Dps proteins, an efficient detoxification and DNA protection machinery in the bacterial response to oxidative stress

The proteins belonging to the Dps (DNA-binding proteins from starved cells) family play an important role within the bacterial defence system against oxidative stress. They act on Fe(II) and hydrogen peroxide that are potentially toxic in the presence of air. Fe(II) forms spontaneously insoluble Fe(III) and reacts with molecular oxygen or its reduced forms to yield the highly damaging hydroxyl radicals. All Dps proteins have the distinctive capacity to annul the toxic combination of iron and hydrogen peroxide as they use the latter compound to oxidise Fe(II). In addition to this intrinsic DNA protection capacity, several members of the family, including the archetypical Escherichia coli Dps, protect DNA physically by shielding it in large Dps-DNA complexes. The structural and functional characteristics that endow Dps proteins with the chemical and physical protection mechanism are presented and discussed also in the framework of the varied situations that may be encountered in different bacterial species.      

The neutrophil-activating Dps protein of Helicobacter pylori, HP-NAP, adopts a mechanism different from Escherichia coli Dps to bind and condense DNA

The Helicobacter pylori neutrophil-activating protein (HP-NAP), a member of the Dps family, is a fundamental virulence factor involved in H.pylori-associated disease. Dps proteins protect bacterial DNA from oxidizing radicals generated by the Fenton reaction and also from various other damaging agents. DNA protection has a chemical component based on the highly conserved ferroxidase activity of Dps proteins, and a physical one based on the capacity of those Dps proteins that contain a positively charged N-terminus to bind and condense DNA. HP-NAP does not possess a positively charged N-terminus but, unlike the other members of the family, is characterized by a positively charged protein surface. To establish whether this distinctive property could be exploited to bind DNA, gel shift, fluorescence quenching and atomic force microscopy (AFM) experiments were performed over the pH range 6.5–8.5. HP-NAP does not self-aggregate in contrast to Escherichia coli Dps, but is able to bind and even condense DNA at slightly acid pH values. The DNA condensation capacity acts in concert with the ferritin-like activity and could be used to advantage by H.pylori to survive during host-infection and other stress challenges. A model for DNA binding/condensation is proposed that accounts for all the experimental observations.     

The multifaceted capacity of Dps proteins to combat bacterial stress conditions: Detoxification of iron and hydrogen peroxide and DNA binding

Background

The widely expressed Dps proteins, so named after the DNA-binding properties of the first characterized member of the family in Escherichia coli, are considered major players in the bacterial response to stress.

Scope of review

The review describes the distinctive features of the “ferritin-like” ferroxidation reaction, which uses hydrogen peroxide as physiological iron oxidant and therefore permits the concomitant removal of the two reactants that give rise to hydroxyl radicals via Fenton chemistry. It also illustrates the structural elements identified to date that render the interaction of some Dps proteins with DNA possible and outlines briefly the significance of Dps–DNA complex formation and of the Dps interaction with other DNA-binding proteins in relation to the organization of the nucleoid and microbial survival.

General significance

Understanding in molecular terms the distinctive role of Dps proteins in bacterial resistance to general and specific stress conditions.

Major conclusions

The state of the art is that the response to oxidative and peroxide-mediated stress is mediated directly by Dps proteins via their ferritin-like activity. In contrast, the response to other stress conditions derives from the concerted interplay of diverse interactions that Dps proteins may establish with DNA and with other DNA-binding proteins.     

No significant paraquat-induced oxidative DNA damage in rats

The metabolism of paraquat generates oxygen radicals. Paraquat has thus been suggested as a model compound to induce oxidative damage to DNA, lipids and proteins in different cells and tissues, although experimental data are inconsistent. In order to explore the possibilities for an animal model of oxidative DNA damage in vivo, rats were treated with 20 mg/kg paraquat or vehicle i.p. One and five days later we measured DNA oxidation in terms of 7-hydro-8-oxo-2′-deoxyguanosine (8-oxodG) in the liver and lung as well as the urinary excretion of 8-oxodG. No significant effects on the level of 8-oxodG in the liver, the lung or the urinary excretion, could be distinguished following paraquat treatment. We found, however, a significant correlation (r=0.69; p<0.0002) between the 8-oxodG level in the lung and the urinary excretion, but no significant correlation between the level in the liver and the urinary excretion or between the levels in the liver and the lung. During the experiment the rats were clearly affected by the paraquat as they were very lethargic compared to the controls. Accordingly, even at toxic doses, paraquat did not cause detectable oxidative damage to DNA. The data do not support the use of paraquat as a model compound in experiments investigating effects or prevention of oxidative damage to DNA.     

Nuclear aggregates of polyamines in a radiation-induced DNA damage model

Polyamines (PA) are believed to protect DNA minimizing the effect of radiation damage either by inducing DNA compaction and aggregation or acting as scavengers of free radicals. Using an in vitro pDNA double strand breakage assay based on gel electrophoretic mobility, we compared the protective capability of PA against γ-radiation with that of compounds generated by the supramolecular self-assembly of nuclear polyamines and phosphates, named Nuclear Aggregates of Polyamines (NAPs). Both unassembled PA and in vitro produced NAPs (ivNAPs) were ineffective in conferring pDNA protection at the sub-mM concentration. Single PA showed an appreciable protective effect only at high (mM) concentrations. However, concentrations of spermine (4+) within a critical range (0.481 mM) induced pDNA precipitation, an event that was not observed with NAPs–pDNA interaction. We conclude that the interaction of individual PA is ineffective to assure DNA protection, simultaneously preserving the flexibility and charge density of the double strand. Furthermore, data obtained by testing polyamine and ivNAPS with the current radiation-induced DNA damage model support the concept that PA-phosphate aggregates are the only forms through which PA interact with DNA.     

Oxidative DNA damage in cultured fibroblasts from patients with hereditary glutathione synthetase deficiency

The SH compound glutathione (GSH) is involved in several fundamental functions in the cell, including protection against reactive oxygen species (ROS). Here, we studied the effect on oxidative DNA damage in cultured skin fibroblasts from patients with hereditary GSH synthetase deficiency. Our hypothesis was that GSH-deficient cells are more prone to DNA damage than control cells. Single cell gel electrophoresis (the comet assay) in combination with the formamidopyrimidine DNA glycosylase enzyme, which recognizes oxidative base modifications, was used on cultured fibroblasts from 11 patients with GSH synthetase deficiency and five control subjects. Contrary to this hypothesis, we found no significant difference in background levels of DNA damage between cells from patients and control subjects. To study the induction of oxidative DNA damage without simultaneous DNA repair, the cells were γ-irradiated on ice and DNA single-strand breaks measured. The patient and control cells were equally sensitive to induction of single strand breaks by γ-irradiation. Therefore, factors other than GSH protect DNA from oxidative damage. However, cells with a high background level of oxidative DNA damage were found to be more sensitive to ionizing radiation. This suggests that differences in background levels of oxidative DNA damage may depend on the cells' intrinsic protection against induction of oxidative damage.     

Structure, function and regulation of the DNA-binding protein Dps and its role in acid and oxidative stress resistance in Escherichia coli: a review

Dps, the DNA‐binding protein from starved cells, is capable of providing protection to cells during exposure to severe environmental assaults; including oxidative stress and nutritional deprivation. The structure and function of Dps have been the subject of numerous studies and have been examined in several bacteria that possess Dps or a structural/functional homologue of the protein. Additionally, the involvement of Dps in stress resistance has been researched extensively as well. The ability of Dps to provide multifaceted protection is based on three intrinsic properties of the protein: DNA binding, iron sequestration, and its ferroxidase activity. These properties also make Dps extremely important in iron and hydrogen peroxide detoxification and acid resistance as well. Regulation of Dps expression in E. coli is complex and partially dependent on the physiological state of the cell. Furthermore, it is proposed that Dps itself plays a role in gene regulation during starvation, ultimately making the cell more resistant to cytotoxic assaults by controlling the expression of genes necessary for (or deleterious to) stress resistance. The current review focuses on the aforementioned properties of Dps in E. coli, its prototypic organism. The consequences of elucidating the protective mechanisms of this protein are far‐reaching, as Dps homologues have been identified in over 1000 distantly related bacteria and Archaea. Moreover, the prevalence of Dps and Dps‐like proteins in bacteria suggests that protection involving DNA and iron sequestration is crucial and widespread in prokaryotes.     

Dps proteins prevent Fenton-mediated oxidative damage by trapping hydroxyl radicals within the protein shell

Dps (DNA-binding proteins from starved cells) proteins belong to a widespread bacterial family of proteins expressed under nutritional and oxidative stress conditions. In particular, Dps proteins protect DNA against Fenton-mediated oxidative stress, as they catalyze iron oxidation by hydrogen peroxide at highly conserved ferroxidase centers and thus reduce significantly hydroxyl radical production. This work investigates the possible generation of intraprotein radicals during the ferroxidation reaction by Escherichia coli and Listeria innocua Dps, two representative members of the family. Stopped-flow analyses show that the conserved tryptophan and tyrosine residues located near the metal binding/oxidation center are in a radical form after iron oxidation by hydrogen peroxide. DNA protection assays indicate that the presence of both residues is necessary to limit release of hydroxyl radicals in solution and the consequent oxidative damage to DNA. In general terms, the demonstration that conserved protein residues act as a trap that dissipates free electrons generated during the oxidative process brings out a novel role for the Dps protein cage.     

Proteins associated with mitochondrial DNA protect it against X-rays and hydrogen peroxide

The high susceptibility of mitochondrial DNA to reactive oxygen species and other damaging agents has been supposed to result from the absence of histones. Here we show that DNA-binding proteins of mitochondrial nucleoids can shield mtDNA from X-ray radiation and hydrogen peroxide just as nuclear histones do. Mitochondria, mitochondrial nucleoid proteins, and histones were isolated from mouse liver and assessed for mtDNA protection by the yield of PCR products. In vitro, mtDNA in complex either with nucleoid proteins or with nuclear histones proved to be much less damaged than naked mtDNA, with little difference in protective efficacy. Most probably, in mitochondria the nucleoid proteins also protect mtDNA against reactive oxygen species and thus attenuate the oxidative damage.     

Assaying DNA Damage in Hippocampal Neurons Using the Comet Assay

A number of drugs target the DNA repair pathways and induce cell kill by creating DNA damage. Thus, processes to directly measure DNA damage have been extensively evaluated. Traditional methods are time consuming, expensive, resource intensive and require replicating cells. In contrast, the comet assay, a single cell gel electrophoresis assay, is a faster, non-invasive, inexpensive, direct and sensitive measure of DNA damage and repair. All forms of DNA damage as well as DNA repair can be visualized at the single cell level using this powerful technique.The principle underlying the comet assay is that intact DNA is highly ordered whereas DNA damage disrupts this organization. The damaged DNA seeps into the agarose matrix and when subjected to an electric field, the negatively charged DNA migrates towards the cathode which is positively charged. The large undamaged DNA strands are not able to migrate far from the nucleus. DNA damage creates smaller DNA fragments which travel farther than the intact DNA. Comet Assay, an image analysis software, measures and compares the overall fluorescent intensity of the DNA in the nucleus with DNA that has migrated out of the nucleus. Fluorescent signal from the migrated DNA is proportional to DNA damage. Longer brighter DNA tail signifies increased DNA damage. Some of the parameters that are measured are tail moment which is a measure of both the amount of DNA and distribution of DNA in the tail, tail length and percentage of DNA in the tail. This assay allows to measure DNA repair as well since resolution of DNA damage signifies repair has taken place. The limit of sensitivity is approximately 50 strand breaks per diploid mammalian cell ^1,2. Cells treated with any DNA damaging agents, such as etoposide, may be used as a positive control. Thus the comet assay is a quick and effective procedure to measure DNA damage.     

Mutagenicity and repair of oxidative DNA damage: insights from studies using defined lesions

Oxidative DNA damage has been implicated in mutagenesis, carcinogenesis and aging. Endogenous cellular processes such as aerobic metabolism generate reactive oxygen species (ROS) that interact with DNA to form dozens of DNA lesions. If unrepaired, these lesions can exert a number of deleterious effects including the induction of mutations. In an effort to understand the genetic consequences of cellular oxidative damage, many laboratories have determined the patterns of mutations generated by the interaction of ROS with DNA. Compilation of these mutational spectra has revealed that GC → AT transitions and GC → TA transversions are the most commonly observed mutations resulting from oxidative damage to DNA. Since mutational spectra convey only the end result of a complex cascade of events, which includes formation of multiple adducts, repair processing, and polymerase errors, it is difficult if not impossible to asses the mutational specificity of individual DNA lesions directly from these spectra. This problem is especially complicated in the case of oxidative DNA damage owing to the multiplicity of lesions formed by a single damaging agent. The task of assigning specific features of mutational spectra to individual DNA lesions has been made possible with the advent of a technology to analyze the mutational properties of single defined adducts, in vitro and in vivo. At the same time, parallel progress in the discovery and cloning of repair enzymes has advanced understanding of the biochemical mechanisms by which cells excise DNA damage. This combination of tools has brought our understanding of DNA lesions to a new level of sophistication. In this review, we summarize the known properties of individual oxidative lesions in terms of their structure, mutagenicity and repairability.     

Effect of gliclazide on DNA damage in human peripheral blood lymphocytes and insulinoma mouse cells

Type 2 diabetes mellitus is associated with increased oxidative stress. Free radicals produced during this stress may damage various cellular components. Gliclazide, a second-generation sulfonylurea, is an oral hypoglycemic drug that possesses antioxidant properties. Therefore, gliclazide may diminish the harmful consequences of oxidative stress in diabetic patients. The aim of our study was to evaluate the action of gliclazide on DNA damage and repair in normal human peripheral blood lymphocytes and insulinoma mouse cells (beta-TC-6). DNA damage and repair were induced by hydrogen peroxide, gamma and ultraviolet radiation and MNNG (N-methyl-N'-nitro-N-nitrosoguanidine) in the presence or absence of gliclazide and were analysed by the alkaline comet assay. DNA double-strand breaks were assayed by pulsed-field gel electrophoresis. Gliclazide protected DNA of both kinds of cells from DNA damage induced by chemicals and radiations. These results suggest that gliclazide may diminish the risk of free radical-related diseases associated with type 2 diabetes mellitus and possibly cancer.     

DNA Strand Breaks in Mitotic Germ Cells of Caenorhabditis elegans Evaluated by Comet Assay

DNA damage responses are important for the maintenance of genome stability and the survival of organisms. Such responses are activated in the presence of DNA damage and lead to cell cycle arrest, apoptosis, and DNA repair. In Caenorhabditis elegans, double-strand breaks induced by DNA damaging agents have been detected indirectly by antibodies against DSB recognizing proteins. In this study we used a comet assay to detect DNA strand breaks and to measure the elimination of DNA strand breaks in mitotic germline nuclei of C. elegans. We found that C. elegans brc-1 mutants were more sensitive to ionizing radiation and camptothecin than the N2 wild-type strain and repaired DNA strand breaks less efficiently than N2. This study is the first demonstration of direct measurement of DNA strand breaks in mitotic germline nuclei of C. elegans. This newly developed assay can be applied to detect DNA strand breaks in different C. elegans mutants that are sensitive to DNA damaging agents.     

Sodium selenite,dietary micronutrient,prevents the lymphocyte DNA damage induced by N-nitrosodiethylamine and phenobarbital promoted experimental hepatocarcinogenesis

Selenium (Se), a micronutrient, has a long history in chemoprevention of mammary and colon cancers in rodent models. Se is a current clinical trial, having shown promise in prevention of prostate and other human cancers. The mechanisms involved in the in vivo anti-carcinogenic activity of Se remain to be elucidated. In the present study, we examined the effect of sodium selenite supplementation in lymphocytes, obtained from hepatoma bearing rats on DNA damage in correlation with oxidative stress. In addition, this study examined the supplementation of Se at 4-ppm levels in the form of sodium selenite either before initiation or during initiation and/or promotion phase's increases lymphocyte Se concentrations. This in turn improves lymphocyte resistance to oxidative stress and protection against the lymphocytes DNA damage. Supplementation of Se increased lymphocyte Se concentration and reduced lymphocytes DNA damage as determined by single cell gel electrophoresis. The enzymatic antioxidants such as superoxide dismutase, glutathione peroxidase, and catalase were found to be decreased while the thiobarbituric acid reactive substances level was increased in the lymphocytes of hepatoma bearing rats. Furthermore, the reactive oxygen species such as superoxide radicals and hydroxyl radicals were also found to be high in lymphocytes. Our present results explain the understanding of unique association between anti-peroxidative effect of Se and ultimately the capability of Se to prevent cancer.     

CUX2 Protein Functions as an Accessory Factor in the Repair of Oxidative DNA Damage

CUX1 and CUX2 proteins are characterized by the presence of three highly similar regions called Cut repeats 1, 2, and 3. Although CUX1 is ubiquitously expressed, CUX2 plays an important role in the specification of neuronal cells and continues to be expressed in postmitotic neurons. Cut repeats from the CUX1 protein were recently shown to stimulate 8-oxoguanine DNA glycosylase 1 (OGG1), an enzyme that removes oxidized purines from DNA and introduces a single strand break through its apurinic/apyrimidinic lyase activity to initiate base excision repair. Here, we investigated whether CUX2 plays a similar role in the repair of oxidative DNA damage. Cux2 knockdown in embryonic cortical neurons increased levels of oxidative DNA damage. In vitro, Cut repeats from CUX2 increased the binding of OGG1 to 7,8-dihydro-8-oxoguanine-containing DNA and stimulated both the glycosylase and apurinic/apyrimidinic lyase activities of OGG1. Genetic inactivation in mouse embryo fibroblasts or CUX2 knockdown in HCC38 cells delayed DNA repair and increased DNA damage. Conversely, ectopic expression of Cut repeats from CUX2 accelerated DNA repair and reduced levels of oxidative DNA damage. These results demonstrate that CUX2 functions as an accessory factor that stimulates the repair of oxidative DNA damage. Neurons produce a high level of reactive oxygen species because of their dependence on aerobic oxidation of glucose as their source of energy. Our results suggest that the persistent expression of CUX2 in postmitotic neurons contributes to the maintenance of genome integrity through its stimulation of oxidative DNA damage repair.     

DNA protecting and genotoxic effects of olive oil related components in cells exposed to hydrogen peroxide

In search for compounds, able to protect nuclear DNA in cells exposed to oxidative stress, extracts from olive leaves, olive fruits, olive oil and olive mill waste water were tested by using the “single cell gel electrophoresis” methodology (comet assay). Jurkat cells in culture were exposed to continuously generated hydrogen peroxide (11.8±1.5 μM per min) by direct addition into the growth medium of the appropriate amount of the enzyme “glucose oxidase” in the presence or absence of the tested total extracts. The protective effects of the tested extracts or isolated compounds were evaluated from their ability to decrease hydrogen peroxide-induced formation of single strand breaks in the nuclear DNA, while the toxic effects were estimated from the increase of DNA damage when the extracts or isolated compounds were incubated directly with the cells. Significant protection was observed in extracts from olive oil and olive mill waste water. However, above a concentration of 100 μg/ml olive oil extracts exerted DNA damaging effects by themselves in the absence of any H₂O₂. Extracts from olive leaves and olive fruits although protective, were also able to induce DNA damage by themselves. Main compounds isolated from the above described total extracts, like oleuropein glucoside, tyrosol, hydroxytyrosol and caffeic acid, were tested in the same experimental system and found to exert cytotoxic (oleuropein glucoside), no effect (tyrosol) or protective effects (hydroxytyrosol and caffeic acid). In conclusion, cytoprotective as well as cytotoxic compounds with potential pharmaceutical properties were detected in extracts from olive oil related sources by using the comet assay methodology.     

The Dps protein of Agrobacterium tumefaciens does not bind to DNA but protects it toward oxidative cleavage: x-ray crystal structure,iron binding,and hydroxyl-radical scavenging properties

Agrobacterium tumefaciens Dps (DNA-binding proteins from starved cells), encoded by the dps gene located on the circular chromosome of this plant pathogen, was cloned, and its structural and functional properties were determined in vitro. In Escherichia coli Dps, the family prototype, the DNA binding properties are thought to be associated with the presence of the lysine-containing N-terminal tail that extends from the protein surface into the solvent. The x-ray crystal structure of A. tumefaciens Dps shows that the positively charged N-terminal tail, which is 11 amino acids shorter than in the E. coli protein, is blocked onto the protein surface. This feature accounts for the lack of interaction with DNA. The intersubunit ferroxidase center characteristic of Dps proteins is conserved and confers to the A. tumefaciens protein a ferritin-like activity that manifests itself in the capacity to oxidize and incorporate iron in the internal cavity and to release it after reduction. In turn, sequestration of Fe(II) correlates with the capacity of A. tumefaciens Dps to reduce the production of hydroxyl radicals from H2O2 through Fenton chemistry. These data demonstrate conclusively that DNA protection from oxidative damage in vitro does not require formation of a Dps-DNA complex. In vivo, the hydroxyl radical scavenging activity of A. tumefaciens Dps may be envisaged to act in concert with catalase A to counteract the toxic effect of H2O2, the major component of the plant defense system when challenged by the bacterium.     

Oxidative stress status of highly prolific sows during gestation and lactation

Elevated oxidative stress is reported to be associated with pregnancy complications in highly prolific sows. Oxidative DNA damage and the antioxidant status were determined in blood samples collected during the course of gestation and lactation in multiparous sows. Blood samples were drawn from sows (n = 5) on days 30, 60, 90 and 110 of gestation (G30, G60, G90 and G110, respectively), on day 3, 10 and 18 of lactation (L3, L10 and L18, respectively) and on day 5 of postweaning (W5). Lymphocytes were isolated from the fresh blood and cryopreserved in each time point. Lymphocyte DNA damage was analyzed by alkaline single-cell gel electrophoresis (comet assay) to determine the single- and double-strand brakes and endogenous antioxidant concentrations using an HPLC system with UV detection. The comet assay showed elevated (P < 0.05) DNA damage (between 38% and 47%) throughout the gestational and lactational periods than during early gestation (G30; 21%). Plasma retinol concentration was reduced (P < 0.05) at the end of gestation (G110) compared with G30. Plasma α-tocopherol concentrations also showed a similar trend as to retinol. This study indicates that there is an increased systemic oxidative stress during late gestation and lactation, which are not fully recovered until the weaning compared with the G30, and that antioxidant nutrients in circulation substantially reduced in the mother pig at G110.     

Metformin does not prevent DNA damage in lymphocytes despite its antioxidant properties against cumene hydroperoxide-induced oxidative stress

Metformin (1-(diaminomethylidene)-3,3-dimethyl-guanidine), which is the most commonly prescribed oral antihyperglycaemic drug in the world, was reported to have several antioxidant properties such as the inhibition of advanced glycation end-products. In addition to its use in the treatment of diabetes, it has been suggested that metformin may be a promising anti-aging agent. The present work was aimed at assessing the possible protective effects of metformin against DNA-damage induction by oxidative stress in vitro. The effects of metformin were compared with those of N-acetylcysteine (NAC). For this purpose, peripheral blood lymphocytes from aged (n = 10) and young (n = 10) individuals were pre-incubated with various concentrations of metformin (10–50 μM), followed by incubation with 15 μM cumene hydroperoxide (CumOOH) for 48 h, under conditions of low oxidant level, which do not induce cell death. Protection against oxidative DNA damage was evaluated by use of the Comet assay and the cytokinesis-block micronucleus technique. Changes in the levels of malondialdehyde + 4-hydroxy-alkenals, an index of oxidative stress, were also measured in lymphocytes. At concentrations ranging from 10 μM to 50 μM, metformin did not protect the lymphocytes from DNA damage, while 50 μM NAC possessed an effective protective effect against CumOOH-induced DNA damage. Furthermore, NAC, but not metformin, inhibited DNA fragmentation induced by CumOOH. In contrast to the lack of protection against oxidative damage in lymphocyte cultures, metformin significantly protected the cells from lipid peroxidation in both age groups, although not as effective as NAC in preventing the peroxidative damage at the highest doses. Within the limitations of this study, the results indicate that pharmacological concentrations of metformin are unable to protect against DNA damage induced by a pro-oxidant stimulus in cultured human lymphocytes, despite its antioxidant properties.     

Expression and mutagenesis of the dpsA gene of Synechococcus sp. PCC7942, encoding a DNA-binding protein involved in oxidative stress protection

The Dps family of proteins are a diverse group of bacterial stress-inducible polypeptides that bind DNA and likely confer resistance to peroxide damage during periods of oxidative stress and long-term nutrient limitation. Some members of the Dps protein family have been shown to form abundant, large (∼150 kD) hexameric complexes that bind chromosomal DNA with little sequence specificity. Previous work from this lab has demonstrated that the Dps proteins are divergent members of the bacterioferritin/bacterioferritin superfamily, and that the Synechococcus sp. PCC7942 Dps homolog, named DpsA, is a DNA-binding hemoprotein having heme-dependent catalytic activity. We speculated that this protein may yield a peroxide-consuming mechanism located on the chromosomal DNA, and we also suggested that this activity may be a necessary feature to handle the endogenous oxidative stresses associated with oxygenic photosynthesis. Current work has examined the expression of dpsA both under nutrient stress and during the growth phase; whereas dpsA mRNA is detectable in the exponential phase, transition to stationary phase yields a 20-fold increase in steady-state mRNA levels. Mapping the promoter region identifies a TAGAAT −10 sequence likely recognized by a cyanobacterial RpoS homolog. Lastly, site-directed mutants lacking dpsA function exhibit a severe phenotype impaired under all conditions yielding photooxidative stress; these include high light and treatment with paraquat. This supports our contention that the DpsA protein serves an important protective function in an obligate photoautotroph.