeoB in vitro enzyme activity, bacterial growth, and virulence factor expression. Genome-editing and metabolomic analyses revealed that GW3965·HCl inhibited FeoB function and affected the associated mechanisms with reduced iron availability in S. aureus. Gentamicin resistance and Caenorhabditis elegans infection assays further demonstrated the power of GW3965·HCl as a safe and efficient antibacterial agent. In addition to S. aureus, GW3965·HCl also presented its effectiveness on inhibition of the FeoB activity and growth of Gram-positive bacteria. This novel inhibitor will provide new insight for developing a next-generation antibacterial therapy.

Genetic Redundancy in Iron and Manganese Transport in the Metabolically Versatile Bacterium Rhodopseudomonas palustris TIE-1

The purple nonsulfur bacterium Rhodopseudomonas palustris TIE-1 can produce useful biochemicals such as bioplastics and biobutanol. Production of such biochemicals requires intracellular electron availability, which is governed by the availability and the transport of essential metals such as iron (Fe). Because of the distinct chemical properties of ferrous [Fe(II)] and ferric iron [Fe(III)], different systems are required for their transport and storage in bacteria. Although Fe(III) transport systems are well characterized, we know much less about Fe(II) transport systems except for the FeoAB system. Iron transporters can also import manganese (Mn). We studied Fe and Mn transport by five putative Fe transporters in TIE-1 under metal-replete, metal-depleted, oxic, and anoxic conditions. We observed that by overexpressing feoAB, efeU, and nramp1AB, the intracellular concentrations of Fe and Mn can be enhanced in TIE-1 under oxic and anoxic conditions, respectively. The deletion of a single gene/operon does not attenuate Fe or Mn uptake in TIE-1 regardless of the growth conditions used. This indicates that genetically dissimilar yet functionally redundant Fe transporters in TIE-1 can complement each other. Relative gene expression analysis shows that feoAB and efeU are expressed during Fe and Mn depletion under both oxic and anoxic conditions. The promoters of these transporter genes contain a combination of Fur and Fnr boxes, suggesting that their expression is regulated by both Fe and oxygen availability. The findings from this study will help us modulate intracellular Fe and Mn concentrations, ultimately improving TIE-1's ability to produce desirable biomolecules.IMPORTANCERhodopseudomonas palustris TIE-1 is a metabolically versatile bacterium that can use various electron donors, including Fe(II) and poised electrodes, for photoautotrophic growth. TIE-1 can produce useful biomolecules, such as biofuels and bioplastics, under various growth conditions. Production of such reduced biomolecules is controlled by intracellular electron availability, which, in turn, is mediated by various iron-containing proteins in the cell. Several putative Fe transporters exist in TIE-1's genome. Some of these transporters can also transport Mn, part of several important cellular enzymes. Therefore, understanding the ability to transport and respond to various levels of Fe and Mn under different conditions is important to improve TIE-1's ability to produce useful biomolecules. Our data suggest that by overexpressing Fe transporter genes via plasmid-based expression, we can increase the import of Fe and Mn in TIE-1. Future work will leverage these data to improve TIE-1 as an attractive microbial chassis and future biotechnological workhorse.

Bacterial ferrous iron transport: the Feo system

To maintain iron homeostasis within the cell, bacteria have evolved various types of iron acquisition systems. Ferric iron (Fe(3+)) is the dominant species in an oxygenated environment, while ferrous iron (Fe(2+)) is more abundant under anaerobic conditions or at low pH. For organisms that must combat oxygen limitation for their everyday survival, pathways for the uptake of ferrous iron are essential. Several bacterial ferrous iron transport systems have been described; however, only the Feo system appears to be widely distributed and is exclusively dedicated to the transport of iron. In recent years, many studies have explored the role of the FeoB and FeoA proteins in ferrous iron transport and their contribution toward bacterial virulence. The three-dimensional structures for the Feo proteins have recently been determined and provide insight into the molecular details of the transport system. A highly select group of bacteria also express the FeoC protein from the same operon. This review will provide a comprehensive look at the structural and functional aspects of the Feo system. In addition, bioinformatics analyses of the feo operon and the Feo proteins have been performed to complement our understanding of this ubiquitous bacterial uptake system, providing a new outlook for future studies.

Proteomic analysis of Yersinia enterocolitica biovar 1A under iron-rich and iron-poor conditions indicate existence of efficiently regulated mechanisms of iron homeostasis

The pathogenicity of Yersinia enterocolitica biovar 1A strains is controversial as these lack most of the known virulence factors. Acquisition of iron and presence of well-regulated iron homeostasis in bacteria represents an important virulence trait. Differential abundance of proteins was examined under iron-rich and iron-poor conditions in a clinical Y. enterocolitica biovar 1A strain IP27407. Whole cell protein profiles were analysed by 2D gel electrophoresis (2D-GE). Following statistical and MALDI-TOF MS analyses, 28 differentially abundant proteins were identified. Significant iron-responsive changes were observed in the proteins involved in iron acquisition or storage namely, hemin receptor (HemR), periplasmic Fe(2+) transport protein (Tpd), periplasmic chelated iron-binding protein (YfeA) and bacterioferritin (Bfr). Quantitative real-time PCR (qRT-PCR) of eight mRNA transcripts revalidated the differential protein abundance. In silico analysis of iron homeostasis mediated by the bacterioferritin and bacterioferritin-associated ferredoxin (Bfr-Bfd) complex suggested two pathways for the release of reserve iron which might be operating under conditions of different iron availability. The study, for the first time, showed the existence of highly competent iron homeostasis mechanisms in Y. enterocolitica biovar 1A and identified the key proteins involved thereof. Such mechanisms might have implications for the pathogenicity of Y. enterocolitica biovar 1A strains.

BIOLOGICAL SIGNIFICANCE

Although, a few studies have identified the differentially abundant bacterial proteins in response to iron starvation, little information is available in this regard for Y. enterocolitica (especially, the biovar 1A strains). In the present study, differential abundance of several proteins was identified under iron-rich and iron-poor conditions by 2D-GE and MALDI-TOF/MS analysis. These included proteins which may not only be directly implicated in iron acquisition or storage but also play crucial role in cellular metabolism. Given the absence of most known virulence factors in Y. enterocolitica biovar 1A strains, demonstration of well-regulated mechanisms for efficient iron homeostasis constitutes an important observation. The proteins, as identified in the present study, provide useful insights to further unravel the potential pathogenicity of the biovar 1A strains.

Inverse regulation of Fe- and Ni-containing SOD genes by a Fur family regulator Nur through small RNA processed from 3'UTR of the sodF mRNA

Superoxide dismutases (SODs) are widely distributed enzymes that convert superoxides to hydrogen peroxide and molecular oxygen, using various metals as cofactors. Many actinobacteria contain genes for both Ni-containing (sodN) and Fe-containing (sodF) SODs. In Streptomyces coelicolor, expression of the sodF and sodN genes is inversely regulated by nickel-specific Nur, a Fur-family regulator. With sufficient nickel, Nur directly represses sodF transcription, while inducing sodN indirectly. Bioinformatic search revealed that a conserved 19-nt stretch upstream of sodN matches perfectly with the sodF downstream sequence. We found that the sodF gene produced a stable small-sized RNA species (s-SodF) of ∼ 90 nt that harbors the anti-sodN sequence complementary to sodN mRNA from the 5'-end up to the ribosome binding site. Absence of nearby promoters and sensitivity to 5'-phosphate-specific exonuclease indicated that the s-SodF RNA is a likely processed product of sodF mRNA. The s-SodF RNA caused a significant decrease in the half-life of the sodN mRNA. Therefore, Nur activates sodN expression through inhibiting the synthesis of sodF mRNA, from which inhibitory s-SodF RNA is generated. This reveals a novel mechanism by which antagonistic regulation of one gene is achieved by small RNA processed from the 3'UTR of another gene's mRNA.

Transcriptional regulation by Ferric Uptake Regulator (Fur) in pathogenic bacteria

In the ancient anaerobic environment, ferrous iron (Fe²⁺) was one of the first metal cofactors. Oxygenation of the ancient world challenged bacteria to acquire the insoluble ferric iron (Fe³⁺) and later to defend against reactive oxygen species (ROS) generated by the Fenton chemistry. To acquire Fe³⁺, bacteria produce low-molecular weight compounds, known as siderophores, which have extremely high affinity for Fe³⁺. However, during infection the host restricts iron from pathogens by producing iron- and siderophore-chelating proteins, by exporting iron from intracellular pathogen-containing compartments, and by limiting absorption of dietary iron. Ferric Uptake Regulator (Fur) is a transcription factor which utilizes Fe²⁺ as a corepressor and represses siderophore synthesis in pathogens. Fur, directly or indirectly, controls expression of enzymes that protect against ROS damage. Thus, the challenges of iron homeostasis and defense against ROS are addressed via Fur. Although the role of Fur as a repressor is well-documented, emerging evidence demonstrates that Fur can function as an activator. Fur activation can occur through three distinct mechanisms (1) indirectly via small RNAs, (2) binding at cis regulatory elements that enhance recruitment of the RNA polymerase holoenzyme (RNAP), and (3) functioning as an antirepressor by removing or blocking DNA binding of a repressor of transcription. In addition, Fur homologs control defense against peroxide stress (PerR) and control uptake of other metals such as zinc (Zur) and manganese (Mur) in pathogenic bacteria. Fur family members are important for virulence within bacterial pathogens since mutants of fur, perR, or zur exhibit reduced virulence within numerous animal and plant models of infection. This review focuses on the breadth of Fur regulation in pathogenic bacteria.

A proteomic analysis of the regulon of the NarP two-component regulatory system response regulator in the bovine pathogen Mannheimia haemolytica A1

Background

The response of the NarQP two-component signal transduction system regulon in response to the presence of nitrate for the bovine pathogen Mannheimia haemolytica A1 was investigated by proteomic analysis. Total proteins from a narP mutant and the parent SH1217 grown with or without NaNO₃ supplement were examined by ISO-DALT 2D electrophoresis and liquid chromatography-mass spectrometry.

Results

Seventeen proteins were differentially expressed in the parent strain SH1217 in response to the addition of NaNO₃ to the growth media. These responses were absent in the narP mutant, indicating that the altered production of these proteins is mediated by NarP_Mh. Interestingly, NarP_Mh mediated the increased production of some proteins which are not generally associated with nitrate respiration, such as the iron transporters FbpA and YfeA. The increased production of proteins such as superoxide dismutase, SodA, and GAPDH were also observed. The increased production of these iron-regulated proteins by NarP_Mh is thought to enhance the swift establishment of the nitrate respiration mechanism of M. haemolytica during pathogenesis.

Conclusion

The data suggested NarP_Mh acts as an important regulator which regulates the expression of a small set of proteins in response to nitrate availability. This may contribute to the prevalence of M. haemolytica A1 in its host during pathogenesis of BPP, through enhancing the effectiveness of nitrate respiration either directly or indirectly.

Iron homeostasis and management of oxidative stress response in bacteria

Iron is both an essential nutrient for the growth of microorganisms, as well as a dangerous metal due to its capacity to generate reactive oxygen species (ROS) via the Fenton reaction. For these reasons, bacteria must tightly control the uptake and storage of iron in a manner that restricts the build-up of ROS. Therefore, it is not surprising to find that the control of iron homeostasis and responses to oxidative stress are coordinated. The mechanisms concerned with these processes, and the interactions involved, are the subject of this review.

Analysis of the ArcA regulon in anaerobically grown Salmonella enterica sv. Typhimurium

Background

Salmonella enterica serovar Typhimurium (S. Typhimurium) is a Gram-negative pathogen that must successfully adapt to the broad fluctuations in the concentration of dissolved dioxygen encountered in the host. In Escherichia coli, ArcA (Aerobic Respiratory Control) helps the cells to sense and respond to the presence of dioxygen. The global role of ArcA in E. coli is well characterized; however, little is known about its role in anaerobically grown S. Typhimurium.

Results

We compared the transcriptional profiles of the virulent wild-type (WT) strain (ATCC 14028s) and its isogenic arcA mutant grown under anaerobic conditions. We found that ArcA directly or indirectly regulates 392 genes (8.5% of the genome); of these, 138 genes are poorly characterized. Regulation by ArcA in S. Typhimurium is similar, but distinct from that in E. coli. Thus, genes/operons involved in core metabolic pathways (e.g., succinyl-CoA, fatty acid degradation, cytochrome oxidase complexes, flagellar biosynthesis, motility, and chemotaxis) were regulated similarly in the two organisms. However, genes/operons present in both organisms, but regulated differently by ArcA in S. Typhimurium included those coding for ethanolamine utilization, lactate transport and metabolism, and succinate dehydrogenases. Salmonella-specific genes/operons regulated by ArcA included those required for propanediol utilization, flagellar genes (mcpAC, cheV), Gifsy-1 prophage genes, and three SPI-3 genes (mgtBC, slsA, STM3784). In agreement with our microarray data, the arcA mutant was non-motile, lacked flagella, and was as virulent in mice as the WT. Additionally, we identified a set of 120 genes whose regulation was shared with the anaerobic redox regulator, Fnr.

Conclusion(s)

We have identified the ArcA regulon in anaerobically grown S. Typhimurium. Our results demonstrated that in S. Typhimurium, ArcA serves as a transcriptional regulator coordinating cellular metabolism, flagella biosynthesis, and motility. Furthermore, ArcA and Fnr share in the regulation of 120 S. Typhimurium genes.

Salmonella-macrophage interactions upon manganese supplementation

Various studies indicate the role of manganese (Mn) in the virulence of pathogens. Salmonella is an intracellular pathogen which is able to multiply within the macrophages. The present study was therefore, designed to assess the effect of Mn supplementation on Salmonella-macrophage interactions particularly in reference to Salmonella virulence and macrophage functions. A 50-fold decrease in the lethal dose 50 (LD(50)) of Salmonella typhimurium was observed when mice were infected with Salmonella grown in the presence of Mn as compared to the LD(50) in the absence of Mn indicating an increase in the virulence of the organism. A significant increase was observed in the levels of superoxide dismutase (SOD) of S. typhimurium grown in presence of manganese. Upon Mn supplementation, macrophage functions were also found to be altered. Decreased phagocytic activity of macrophages interacted with Salmonella was observed in presence of Mn as compared to the activity in the absence of Mn. A significant increase was observed in the extent of lipid peroxidation along with significant decreases in the activities of SOD and catalase as well as nitrite levels of macrophages interacted with S. typhimurium upon supplementation with Mn. These observations indicate that Mn supplementation might have increased the expression of Mn transporters in Salmonella resulting in increased levels of its superoxide dismutase. The altered Salmonella function in turn might have been responsible for inhibiting phagocytosis and impairing the balance between the oxidant and antioxidant profile of macrophages, thus protecting itself by exhibiting exalted virulence.

Galleria mellonella as an alternative infection model for Yersinia pseudotuberculosis

We report that larvae of the wax moth (Galleria mellonella) are susceptible to infection with the human enteropathogen Yersinia pseudotuberculosis at 37 degrees C. Confocal microscopy demonstrated that in the initial stages of infection the bacteria were taken up into haemocytes. To evaluate the utility of this model for screening Y. pseudotuberculosis mutants we constructed and tested a superoxide dismutase C (sodC) mutant. This mutant showed increased susceptibility to superoxide, a key mechanism of killing in insect haemocytes and mammalian phagocytes. It showed reduced virulence in the murine yersiniosis infection model and in contrast to the wild-type strain IP32953 was unable to kill G. mellonella. The complemented mutant regained all phenotypic properties associated with SodC, confirming the important role of this metalloenzyme in two Y. pseudotuberculosis infection models.

Effects of clotrimazole on experimental spinal cord ischemia/reperfusion injury in rats

OBJECTIVE

The effect of clotrimazole was examined using a spinal cord ischemia/reperfusion model.

METHODS

Twenty albino Wistar rats weighing 234 +/- 12.3 g were used in this study. Rats were anesthetized intraperitoneally with 50 mg/kg ketamine HCl. All animals underwent laparotomy under aseptic conditions. Abdominal aortas of the animals in all but the sham group were exposed. After opening the retroperitoneum, the infrarenal abdominal aorta was clipped for 45 minutes to produce ischemia/reperfusion injury. Polyethylene glycol (PEG, 1 mL) was administrated to the vehicle group. PEG (1 mL) and clotrimazole (30 mg/kg) were administered intraperitoneally in the clotrimazole group. Total laminectomy of T8-T12 was performed on all rats under a microscope. Spinal cords were excised for a length of 2-cm rostrally and 1-cm caudally to the injury site and deep frozen at -76 degrees C for biochemical studies. The levels of malondialdehyde, glutathione-peroxidase, superoxide dismutase, and catalase were measured as an indicator of ischemia level. The most cranial part of the specimens was evaluated morphologically.

RESULTS

Treatment with clotrimazole significantly decreased malondialdehyde, glutathione-peroxidase, superoxide dismutase, and catalase levels in comparison with other groups (P = 0.008). Morphologic evaluation revealed that clotrimazole protected the axons and their myelin sheaths from ischemic damage.

CONCLUSION

This study showed the neuroprotective effects of clotrimazole on spinal cord ischemia/reperfusion injury.

Status of some free radical scavenging enzymes in the blood of myocardial infarction patients

Pro-oxidant and anti-oxidant systems and their levels have significant roles in occlusive vascular diseases. In the present communication, we have measured the levels of some representative anti-oxidant enzymes in the blood of the patients of myocardial infarction after reperfusion and compared them to age and sex matched healthy persons. Our findings show that the activities of anti-oxidant enzymes (viz. SOD, catalase and glutathione reductase) are significantly decreased whereas there is significant increase in the levels of malonaldialdehyde (a marker of free radical-mediated damage) in the patients. The findings point out that ischemic myocardial disorders are associated with excessive free radical generation and free radical-mediated damage of lipids.

Rapid and quantitative activation of Chlamydia trachomatis ribonucleotide reductase by hydrogen peroxide

We recently showed that the class Ic ribonucleotide reductase (RNR) from the human pathogen Chlamydia trachomatis ( Ct) uses a Mn (IV)/Fe (III) cofactor in its R2 subunit to initiate catalysis [Jiang, W., Yun, D., Saleh, L., Barr, E. W., Xing, G., Hoffart, L. M., Maslak, M.-A., Krebs, C., and Bollinger, J. M., Jr. (2007) Science 316, 1188-1191]. The Mn (IV) site of the novel cofactor functionally replaces the tyrosyl radical used by conventional class I RNRs to initiate substrate radical production. As a first step in evaluating the hypothesis that the use of the alternative cofactor could make the RNR more robust to reactive oxygen and nitrogen species [RO(N)S] produced by the host's immune system [Högbom, M., Stenmark, P., Voevodskaya, N., McClarty, G., Gräslund, A., and Nordlund, P. (2004) Science 305, 245-248], we have examined the reactivities of three stable redox states of the Mn/Fe cluster (Mn (II)/Fe (II), Mn (III)/Fe (III), and Mn (IV)/Fe (III)) toward hydrogen peroxide. Not only is the activity of the Mn (IV)/Fe (III)-R2 intermediate stable to prolonged (>1 h) incubations with as much as 5 mM H 2O 2, but both the fully reduced (Mn (II)/Fe (II)) and one-electron-reduced (Mn (III)/Fe (III)) forms of the protein are also efficiently activated by H 2O 2. The Mn (III)/Fe (III)-R2 species reacts with a second-order rate constant of 8 +/- 1 M (-1) s (-1) to yield the Mn (IV)/Fe (IV)-R2 intermediate previously observed in the reaction of Mn (II)/Fe (II)-R2 with O 2 [Jiang, W., Hoffart, L. M., Krebs, C., and Bollinger, J. M., Jr. (2007) Biochemistry 46, 8709-8716]. As previously observed, the intermediate decays by reduction of the Fe site to the active Mn (IV)/Fe (III)-R2 complex. The reaction of the Mn (II)/Fe (II)-R2 species with H 2O 2 proceeds in three resolved steps: sequential oxidation to Mn (III)/Fe (III)-R2 ( k = 1.7 +/- 0.3 mM (-1) s (-1)) and Mn (IV)/Fe (IV)-R2, followed by decay of the intermediate to the active Mn (IV)/Fe (III)-R2 product. The efficient reaction of both reduced forms with H 2O 2 contrasts with previous observations on the conventional class I RNR from Escherichia coli, which is efficiently converted from the fully reduced (Fe 2 (II/II)) to the "met" (Fe 2 (III/III)) form [Gerez, C., and Fontecave, M. (1992) Biochemistry 31, 780-786] but is then only very inefficiently converted from the met to the active (Fe 2 (III/III)-Y (*)) form [Sahlin, M., Sjöberg, B.-M., Backes, G., Loehr, T., and Sanders-Loehr, J. (1990) Biochem. Biophys. Res. Commun. 167, 813-818].

A manganese(IV)/iron(III) cofactor in Chlamydia trachomatis ribonucleotide reductase

In a conventional class I ribonucleotide reductase (RNR), a diiron(II/II) cofactor in the R2 subunit reacts with oxygen to produce a diiron(III/IV) intermediate, which generates a stable tyrosyl radical (Y*). The Y* reversibly oxidizes a cysteine residue in the R1 subunit to a cysteinyl radical (C*), which abstracts the 3'-hydrogen of the substrate to initiate its reduction. The RNR from Chlamydia trachomatis lacks the Y*, and it had been proposed that the diiron(III/IV) complex in R2 directly generates the C* in R1. By enzyme activity measurements and spectroscopic methods, we show that this RNR actually uses a previously unknown stable manganese(IV)/iron(III) cofactor for radical initiation.

FNR is a global regulator of virulence and anaerobic metabolism in Salmonella enterica serovar Typhimurium (ATCC 14028s)

Salmonella enterica serovar Typhimurium must successfully transition the broad fluctuations in oxygen concentrations encountered in the host. In Escherichia coli, FNR is one of the main regulatory proteins involved in O2 sensing. To assess the role of FNR in serovar Typhimurium, we constructed an isogenic fnr mutant in the virulent wild-type strain (ATCC 14028s) and compared their transcriptional profiles and pathogenicities in mice. Here, we report that, under anaerobic conditions, 311 genes (6.80% of the genome) are regulated directly or indirectly by FNR; of these, 87 genes (28%) are poorly characterized. Regulation by FNR in serovar Typhimurium is similar to, but distinct from, that in E. coli. Thus, genes/operons involved in aerobic metabolism, NO. detoxification, flagellar biosynthesis, motility, chemotaxis, and anaerobic carbon utilization are regulated by FNR in a fashion similar to that in E. coli. However, genes/operons existing in E. coli but regulated by FNR only in serovar Typhimurium include those coding for ethanolamine utilization, a universal stress protein, a ferritin-like protein, and a phosphotransacetylase. Interestingly, Salmonella-specific genes/operons regulated by FNR include numerous virulence genes within Salmonella pathogenicity island 1 (SPI-1), newly identified flagellar genes (mcpAC, cheV), and the virulence operon (srfABC). Furthermore, the role of FNR as a positive regulator of motility, flagellar biosynthesis, and pathogenesis was confirmed by showing that the mutant is nonmotile, lacks flagella, is attenuated in mice, and does not survive inside macrophages. The inability of the mutant to survive inside macrophages is likely due to its sensitivity to the reactive oxygen species generated by NADPH phagocyte oxidase.

Computational reconstruction of iron- and manganese-responsive transcriptional networks in alpha-proteobacteria

We used comparative genomics to investigate the distribution of conserved DNA-binding motifs in the regulatory regions of genes involved in iron and manganese homeostasis in alpha-proteobacteria. Combined with other computational approaches, this allowed us to reconstruct the metal regulatory network in more than three dozen species with available genome sequences. We identified several classes of cis-acting regulatory DNA motifs (Irr-boxes or ICEs, RirA-boxes, Iron-Rhodo-boxes, Fur-alpha-boxes, Mur-box or MRS, MntR-box, and IscR-boxes) in regulatory regions of various genes involved in iron and manganese uptake, Fe-S and heme biosynthesis, iron storage, and usage. Despite the different nature of the iron regulons in selected lineages of alpha-proteobacteria, the overall regulatory network is consistent with, and confirmed by, many experimental observations. This study expands the range of genes involved in iron homeostasis and demonstrates considerable interconnection between iron-responsive regulatory systems. The detailed comparative and phylogenetic analyses of the regulatory systems allowed us to propose a theory about the possible evolution of Fe and Mn regulons in alpha-proteobacteria. The main evolutionary event likely occurred in the common ancestor of the Rhizobiales and Rhodobacterales, where the Fur protein switched to regulating manganese transporters (and hence Fur had become Mur). In these lineages, the role of global iron homeostasis was taken by RirA and Irr, two transcriptional regulators that act by sensing the physiological consequence of the metal availability rather than its concentration per se, and thus provide for more flexible regulation.

The availability of hundreds of complete genomes allows one to use comparative genomics to describe key metabolic processes and regulatory gene networks. Genome context analyses and comparisons of transcription factor binding sites between genomes offer a powerful approach for functional gene annotation. Reconstruction of transcriptional regulatory networks allows for better understanding of cellular processes, which can be substantiated by direct experimentation. Iron homeostasis in bacteria is conferred by the regulation of various iron uptake transporters, iron storage ferritins, and iron-containing enzymes. In high concentrations, iron is poisonous for the cell, so strict control of iron homeostasis is maintained, mostly at the level of transcription by iron-responsive regulators. Despite their general importance, iron regulatory networks in most bacterial species are not well-understood. In this study, Rodionov and colleagues applied comparative genomic approaches to describe the regulatory network formed by genes involved in iron homeostasis in the alpha subclass of proteobacteria, which have extremely versatile lifestyles. These networks are mediated by a set of various DNA motifs (or regulatory signals) that occur in 5′ gene regions and involve at least six different metal-responsive regulators. This study once again shows the power of comparative genomics in the analysis of complex regulatory networks and their evolution.

Probing the ArcA-P modulon of Escherichia coli by whole genome transcriptional analysis and sequence recognition profiling

The ArcB/ArcA two-component signal transduction system of Escherichia coli regulates gene expression in response to the redox conditions of growth. Over the years, genetic screens have lead to the identification of about 30 ArcA-P-controlled operons that are involved in redox metabolism. However, the discovery of 3 targets that are not implicated in respiratory metabolism (the tra operon for plasmid conjugation, psi site for Xer-based recombination, and oriC site for chromosome replication) suggests that the Arc modulon may comprise additional operons that are involved in a myriad of functions. To identify these operons, we derived the ArcA-P-dependent transcription profile of E. coli using oligonucleotide-based microarray analysis. The findings indicated that 9% of all open reading frames in E. coli are affected either directly or indirectly by ArcA-P. To identify which operons are under the direct control of ArcA-P, we developed the ArcA-P recognition weight matrix from footprinting data and used it to scan the genome, yielding an ArcA-P sequence affinity map. By overlaying both methods, we identified 55 new Arc-regulated operons that are implicated in energy metabolism, transport, survival, catabolism, and transcriptional regulation. The data also suggest that the Arc response pathway, which translates into a net global downscaling of gene expression, overlaps partly with the FNR regulatory network. A conservative but reasonable assessment is that the Arc pathway recruits 100-150 operons to mediate a role in cellular adaptation that is more extensive than hitherto anticipated.

Role and regulation of the superoxide dismutases of Staphylococcus aureus

Staphylococcus aureus has two superoxide dismutases (SODs), encoded by the sodA and sodM genes, which inactivate harmful superoxide radicals () encountered during host infection or generated from aerobic metabolism. The transcriptional start sites have been mapped and expression analysis on reporter fusions in both genes has been carried out. Under standard growth conditions, manganese (Mn), a mineral superoxide scavenger, elevated total SOD activity but had no effect on the transcription of either gene. Transcription of sodA and sodM was most strongly induced by either internally or externally generated, respectively. Sensitivity to internally generated was linked with SodA deficiency. Mn supplementation completely rescued a sodA mutant when challenged by internally generated, and this was growth-phase-dependent. Sensitivity to externally generated stress was only observed in a sodA sodM mutant and was Mn-independent. In a mouse abscess model of infection, isogenic sodA, sodM and sodA sodM mutants had reduced virulence compared to the parental strain, showing the importance of the enzymic scavenging system for the survival of the pathogen.

Molecular characterization and quantitative analysis of superoxide dismutases in virulent and avirulent strains of Aeromonas salmonicida subsp. salmonicida

Aeromonas salmonicida subsp. salmonicida is a facultatively intracellular gram-negative bacterium that is the etiological agent of furunculosis, a bacterial septicemia of salmonids that causes significant economic loss to the salmon farming industry. The mechanisms by which A. salmonicida evades intracellular killing may be relevant in understanding virulence and the eventual design of appropriate treatment strategies for furunculosis. We have identified two open reading frames (ORFs) and related upstream sequences that code for two putative superoxide dismutases (SODs), sodA and sodB. The sodA gene encoded a protein of 204 amino acids with a molecular mass of approximately 23.0 kDa (SodA) that had high similarity to other prokaryotic Mn-SODs. The sodB gene encoded a protein of 194 amino acids with a molecular mass of approximately 22.3 kDa that had high similarity to other prokaryotic Fe-SODs. Two enzymes with activities consistent with both these ORFs were identified by inhibition of O(2)(-)-catalyzed tetrazolium salt reduction in both gels and microtiter plate assays. The two enzymes differed in their expression patterns in in vivo- and in vitro-cultured bacteria. The regulatory sequences upstream of putative sodA were consistent with these differences. We could not identify other SOD isozymes such as sodC either functionally or through data mining. Levels of SOD were significantly higher in virulent than in avirulent strains of A. salmonicida subsp. salmonicida strain A449 when cultured in vitro and in vivo.

Emerging themes in manganese transport, biochemistry and pathogenesis in bacteria

Though an essential trace element, manganese is generally accorded little importance in biology other than as a cofactor for some free radical detoxifying enzymes and in the photosynthetic photosystem II. Only a handful of other Mn2+-dependent enzymes are known. Recent data, primarily in bacteria, suggest that Mn2+-dependent processes may have significantly greater physiological importance. Two major classes of prokaryotic Mn2+ uptake systems have now been described, one homologous to eukaryotic Nramp transporters and one a member of the ABC-type ATPase superfamily. Each is highly selective for Mn2+ over Fe2+ or other transition metal divalent cations, and each can accumulate millimolar amounts of intracellular Mn2+ even when environmental Mn2+ is scarce. In Salmonella enterica serovar Typhimurium, simultaneous mutation of both types of transporter results in avirulence, implying that one or more Mn2+-dependent enzymes is essential for pathogenesis. This review summarizes current literature on Mn2+ transport, primarily in the Bacteria but with relevant comparisons to the Archaea and Eukaryota. Mn2+-dependent enzymes are then discussed along with some speculations as to their role(s) in cellular physiology, again primarily in Bacteria. It is of particular interest that most of the enzymes which interconvert phosphoglycerate, pyruvate, and oxaloacetate intermediates are either strictly Mn2+-dependent or highly stimulated by Mn2+. This suggests that Mn2+ may play an important role in central carbon metabolism. Further studies will be required, however, to determine whether these or other actions of Mn2+ within the cell are the relevant factors in pathogenesis.

Regulation of riboflavin biosynthesis and transport genes in bacteria by transcriptional and translational attenuation

The riboflavin biosynthesis in bacteria was analyzed using comparative analysis of genes, operons and regulatory elements. A model for regulation based on formation of alternative RNA structures involving the RFN elements is suggested. In Gram-positive bacteria including actinomycetes, Thermotoga, Thermus and Deinococcus, the riboflavin metabolism and transport genes are predicted to be regulated by transcriptional attenuation, whereas in most Gram-negative bacteria, the riboflavin biosynthesis genes seem to be regulated on the level of translation initiation. Several new candidate riboflavin transporters were identified (impX in Desulfitobacterium halfniense and Fusobacterium nucleatum; pnuX in several actinomycetes, including some Corynebacterium species and Strepto myces coelicolor; rfnT in Rhizobiaceae). Traces of a number of likely horizontal transfer events were found: the complete riboflavin operon with the upstream regulatory element was transferred to Haemophilus influenzae and Actinobacillus pleuropneumoniae from some Gram-positive bacterium; non-regulated riboflavin operon in Pyrococcus furiousus was likely transferred from Thermotoga; and the RFN element was inserted into the riboflavin operon of Pseudomonas aeruginosa from some other Pseudomonas species, where it had regulated the ribH2 gene.

Manganese deficiency can replace high oxygen levels needed for lignin peroxidase formation by Phanerochaete chrysosporium

The combined effects of Mn and oxygen on lignin peroxidase (LIP) activity and isozyme composition in Phanerochaete chrysosporium were studied by using shallow stationary cultures grown in the presence of limited or excess N. When no Mn was added, LIP was formed in both N-limited and N-excess cultures exposed to air, but no LIP activity was observed at Mn concentrations greater than 13 mg/liter. In oxygen-flushed, N-excess cultures, LIP was formed at all Mn concentrations, and the peak LIP activity values in the extracellular fluid were nearly identical in the presence of Mn concentrations ranging from 3 to 1,500 mg/liter. When the availability of oxygen to cultures exposed to air was increased by growing the fungus under nonimmersed liquid conditions, higher levels of Mn were needed to suppress LIP formation compared with the levels needed in shallow stationary cultures. The composition of LIP isozymes was affected by the levels of N and Mn. Addition of veratryl alcohol to cultures exposed to air did not eliminate the suppressive effect of Mn on LIP formation. A deficiency of Mn in N-excess cultures resulted in lower biomass and a lower rate of glucose consumption than in the presence of Mn. In addition, almost no activity of the antioxidant enzyme Mn superoxide dismutase was observed in Mn-deficient, N-excess cultures, but the activity of this enzyme increased as the Mn concentration increased from 3 to 13 mg/liter. No Zn/Cu superoxide dismutase activity was observed in N-excess cultures regardless of the Mn concentration.

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.

Antioxidant vitamins C and E affect the superoxide-mediated induction of the soxRS regulon of Escherichia coli

The mechanism of activation of Escherichia coli redox sensory protein SoxR is still unclear: a [2Fe-2S] cluster contained in a SoxR dimer is potentially redox-sensitive, but the nature of the signal is unknown. Antioxidant vitamins C (ascorbate) and E (alpha-tocopherol) were used to explore the mechanism of activation of the SoxR protein in vivo. Treating E. coli cells with ascorbate or alpha-tocopherol increased their tolerance to paraquat (PQ, a redox-cycling compound), even in the absence of the soxRS locus, suggesting a radical-quenching activity. When using a soxS'::lacZ fusion, whose expression is governed by activated SoxR, ascorbate and alpha-tocopherol also prevented the expression of beta-galactosidase after PQ treatment. A secondary activity was observed in cells carrying soxR101, a mutation resulting in the constitutive expression of the sox regulon, where the overexpression of soxS'::lacZ was also reduced by ascorbate or alpha-tocopherol treatment. Additionally, different mechanisms of action were revealed as alpha-tocopherol was capable of preventing both PQ and meanadione (MD) lethality, whilst ascorbate prevented PQ lethality but increased MD-mediated cell death. It is proposed that alpha-tocopherol, positioned in membranes, can prevent superoxide-dependent membrane damage; however, water-soluble ascorbate is unable to do so and can even increase the concentration of oxygen radicals reacting with released membrane-associated Fe(II).

Mechanism of regulation of 8-hydroxyguanine endonuclease by oxidative stress: roles of FNR, ArcA, and Fur

We found previously that 8-hydroxyguanine (oh8Gua) endonuclease in E. coli is induced in response to oxidative stress in a fashion similar to the oxidative response of the Mn-superoxide dismutase (MnSOD). In this study, attempts were made to identify the genes involved in the co-regulation of E. coli endonuclease and MnSOD (sodA). oh8Gua nuclease is induced by molecular oxygen and a superoxide radical generator (paraquat) but not by H2O2, suggesting that the regulation of this endonuclease is dependent on SoxRS but independent of OxyR. This enzyme was induced by paraquat in all of the soxRS mutant strains used (soxR-, soxS- and soxRc), whereas glucose-6-phosphate dehydrogenase (a member of the soxRS regulon) showed the expected responses; therefore, this possibility was excluded. The presence of metal chelators in the growth medium caused the induction of this enzyme, and this induction was suppressed by the addition of Fe++. Consistent with this finding, this enzyme was expressed under anaerobiosis in all of the mutant strains of fnr in particular, as well as fur, arcA, and combinations thereof. These findings suggest that the oxidative regulation of oh8Gua endonuclease is under control of fnr, fur, and arcA, where fnr plays a predominant role. The multiple involvement of regulatory genes as well as co-regulation with antioxidant enzyme will enhance the efficiency of cellular growth and survival in the aerobic environment.

Regulation of high affinity iron uptake in the yeast Saccharomyces cerevisiae. Role of dioxygen and Fe

High affinity iron uptake in Saccharomyces cerevisiae requires a metal reductase, a multicopper ferroxidase, and an iron permease. Fet3, the apparent ferroxidase, is proposed to facilitate iron uptake by catalyzing the oxidation of reductase-generated Fe(II) to Fe(III) by O2; in this model, Fe(III) is the substrate for the iron permease, encoded by FTR1 (Kaplan, J., and O'Halloran, T. V. (1996) Science 271, 1510-1512). We show here that dioxygen also plays an essential role in the expression of these iron uptake activities. Cells grown anaerobically exhibited no Fe(III) reductase or high affinity iron uptake activity, even if assayed for these activities under air. Northern blot analysis showed that the amount of those mRNAs encoding proteins associated with this uptake was repressed in anaerobic cultures but was rapidly induced by exposure of the culture to dioxygen. The anaerobic repression was reduced in cells expressing an iron-independent form of the trans-activator, Aft1, a protein that regulates the expression of these proteins. Thus, the effect of oxygenation on this expression appeared due at least in part to the state or distribution of iron in the cells. In support of this hypothesis, the membrane-permeant Fe(II) chelator, 2, 2'-bipyridyl, in contrast to the impermeant chelator bathophenanthroline disulfonate, caused a strong and rapid induction of these transcripts under anaerobic conditions. An increase in the steady-state levels of iron-regulated transcripts upon oxygenation or 2,2'-bipyridyl addition occurred within 5 min, indicating that a relatively small, labile intracellular pool of Fe(II) regulates the expression of these activities. The strength of the anaerobic repression was dependent on the low affinity, Fe(II)-specific iron transporter, encoded by FET4, suggesting that this Fe(II) pool was linked in part to iron brought into the cell via Fet4 protein. The data suggest a model in which dioxygen directly or indirectly modulates the Fe(III)/Fe(II) ratio in an iron pool linked to Aft1 protein while bipyridyl increases this ratio by chelating Fe(II). These results indicate that dioxygen both modulates the sensitivity to iron-dependent transcriptional regulation and acts as substrate for Fet3 in the ferroxidase reaction catalyzed by this ceruloplasmin homologue.

Cloning and characterization of the fur gene from Helicobacter pylori

The fur homologue of Helicobacter pylori was isolated by screening a plasmid-based, genomic DNA library using the Fur titration assay (FURTA). The analysis of the DNA sequence revealed significant homology with Fur proteins from various other bacterial species. The highest degree of homology was observed for the Fur protein from Campylobacter jejuni. The H. pylori fur gene on a plasmid could partially complement the fur mutation in Escherichia coli strain H1681. The repressor activity depended on addition of iron to the medium indicating that iron acts as a co-repressor for the H. pylori protein similar to Fur from other bacteria. Comparison of Fur from H. pylori strain NCTC11638 with the recently published genomic DNA sequence of another strain (26695) confirmed the identity of the fur homologue and revealed that the fur locus is highly conserved in both strains.

The adhesion-associated sca operon in Streptococcus gordonii encodes an inducible high-affinity ABC transporter for Mn2+ uptake

ScaA lipoprotein in Streptococcus gordonii is a member of the LraI family of homologous polypeptides found among streptococci, pneumococci, and enterococci. It is the product of the third gene within the scaCBA operon encoding the components of an ATP-binding cassette (ABC) transporter system. Inactivation of scaC (ATP-binding protein) or scaA (substrate-binding protein) genes resulted in both impaired growth of cells and > 70% inhibition of 54Mn2+ uptake in media containing < 0.5 microM Mn2+. In wild-type and scaC mutant cells, production of ScaA was induced at low concentrations of extracellular Mn2+ (< 0.5 microM) and by the addition of > or = 20 microM Zn2+. Sca permease-mediated uptake of 54Mn2+ was inhibited by Zn2+ but not by Ca2+, Mg2+, Fe2+, or Cu2+. Reduced uptake of 54Mn2+ by sca mutants and by wild-type cells in the presence of Zn2+ was abrogated by the uncoupler carbonylcyanide m-chlorophenylhydrazone, suggesting that Mn2+ uptake under these conditions was proton motive force dependent. The frequency of DNA-mediated transformation was reduced > 20-fold in sca mutants. The addition of 0.1 mM Mn2+ to the transformation medium restored only partly the transformability of mutant cells, implying an alternate role for Sca proteins in the transformation process. Cells of sca mutants were unaffected in other binding properties tested and were unaffected in sensitivity to oxidants. The results show that Sca permease is a high-affinity mechanism for the acquisition of Mn2+ and is essential for growth of streptococci under Mn2+-limiting conditions.

Induction of superoxide dismutase synthesis in Humicola lutea 110 by pentachlorophenol

Pentachlorophenol (PCP) caused a rapid and pronounced increase in the rate of biosynthesis of the superoxide dismutase (SOD) in fungal strain Humicola lutea 110. Mn-containing SOD (Mn-SOD) was mainly responsible for modulating total cell SOD. The kinetics of SOD synthesis in the presence of PCP demonstrated the induction model of enzyme formation. This model was also supported by deinduction experiments, because the removal of the PCP was followed by a marked decrease in SOD activity. PCP also caused a moderate induction of catalase. The concentrations, which were effective in inducing the Mn-SOD, increased the cyanide-resistant respiration. It seems likely that PCP increased the rate of intracellular production of superoxide [Formula: see text]. Addition of inhibitors of transcription and translation to cultures in the presence of inducer (PCP) inhibited further accumulation of SOD activity. These data suggest that PCP, probably by the increase of [Formula: see text] content, accelerates new enzyme synthesis in fungal strain Humicola lutea 110.Key words: superoxide dismutase, superoxide, induction, pentachlorophenol, fungi, Humicola lutea.

Differential regulation of the leukotoxin operon in highly leukotoxic and minimally leukotoxic strains of Actinobacillus actinomycetemcomitans

The expression of the leukotoxin (ltx) operon varies significantly among Actinobacillus actinomycetemcomitans strains. The dual promoters driving ltx expression in the highly toxic strain JP2 have been previously characterized (J. M. Brogan, E. T. Lally, K. Poulsen, M. Kilian, and D. R. Demuth, Infect. Immun. 62:501-508, 1994), and genetic analyses of A. actinomycetemcomitans suggest that highly toxic strains like JP2 arose from minimally toxic strains, presumably by deletion of a 530-bp domain within the ltx promoter region (K. Poulsen, E. Theilade, E.T. Lally, D. R. Demuth, and M. Kilian, Microbiology 140:2049-2060, 1994). However, the ltx promoter of minimally toxic A. actinomycetemcomitans strains has not been well characterized. In this study, deletion and primer extension analyses showed that the ltx promoter of A. actinomycetemcomitans 652 is situated approximately 150 bp upstream of the ltxC gene and initiates transcription 138 nucleotides upstream of ltxC. In contrast to strain JP2, only a single promoter appears to drive ltx expression in 652. The 652 promoter resides within the 530-bp region that is absent from the JP2 promoter sequence, suggesting that the specific sequences controlling ltx expression differ in highly toxic and minimally toxic A. actinomycetemcomitans strains. In addition, ltx expression in strain 652 was shown to be induced three- to fourfold when cells were grown under anaerobic conditions. The induction of whole cell leukotoxicity, was accompanied by increases in the levels of Ltx polypeptide and the steady-state levels of ltx mRNA, suggesting that regulation occurred at the level of transcription. In contrast, the levels of leukotoxicity, Ltx polypeptide, and fix mRNA in strain JP2 were unaffected by anaerobic growth. These results suggest that the ltx operon is differentially regulated in highly toxic and minimally toxic A. actinomycetemcomitans strains and that the sequences controlling the oxygen-dependent regulation of ltx expression may reside within the 530-bp domain that is not present in highly toxic A. actinomycetemcomitans.

Cloning and expression of the Mycobacterium fortuitum superoxide dismutase gene

In this paper we report the cloning, sequencing and expression of the superoxide dismutase (sod) gene from Mycobacterium fortuitum. A single gene was found to code for superoxide dismutase activity with its identity being confirmed by expression in M. aurum. The amino acid sequence was found to be similar to that of superoxide dismutases of several other origins. A region downstream of the sod gene also showed similarities to the corresponding sequences of the two main mycobacterial pathogens: M. leprae and M. tuberculosis. Analysis of enzymatic activity showed this enzyme in M. fortuitum required manganese as cofactor.

Stress response in Lactococcus lactis: cloning, expression analysis, and mutation of the lactococcal superoxide dismutase gene

In an analysis of the stress response of Lactococcus lactis, three proteins that were induced under low pH culture conditions were detected. One of these was identified as the lactococcal superoxide dismutase (SodA) by N-terminal amino acid sequence analysis. The gene encoding this protein, designated sodA, was cloned by the complementation of a sodA sodB Escherichia coli strain. The deduced amino acid sequence of L. lactis SodA showed the highest degree of similarity to the manganese-containing Sod (MnSod) of Bacillus stearothermophilus. A promoter upstream of the sodA gene was identified by primer extension analysis, and an inverted repeat surrounding the -35 hexanucleotide of this promoter is possibly involved in the regulation of the expression of sodA. The expression of sodA was analyzed by transcriptional fusions with a promoterless lacZ gene. The induction of beta-galactosidase activity occurred in aerated cultures. Deletion experiments revealed that a DNA fragment of more than 130 bp surrounding the promoter was needed for the induction of lacZ expression by aeration. The growth rate of an insertion mutant of sodA did not differ from that of the wild type in standing cultures but was decreased in aerated cultures.