DNA protection mechanisms are not involved in the radioresistance of the hyperthermophilic archaea Pyrococcus abyssi and P. furiosus

Antioxidant defense of Deinococcus radiodurans: how does it contribute to extreme radiation resistance?

PURPOSE

Deinococcus radiodurans is an extremely radioresistant bacterium characterized by D10 of 10 kGy, and able to grow luxuriantly under chronic ionizing radiation of 60 Gy/h. The aim of this article is to review the antioxidant system of D. radiodurans and its possible role in the unusual resistance of this bacterium to ionizing radiation.

CONCLUSIONS

The unusual radiation resistance of D. radiodurans has apparently evolved as a side effect of the adaptation of this extremophile to other damaging environmental factors, especially desiccation. The antioxidant proteins and low-molecular antioxidants (especially low-molecular weight Mn2+ complexes and carotenoids, in particular, deinoxanthin), as well as protein and non-protein regulators, are important for the antioxidant defense of this species. Antioxidant protection of proteins from radiation inactivation enables the repair of DNA damage caused by ionizing radiation.

Archaea: A Gold Mine for Topoisomerase Diversity

The control of DNA topology is a prerequisite for all the DNA transactions such as DNA replication, repair, recombination, and transcription. This global control is carried out by essential enzymes, named DNA-topoisomerases, that are mandatory for the genome stability. Since many decades, the Archaea provide a significant panel of new types of topoisomerases such as the reverse gyrase, the type IIB or the type IC. These more or less recent discoveries largely contributed to change the understanding of the role of the DNA topoisomerases in all the living world. Despite their very different life styles, Archaea share a quasi-homogeneous set of DNA-topoisomerases, except thermophilic organisms that possess at least one reverse gyrase that is considered a marker of the thermophily. Here, we discuss the effect of the life style of Archaea on DNA structure and topology and then we review the content of these essential enzymes within all the archaeal diversity based on complete sequenced genomes available. Finally, we discuss their roles, in particular in the processes involved in both the archaeal adaptation and the preservation of the genome stability.

Comparative Proteomics Analysis Reveals New Features of the Oxidative Stress Response in the Polyextremophilic Bacterium Deinococcus radiodurans

Deinococcus radiodurans is known for its extreme resistance to ionizing radiation, oxidative stress, and other DNA-damaging agents. The robustness of this bacterium primarily originates from its strong oxidative resistance mechanisms. Hundreds of genes have been demonstrated to contribute to oxidative resistance in D. radiodurans; however, the antioxidant mechanisms have not been fully characterized. In this study, comparative proteomics analysis of D. radiodurans grown under normal and oxidative stress conditions was conducted using label-free quantitative proteomics. The abundances of 852 of 1700 proteins were found to significantly differ between the two groups. These differential proteins are mainly associated with translation, DNA repair and recombination, response to stresses, transcription, and cell wall organization. Highly upregulated expression was observed for ribosomal proteins such as RplB, Rpsl, RpsR, DNA damage response proteins (DdrA, DdrB), DNA repair proteins (RecN, RecA), and transcriptional regulators (members of TetR, AsnC, and GntR families, DdrI). The functional analysis of proteins in response to oxidative stress is discussed in detail. This study reveals the global protein expression profile of D. radiodurans in response to oxidative stress and provides new insights into the regulatory mechanism of oxidative resistance in D. radiodurans.

Antioxidative system of Deinococcus radiodurans

Deinococcus radiodurans is famous for its extreme resistance to various stresses such as ionizing radiation (IR), desiccation and oxidative stress. The underlying mechanism of exceptional resistance of this robust bacterium still remained unclear. However, the antioxidative system of D. radiodurans has been considered to be the determinant factor for its unparalleled resistance and protects the proteome during stress, then the DNA repair system and metabolic system exert their functions to restore the cell to normal physiological state. The antioxidative system not only equipped with the common reactive oxygen species (ROS) scavenging enzymes (e.g., catalase and superoxide dismutase) but also armed with a variety of non-enzyme antioxidants (e.g., carotenoids and manganese species). And the small manganese complexes play an important role in the antioxidative system of D. radiodurans. Recent studies have characterized several regulators (e.g., PprI and PprM) in D. radiodurans, which play critical roles in the protection of the bacteria from various stresses. In this review, we offer a panorama of the progress regarding the characteristics of the antioxidative system in D. radiodurans and its application in the future.

N 4-Cytosine DNA Methylation Is Involved in the Maintenance of Genomic Stability in Deinococcus radiodurans

DNA methylation serves as a vital component of restriction-modification (R-M) systems in bacteria, where it plays a crucial role in defense against foreign DNA. Recent studies revealed that DNA methylation has a global impact on gene expression. Deinococcus radiodurans, an ideal model organism for studying DNA repair and genomic stability, possesses unparalleled resistance to DNA-damaging agents such as irradiation and strong oxidation. However, details on the methylome of this bacterium remain unclear. Here, we demonstrate that N ⁴-cytosine is the major methylated form (4mC) in D. radiodurans. A novel methylated motif, "C^4mCGCGG" was identified that was fully attributed to M.DraR1 methyltransferase. M.DraR1 can specifically bind and methylate the second cytosine at N ⁴ atom of "CCGCGG" motif, preventing its digestion by a cognate restriction endonuclease. Cells deficient in 4mC modification displayed higher spontaneous rifampin mutation frequency and enhanced DNA recombination and transformation efficiency. And genes involved in the maintenance of genomic stability were differentially expressed in conjunction with the loss of M.DraR1. This study provides evidence that N ⁴-cytosine DNA methylation contributes to genomic stability of D. radiodurans and lays the foundation for further research on the mechanisms of epigenetic regulation by R-M systems in bacteria.

Physical and functional interplay between PCNA DNA clamp and Mre11-Rad50 complex from the archaeon Pyrococcus furiosus

Several archaeal species prevalent in extreme environments are particularly exposed to factors likely to cause DNA damages. These include hyperthermophilic archaea (HA), living at temperatures >70°C, which arguably have efficient strategies and robust genome guardians to repair DNA damage threatening their genome integrity. In contrast to Eukarya and other archaea, homologous recombination appears to be a vital pathway in HA, and the Mre11–Rad50 complex exerts a broad influence on the initiation of this DNA damage response process. In a previous study, we identified a physical association between the Proliferating Cell Nuclear Antigen (PCNA) and the Mre11–Rad50 (MR) complex. Here, by performing co-immunoprecipitation and SPR analyses, we identified a short motif in the C- terminal portion of Pyrococcus furiosus Mre11 involved in the interaction with PCNA. Through this work, we revealed a PCNA-interaction motif corresponding to a variation on the PIP motif theme which is conserved among Mre11 sequences of Thermococcale species. Additionally, we demonstrated functional interplay in vitro between P. furiosus PCNA and MR enzymatic functions in the DNA end resection process. At physiological ionic strength, PCNA stimulates MR nuclease activities for DNA end resection and promotes an endonucleolytic incision proximal to the 5′ strand of double strand DNA break.

Review of microbial resistance to chronic ionizing radiation exposure under environmental conditions

Ionizing radiation (IR) produces multiple types of damage to nucleic acids, proteins and other crucial cellular components. Nevertheless, various microorganisms from phylogenetically distant taxa (bacteria, archaea, fungi) can resist IR levels many orders of magnitude above natural background. This intriguing phenomenon of radioresistance probably arose independently many times throughout evolution as a byproduct of selective pressures from other stresses (e.g. desiccation, UV radiation, chemical oxidants). Most of the literature on microbial radioresistance is based on acute γ-irradiation experiments performed in the laboratory, typically involving pure cultures grown under near-optimal conditions. There is much less information about the upper limits of radioresistance in the field, such as in radioactively-contaminated areas, where several radiation types (e.g. α and β, as well as γ) and other stressors (e.g. non-optimal temperature and nutrient levels, toxic chemicals, interspecific competition) act over multiple generations. Here we discuss several examples of radioresistant microbes isolated from extremely radioactive locations (e.g. Chernobyl and Mayak nuclear plant sites) and estimate the radiation dose rates they were able to tolerate. Some of these organisms (e.g. the fungus Cladosporium cladosporioides, the cyanobacterium Geitlerinema amphibium) are widely-distributed and colonize a variety of habitats. These examples suggest that resistance to chronic IR and chemical contamination is not limited to rare specialized strains from extreme environments, but can occur among common microbial taxa, perhaps due to overlap between mechanisms of resistance to IR and other stressors.

Increase of positive supercoiling in a hyperthermophilic archaeon after UV irradiation

Diverse DNA repair mechanisms are essential to all living organisms. Some of the most widespread repair systems allow recovery of genome integrity in the face of UV radiation. Here, we show that the hyperthermophilic archaeon Thermococcus nautili possesses a remarkable ability to recovery from extreme chromosomal damage. Immediately following UV irradiation, chromosomal DNA of T. nautili is fragmented beyond recognition. However, the extensive UV-induced double-stranded breaks (DSB) are repaired over the course of several hours, allowing restoration of growth. DSBs also disrupted plasmid DNA in this species. Similar to the chromosome, plasmid integrity was restored during an outgrowth period. Intriguingly, the topology of recovered pTN1 plasmids differed from control strain by being more positively supercoiled. As reverse gyrase (RG) is the only enzyme capable of inducing positive supercoiling, our results suggest the activation of RG activity by UV-induced stress. We suggest simple UV stress could be used to study archaeal DNA repair and responses to DSB.

Microbial radiation-resistance mechanisms

Organisms living in extreme environments have evolved a wide range of survival strategies by changing biochemical and physiological features depending on their biological niches. Interestingly, organisms exhibiting high radiation resistance have been discovered in the three domains of life (Bacteria, Archaea, and Eukarya), even though a naturally radiationintensive environment has not been found. To counteract the deleterious effects caused by radiation exposure, radiation- resistant organisms employ a series of defensive systems, such as changes in intracellular cation concentration, excellent DNA repair systems, and efficient enzymatic and non-enzymatic antioxidant systems. Here, we overview past and recent findings about radiation-resistance mechanisms in the three domains of life for potential usage of such radiationresistant microbes in the biotechnology industry.

Mechanisms of Stress Resistance and Gene Regulation in the Radioresistant Bacterium Deinococcus radiodurans

The bacterium Deinococcus radiodurans reveals extraordinary resistance to ionizing radiation, oxidative stress, desiccation, and other damaging conditions. In this review, we consider the main molecular mechanisms underlying such resistance, including the action of specific DNA repair and antioxidation systems, and transcription regulation during the anti-stress response.

Unraveling the mechanisms of extreme radioresistance in prokaryotes: Lessons from nature

The last 50 years, a variety of archaea and bacteria able to withstand extremely high doses of ionizing radiation, have been discovered. Several lines of evidence suggest a variety of mechanisms explaining the extreme radioresistance of microorganisms found usually in isolated environments on Earth. These findings are discussed thoroughly in this study. Although none of the strategies discussed here, appear to be universal against ionizing radiation, a general trend was found. There are two cellular mechanisms by which radioresistance is achieved: (a) protection of the proteome and DNA from damage induced by ionizing radiation and (b) recruitment of advanced and highly sophisticated DNA repair mechanisms, in order to reconstruct a fully functional genome. In this review, we critically discuss various protecting (antioxidant enzymes, presence or absence of certain elements, high metal ion or salt concentration etc.) and repair (Homologous Recombination, Single-Strand Annealing, Extended Synthesis-Dependent Strand Annealing) mechanisms that have been proposed to account for the extraordinary abilities of radioresistant organisms and the homologous radioresistance signature genes in these organisms. In addition, and based on structural comparative analysis of major radioresistant organisms, we suggest future directions and how humans could innately improve their resistance to radiation-induced toxicity, based on this knowledge.

A specific proteomic response of Sulfolobus solfataricus P2 to gamma radiations

Sulfolobus solfataricus is an acidophilic hyperthermophilic crenarchaeon living at 80 °C in aerobic conditions. As other thermophilic organisms, S. solfataricus is resistant to gamma irradiation and we studied the response of this microorganism to this ionizing irradiation by monitoring cell growth, DNA integrity and proteome variations. In aerobic conditions, the S. solfataricus genome was fragmented due to the multiple DNA double strand breakages induced by γ-rays and was fully restored within a couple of hours. Comparison of irradiated and unirradiated cell proteomes indicated that only few proteins changed. The proteins identified by mass spectrometry are involved in different cellular pathways including DNA replication, recombination and repair. Interestingly, we observed that some proteins are irradiation dose-specific while others are common to the cell response regardless of the irradiation dose. Most of the proteins highlighted in these conditions seem to act together to allow an efficient cell response to γ-irradiation.

Biology of extreme radiation resistance: the way of Deinococcus radiodurans

The bacterium Deinococcus radiodurans is a champion of extreme radiation resistance that is accounted for by a highly efficient protection against proteome, but not genome, damage. A well-protected functional proteome ensures cell recovery from extensive radiation damage to other cellular constituents by molecular repair and turnover processes, including an efficient repair of disintegrated DNA. Therefore, cell death correlates with radiation-induced protein damage, rather than DNA damage, in both robust and standard species. From the reviewed biology of resistance to radiation and other sources of oxidative damage, we conclude that the impact of protein damage on the maintenance of life has been largely underestimated in biology and medicine.

Oxidative stress resistance in Deinococcus radiodurans

Deinococcus radiodurans is a robust bacterium best known for its capacity to repair massive DNA damage efficiently and accurately. It is extremely resistant to many DNA-damaging agents, including ionizing radiation and UV radiation (100 to 295 nm), desiccation, and mitomycin C, which induce oxidative damage not only to DNA but also to all cellular macromolecules via the production of reactive oxygen species. The extreme resilience of D. radiodurans to oxidative stress is imparted synergistically by an efficient protection of proteins against oxidative stress and an efficient DNA repair mechanism, enhanced by functional redundancies in both systems. D. radiodurans assets for the prevention of and recovery from oxidative stress are extensively reviewed here. Radiation- and desiccation-resistant bacteria such as D. radiodurans have substantially lower protein oxidation levels than do sensitive bacteria but have similar yields of DNA double-strand breaks. These findings challenge the concept of DNA as the primary target of radiation toxicity while advancing protein damage, and the protection of proteins against oxidative damage, as a new paradigm of radiation toxicity and survival. The protection of DNA repair and other proteins against oxidative damage is imparted by enzymatic and nonenzymatic antioxidant defense systems dominated by divalent manganese complexes. Given that oxidative stress caused by the accumulation of reactive oxygen species is associated with aging and cancer, a comprehensive outlook on D. radiodurans strategies of combating oxidative stress may open new avenues for antiaging and anticancer treatments. The study of the antioxidation protection in D. radiodurans is therefore of considerable potential interest for medicine and public health.

Bacterial killing by dry metallic copper surfaces

Metallic copper surfaces rapidly and efficiently kill bacteria. Cells exposed to copper surfaces accumulated large amounts of copper ions, and this copper uptake was faster from dry copper than from moist copper. Cells suffered extensive membrane damage within minutes of exposure to dry copper. Further, cells removed from copper showed loss of cell integrity. Acute contact with metallic copper surfaces did not result in increased mutation rates or DNA lesions. These findings are important first steps for revealing the molecular sensitive targets in cells lethally challenged by exposure to copper surfaces and provide a scientific explanation for the use of copper surfaces as antimicrobial agents for supporting public hygiene.

A model of interactions between radiation-induced oxidative stress, protein and DNA damage in Deinococcus radiodurans

Ionizing radiation triggers oxidative stress, which can have a variety of subtle and profound biological effects. Here we focus on mathematical modeling of potential synergistic interactions between radiation damage to DNA and oxidative stress-induced damage to proteins involved in DNA repair/replication. When sensitive sites on these proteins are attacked by radiation-induced radicals, correct repair of dangerous DNA lesions such as double strand breaks (DSBs) can be compromised. In contrast, if oxidation of important proteins is prevented by strong antioxidant defenses, DNA repair may function more efficiently. These processes probably occur to some extent even at low doses of radiation/oxidative stress, but they are easiest to investigate at high doses, where both DNA and protein damage are extensive. As an example, we use data on survival of Deinococcus radiodurans after high doses (thousands of Gy) of acute and chronic irradiation. Our model of radiogenic oxidative stress is consistent with these data and can potentially be generalized to other organisms and lower radiation doses.

Genome analysis and genome-wide proteomics of Thermococcus gammatolerans, the most radioresistant organism known amongst the Archaea

The genome sequence of Thermococcus gammatolerans, a radioresistant archaeon, is described; a proteomic analysis reveals that radioresistance may be due to unknown DNA repair enzymes.

Background

Thermococcus gammatolerans was isolated from samples collected from hydrothermal chimneys. It is one of the most radioresistant organisms known amongst the Archaea. We report the determination and annotation of its complete genome sequence, its comparison with other Thermococcales genomes, and a proteomic analysis.

Results

T. gammatolerans has a circular chromosome of 2.045 Mbp without any extra-chromosomal elements, coding for 2,157 proteins. A thorough comparative genomics analysis revealed important but unsuspected genome plasticity differences between sequenced Thermococcus and Pyrococcus species that could not be attributed to the presence of specific mobile elements. Two virus-related regions, tgv1 and tgv2, are the only mobile elements identified in this genome. A proteogenome analysis was performed by a shotgun liquid chromatography-tandem mass spectrometry approach, allowing the identification of 10,931 unique peptides corresponding to 951 proteins. This information concurrently validates the accuracy of the genome annotation. Semi-quantification of proteins by spectral count was done on exponential- and stationary-phase cells. Insights into general catabolism, hydrogenase complexes, detoxification systems, and the DNA repair toolbox of this archaeon are revealed through this genome and proteome analysis.

Conclusions

This work is the first archaeal proteome investigation done at the stage of primary genome annotation. This archaeon is shown to use a large variety of metabolic pathways even under a rich medium growth condition. This proteogenomic study also indicates that the high radiotolerance of T. gammatolerans is probably due to proteins that remain to be characterized rather than a larger arsenal of known DNA repair enzymes.

A new perspective on radiation resistance based on Deinococcus radiodurans

In classical models of radiation toxicity, DNA is the molecule that is most affected by ionizing radiation (IR). However, recent data show that the amount of protein damage caused during irradiation of bacteria is better related to survival than to DNA damage. In this Opinion article, a new model is presented in which proteins are the most important target in the hierarchy of macromolecules affected by IR. A first line of defence against IR in extremely radiation-resistant bacteria might be the accumulation of manganese complexes, which can prevent the production of iron-dependent reactive oxygen species. This would allow an irradiated cell to protect sufficient enzymatic activity needed to repair DNA and survive.

Deinococcus radiodurans: what belongs to the survival kit?

Deinococcus radiodurans, one of the most radioresistant organisms known to date, is able to repair efficiently hundreds of DNA double- and single-strand breaks as well as other types of DNA damages promoted by ionizing or ultraviolet radiation. We review recent discoveries concerning several aspects of radioresistance and survival under high genotoxic stress. We discuss different hypotheses and possibilities that have been suggested to contribute to radioresistance and propose that D. radiodurans combines a variety of physiological tools that are tightly coordinated. A complex network of regulatory proteins may be discovered in the near future that might allow further understanding of radioresistance.

Extreme resistance of bdelloid rotifers to ionizing radiation

Rotifers of class Bdelloidea are common invertebrate animals with highly unusual characteristics, including apparently obligate asexuality, the ability to resume reproduction after desiccation at any life stage, and a paucity of transposable genetic elements of types not prone to horizontal transmission. We find that bdelloids are also extraordinarily resistant to ionizing radiation (IR). Reproduction of the bdelloids Adineta vaga and Philodina roseola is much more resistant to IR than that of Euchlanis dilatata, a rotifer belonging to the desiccation-intolerant and facultatively sexual class Monogononta, and all other animals for which we have found relevant data. By analogy with the desiccation- and radiation-resistant bacterium Deinococcus radiodurans, we suggest that the extraordinary radiation resistance of bdelloid rotifers is a consequence of their evolutionary adaptation to survive episodes of desiccation encountered in their characteristic habitats and that the damage incurred in such episodes includes DNA breakage that is repaired upon rehydration. Such breakage and repair may have maintained bdelloid chromosomes as colinear pairs and kept the load of transposable genetic elements low and may also have contributed to the success of bdelloid rotifers in avoiding the early extinction suffered by most asexuals.

The Mre11 protein interacts with both Rad50 and the HerA bipolar helicase and is recruited to DNA following gamma irradiation in the archaeon Sulfolobus acidocaldarius

BACKGROUND

The ubiquitous Rad50 and Mre11 proteins play a key role in many processes involved in the maintenance of genome integrity in Bacteria and Eucarya, but their function in the Archaea is presently unknown. We showed previously that in most hyperthermophilic archaea, rad50-mre11 genes are linked to nurA encoding both a single-strand endonuclease and a 5' to 3' exonuclease, and herA, encoding a bipolar DNA helicase which suggests the involvement of the four proteins in common molecular pathway(s). Since genetic tools for hyperthermophilic archaea are just emerging, we utilized immuno-detection approaches to get the first in vivo data on the role(s) of these proteins in the hyperthermophilic crenarchaeon Sulfolobus acidocaldarius.

RESULTS

We first showed that S. acidocaldarius can repair DNA damage induced by high doses of gamma rays, and we performed a time course analysis of the total levels and sub-cellular partitioning of Rad50, Mre11, HerA and NurA along with the RadA recombinase in both control and irradiated cells. We found that during the exponential phase, all proteins are synthesized and display constant levels, but that all of them exhibit a different sub-cellular partitioning. Following gamma irradiation, both Mre11 and RadA are immediately recruited to DNA and remain DNA-bound in the course of DNA repair. Furthermore, we show by immuno-precipitation assays that Rad50, Mre11 and the HerA helicase interact altogether.

CONCLUSION

Our analyses strongly support that in Sulfolobus acidocaldarius, the Mre11 protein and the RadA recombinase might play an active role in the repair of DNA damage introduced by gamma rays and/or may act as DNA damage sensors. Moreover, our results demonstrate the functional interaction between Mre11, Rad50 and the HerA helicase and suggest that each protein play different roles when acting on its own or in association with its partners. This report provides the first in vivo evidence supporting the implication of the Mre11 protein in DNA repair processes in the Archaea and showing its interaction with both Rad50 and the HerA bipolar helicase. Further studies on the functional interactions between these proteins, the NurA nuclease and the RadA recombinase, will allow us to define their roles and mechanism of action.

Protein oxidation implicated as the primary determinant of bacterial radioresistance

In the hierarchy of cellular targets damaged by ionizing radiation (IR), classical models of radiation toxicity place DNA at the top. Yet, many prokaryotes are killed by doses of IR that cause little DNA damage. Here we have probed the nature of Mn-facilitated IR resistance in Deinococcus radiodurans, which together with other extremely IR-resistant bacteria have high intracellular Mn/Fe concentration ratios compared to IR-sensitive bacteria. For in vitro and in vivo irradiation, we demonstrate a mechanistic link between Mn(II) ions and protection of proteins from oxidative modifications that introduce carbonyl groups. Conditions that inhibited Mn accumulation or Mn redox cycling rendered D. radiodurans radiation sensitive and highly susceptible to protein oxidation. X-ray fluorescence microprobe analysis showed that Mn is globally distributed in D. radiodurans, but Fe is sequestered in a region between dividing cells. For a group of phylogenetically diverse IR-resistant and IR-sensitive wild-type bacteria, our findings support the idea that the degree of resistance is determined by the level of oxidative protein damage caused during irradiation. We present the case that protein, rather than DNA, is the principal target of the biological action of IR in sensitive bacteria, and extreme resistance in Mn-accumulating bacteria is based on protein protection.

One original goal of radiobiology was to explain why cells are so sensitive to ionizing radiation (IR). Early studies in bacteria incriminated DNA as the principal radiosensitive target, an assertion that remains central to modern radiation toxicity models. More recently, the emphasis has shifted to understanding why bacteria such as Deinococcus radiodurans are extremely resistant to IR, by focusing on DNA repair systems expressed during recovery from high doses of IR. Unfortunately, as key features of DNA-centric hypotheses of extreme resistance have grown weaker, the study of alternative cellular targets has lagged far behind, mostly because of their relative biological complexity. Recent studies have shown that extreme levels of bacterial IR resistance correlate with high intracellular Mn(II) concentrations, and resistant and sensitive bacteria are equally susceptible to IR-induced DNA damage. The current work establishes a mechanistic link between Mn(II) and protection of proteins from radiation damage. In contrast to resistant bacteria, naturally sensitive bacteria are highly susceptible to IR-induced protein oxidation. We propose that sensitive bacteria sustain lethal levels of protein damage at radiation doses that elicit relatively little DNA damage, and that extreme resistance in bacteria is dependent on protein protection.

A high intracellular concentration of manganese inDeinococcus radiodurans protects proteins, but not DNA, from ionizing radiation-induced oxidative damage. Protein protection may be critical to the known radiation resistance of these bacteria.

Deinococcus geothermalis: the pool of extreme radiation resistance genes shrinks

Bacteria of the genus Deinococcus are extremely resistant to ionizing radiation (IR), ultraviolet light (UV) and desiccation. The mesophile Deinococcus radiodurans was the first member of this group whose genome was completely sequenced. Analysis of the genome sequence of D. radiodurans, however, failed to identify unique DNA repair systems. To further delineate the genes underlying the resistance phenotypes, we report the whole-genome sequence of a second Deinococcus species, the thermophile Deinococcus geothermalis, which at its optimal growth temperature is as resistant to IR, UV and desiccation as D. radiodurans, and a comparative analysis of the two Deinococcus genomes. Many D. radiodurans genes previously implicated in resistance, but for which no sensitive phenotype was observed upon disruption, are absent in D. geothermalis. In contrast, most D. radiodurans genes whose mutants displayed a radiation-sensitive phenotype in D. radiodurans are conserved in D. geothermalis. Supporting the existence of a Deinococcus radiation response regulon, a common palindromic DNA motif was identified in a conserved set of genes associated with resistance, and a dedicated transcriptional regulator was predicted. We present the case that these two species evolved essentially the same diverse set of gene families, and that the extreme stress-resistance phenotypes of the Deinococcus lineage emerged progressively by amassing cell-cleaning systems from different sources, but not by acquisition of novel DNA repair systems. Our reconstruction of the genomic evolution of the Deinococcus-Thermus phylum indicates that the corresponding set of enzymes proliferated mainly in the common ancestor of Deinococcus. Results of the comparative analysis weaken the arguments for a role of higher-order chromosome alignment structures in resistance; more clearly define and substantially revise downward the number of uncharacterized genes that might participate in DNA repair and contribute to resistance; and strengthen the case for a role in survival of systems involved in manganese and iron homeostasis.

Identification homologous recombination function from haloarchaea plasmid pHH205

Homologous recombination (HR) was found to be so frequent in haloarchaea that its significance in evolution and diversity of this clade of life might have been underestimated. However, so far there has been no report on recombination function carried on plasmid. Here we report that a 4.8-kb SnaBI-PvuII digested segment from pHH205 might carry such a function. Four constructed plasmids: pUN, pUN-205, pUM and pUM-205, with pUN and pUN205 containing Nov(R) gene, pUM and pUM-205 carrying Mev(R) gene, were used to transform Haloferax volcanii DS52 (radA(-)). The results showed that only pUN-205 and pUM-205 containing the 4.8-kb SnaBI-PvuII digested segment from pHH205 were able to shift Nov(R) and Mev(R) gene into the chromosome of Haloferax volcanii DS52 through HR, whereas those in pUN and pUM could not, which indicated that the segment from pHH205 does contain a recombination function.

Modulating radiation resistance: Insights based on defenses against reactive oxygen species in the radioresistant bacterium Deinococcus radiodurans

The classical dogma of radiation biology asserts that the cytotoxic effects of ionizing radiation (IR) are principally the result of DNA damage. Yet many organisms that encode a complement of DNA repair functions are killed by IR doses that cause little DNA damage. Instead, proteins likely are the first major class of molecules damaged by IR. This article presents a new perspective on extreme IR resistance in the eubacterium Deinococcus radiodurans, reevaluates the role of superoxide (02*-) ions in IR toxicity, and speculates on potential strategies for controlling resistance in prokaryotes and eukaryotes based on scavenging IR-induced 02*-.

Microarray analysis of the hyperthermophilic archaeon Pyrococcus furiosus exposed to gamma irradiation

The remarkable survival of the hyperthermophilic archaeon Pyrococcus furiosus to ionizing radiation was previously demonstrated. Using a time course study and whole-genome microarray analyses of mRNA transcript levels, the genes and regulatory pathways involved in the repair of lesions produced by ionizing irradiation (oxidative damage and DNA strand breaks) in P. furiosus were investigated. Data analyses showed that radA, encoding the archaeal homolog of the RecA/Rad51 recombinase, was moderately up regulated by irradiation and that a putative DNA-repair gene cluster was specifically induced by exposure to ionizing radiation. This novel repair system appears to be unique to thermophilic archaea and bacteria and is suspected to be involved in translesion synthesis. Genes that encode for a putative Dps-like iron-chelating protein and two membrane-bound oxidoreductases were differentially expressed following gamma irradiation, potentially in response to oxidative stress. Surprisingly, the many systems involved in oxygen detoxification and redox homeostasis appeared to be constitutively expressed. Finally, we identified several transcriptional regulators and protein kinases highly regulated in response to gamma irradiation.

Deinococcus radiodurans — the consummate survivor

Relatively little is known about the biochemical basis of the capacity of Deinococcus radiodurans to endure the genetic insult that results from exposure to ionizing radiation and can include hundreds of DNA double-strand breaks. However, recent reports indicate that this species compensates for extensive DNA damage through adaptations that allow cells to avoid the potentially detrimental effects of DNA strand breaks. It seems that D. radiodurans uses mechanisms that limit DNA degradation and that restrict the diffusion of DNA fragments that are produced following irradiation, to preserve genetic integrity. These mechanisms also increase the efficiency of the DNA-repair proteins.

Physiological responses of the halophilic archaeon Halobacterium sp. strain NRC1 to desiccation and gamma irradiation

We report that the halophilic archaeon Halobacterium sp. strain NRC-1 is highly resistant to desiccation, high vacuum and 60Co gamma irradiation. Halobacterium sp. was able to repair extensive double strand DNA breaks (DSBs) in its genomic DNA, produced both by desiccation and by gamma irradiation, within hours of damage induction. We propose that resistance to high vacuum and 60Co gamma irradiation is a consequence of its adaptation to desiccating conditions. Gamma resistance in Halobacterium sp. was dependent on growth stage with cultures in earlier stages exhibiting higher resistance. Membrane pigments, specifically bacterioruberin, offered protection against cellular damages induced by high doses (5 kGy) of gamma irradiation. High-salt conditions were found to create a protective environment against gamma irradiation in vivo by comparing the amount of DSBs induced by ionizing radiation in the chromosomal DNA of Halobacterium sp. to that of the more radiation-sensitive Escherichia coli that grows in lower-salt conditions. No inducible response was observed after exposing Halobacterium sp. to a nonlethal dose (0.5 kGy) of gamma ray and subsequently exposing the cells to either a high dose (5 kGy) of gamma ray or desiccating conditions. We find that the hypersaline environment in which Halobacterium sp. flourishes is a fundamental factor for its resistance to desiccation, damaging radiation and high vacuum.

MlaA, a hexameric ATPase linked to the Mre11 complex in archaeal genomes

We identify and characterize MlaA, a novel protein, which is found in a conserved operon with Mre11 and Rad50 in archaeal genomes. MlaA is fused with Mre11 in Methanobacter thermoautotrophicus, suggesting the MlaA is functionally linked to the Mre11 complex. MlaA preferentially and cooperatively binds double-stranded and secondary structure containing DNA and has double-stranded but not single-stranded DNA-stimulated ATPase activity. Electron microscopy reveals that MlaA forms a 360-kDa hexameric ring structure with a central hole. Our data suggest that the archaeal Mre11 complex is associated with a novel hexameric ATPase that could be required for the processing of DNA double-stranded breaks and recombination intermediates.

Thermococcus marinus sp. nov. and Thermococcus radiotolerans sp. nov., two hyperthermophilic archaea from deep-sea hydrothermal vents that resist ionizing radiation

Enrichments for anaerobic, organotrophic hyperthermophiles were performed with hydrothermal chimney samples collected from the Mid-Atlantic Ridge at a depth of 3,550 m (23 degrees 22'N, 44 degrees 57'W) and the Guaymas Basin (27 degrees 01'N, 111 degrees 24'W) at a depth of 2,616 m. Positive enrichments were submitted to gamma-irradiation at doses of 20 and 30 kGy. Two hyperthermophilic, anaerobic, sulfur-metabolizing archaea were isolated. Strain EJ1T was isolated from chimney samples collected from the Mid-Atlantic Ridge after gamma-irradiation at 20 kGy, and strain EJ2T was isolated from the Guaymas Basin after gamma-irradiation at 30 kGy. Only strain EJ2T was motile, and both formed regular cocci. These new strains grew between 55 and 95 degrees C with the optimal temperature being 88 degrees C. The optimal pH for growth was 6.0, and the optimal NaCl concentration for growth was around 20 g l(-1). These strains were obligate anaerobic heterotrophs that utilized yeast extract, tryptone, and peptone as a carbon source for growth. Ten amino acids were essential for the growth of strain EJ1), such as arginine, aspartic acid, isoleucine, leucine, methionine, phenylalanine, proline, threonine, tyrosine, and valine, while strain EJ2T was unable to grow on a mixture of amino acids. Elemental sulfur or cystine was required for EJ2T growth and was reduced to hydrogen sulfide. Rifampicin inhibited growth for both strains EJ1T and EJ2T. The G + C contents of the genomic DNA were 52.3 and 54.5 mol% for EJ1T and EJ2T, respectively. As determined by 16S rRNA gene sequence analysis, these strains were more closely related to Thermococcus gorgonarius, T. celer, T. guaymasensis, T. profundus, and T. hydrothermalis. However, no significant homology was observed between them with DNA-DNA hybridization. These novel organisms also possess phenotypic traits that differ from those of its closest phylogenetic relatives. Therefore, it is proposed that these isolates, which are amongst the most radioresistant hyperthermophilic archaea known to date with T. gammatolerans (Jolivet et al. 2003a), should be described as novel species T. marinus sp. nov. and T. radiotolerans sp. nov. The type strain of T. marinus is strain EJ1T (= DSM 15227T = JCM 11825T) and the type strain of T. radiotolerans is strain EJ2T (= DSM 15228T = JCM 11826T).

Physiological responses of the hyperthermophilic archaeon "Pyrococcus abyssi" to DNA damage caused by ionizing radiation

The mechanisms by which hyperthermophilic Archaea, such as "Pyrococcus abyssi" and Pyrococcus furiosus, survive high doses of ionizing gamma irradiation are not thoroughly elucidated. Following gamma-ray irradiation at 2,500 Gy, the restoration of "P. abyssi" chromosomes took place within chromosome fragmentation. DNA synthesis in irradiated "P. abyssi" cells during the DNA repair phase was inhibited in comparison to nonirradiated control cultures, suggesting that DNA damage causes a replication block in this organism. We also found evidence for transient export of damaged DNA out of irradiated "P. abyssi" cells prior to a restart of chromosomal DNA synthesis. Our cell fractionation assays further suggest that "P. abyssi" contains a highly efficient DNA repair system which is continuously ready to repair the DNA damage caused by high temperature and/or ionizing radiation.

Thermococcus gammatolerans sp. nov., a hyperthermophilic archaeon from a deep-sea hydrothermal vent that resists ionizing radiation

Enrichments for anaerobic organotrophic hyperthermophiles were performed with hydrothermal chimney samples collected at the Guaymas Basin (27 degrees 01' N, 111 degrees 24' W). Positive enrichments were submitted to gamma-irradiation at a dose of 30 kGy. One of the resistant strains, designated strain EJ3(T), formed regular motile cocci. The new strain grew between 55 and 95 degrees C, with an optimum growth temperature of 88 degrees C. The optimal pH for growth was 6.0, and the optimum NaCl concentration for growth was around 20 g l(-1). Strain EJ3(T) was an obligately anaerobic heterotroph that utilized yeast extract, tryptone and peptone. Elemental sulfur or cystine was required for growth and reduced to hydrogen sulfide. The G + C content of the genomic DNA was 51.3 mol%. As determined by 16S rRNA gene sequence analysis, the organism was most closely related to Thermococcus celer, Thermococcus guaymasensis, Thermococcus hydrothermalis, Thermococcus profundus and Thermococcus gorgonarius. However, no significant homology was observed between them by DNA-DNA hybridization. The novel organism also possessed phenotypic traits that differ from those of its closest phylogenetic relatives. Therefore, it is proposed that this isolate, which constitutes the most radioresistant hyperthermophilic archaeon known to date, should be described as the type strain of a novel species, Thermococcus gammatolerans sp. nov. The type strain is EJ3(T) (= DSM 15229(T) = JCM 11827(T)).

An integrated analysis of the genome of the hyperthermophilic archaeon Pyrococcus abyssi

The hyperthermophilic euryarchaeon Pyrococcus abyssi and the related species Pyrococcus furiosus and Pyrococcus horikoshii, whose genomes have been completely sequenced, are presently used as model organisms in different laboratories to study archaeal DNA replication and gene expression and to develop genetic tools for hyperthermophiles. We have performed an extensive re-annotation of the genome of P. abyssi to obtain an integrated view of its phylogeny, molecular biology and physiology. Many new functions are predicted for both informational and operational proteins. Moreover, several candidate genes have been identified that might encode missing links in key metabolic pathways, some of which have unique biochemical features. The great majority of Pyrococcus proteins are typical archaeal proteins and their phylogenetic pattern agrees with its position near the root of the archaeal tree. However, proteins probably from bacterial origin, including some from mesophilic bacteria, are also present in the P. abyssi genome.

Microbial survival of space vacuum and extreme ultraviolet irradiation: strain isolation and analysis during a rocket flight

We have recovered new isolates from hot springs, in Yellowstone National Park and the Kamchatka Peninsula, after gamma-irradiation and exposure to high vacuum (10(-6) Pa) of the water and sediment samples. The resistance to desiccation and ionizing radiation of one of the isolates, Bacillus sp. strain PS3D, was compared to that of the mesophilic bacterium, Deinococcus radiodurans, a species well known for its extraordinary resistance to desiccation and high doses of ionizing radiation. Survival of these two microorganisms was determined in real and simulated space conditions, including exposure to extreme UV radiation (10-100 nm) during a rocket flight. We found that up to 15 days of desiccation alone had little effect on the viability of either bacterium. In contrast, exposure to space vacuum ( approximately 10(-6) Pa) decreased cell survival by two and four orders of magnitude for Bacillus sp. strain PS3D and D. radiodurans, respectively. Simultaneous exposure to space vacuum and extreme UV radiation further decreased the survival of both organisms, compared to unirradiated controls. This is the first report on the isolated effect of extreme UV at 30 nm on cell survival. Extreme UV can only be transmitted through high vacuum, therefore its penetration into the cells may only be superficial, suggesting that in contrast to near UV, membrane proteins rather than DNA were damaged by the radiation.

NurA, a novel 5'-3' nuclease gene linked to rad50 and mre11 homologs of thermophilic Archaea

We isolated and characterized a new nuclease (NurA) exhibiting both single-stranded endonuclease activity and 5'-3' exonuclease activity on single-stranded and double-stranded DNA from the hyperthermophilic archaeon Sulfolobus acidocaldarius. Nuclease homologs are detected in all thermophilic archaea and, in most species, the nurA gene is organized in an operon-like structure with rad50 and mre11 archaeal homologs. This nuclease might thus act in concert with Rad50 and Mre11 proteins in archaeal recombination/repair. To our knowledge, this is the first report of a 5'-3' nuclease potentially associated with Rad50 and Mre11-like proteins that may lead to the processing of double-stranded breaks in 3' single-stranded tails.

Biological effects of DNA damage in the hyperthermophilic archaeon Sulfolobus acidocaldarius

To investigate the generality of efficient double-strand break repair and damage-induced mutagenesis in hyperthermophilic archaea, we systematically measured the effects of five DNA-damaging agents on Sulfolobus acidocaldarius and compared the results to those obtained for Escherichia coli under corresponding conditions. The observed lethality of gamma-radiation was very similar for S. acidocaldarius and E. coli, arguing against unusually efficient double-strand break repair in S. acidocaldarius. In addition, DNA-strand-breaking agents (gamma-radiation or bleomycin), as well as DNA-cross-linking agents (mechlorethamine, butadiene diepoxide or cisplatin) stimulated forward mutation, reverse mutation, and formation of recombinants via conjugation in Sulfolobus cells. Although two of the five DNA-damaging agents failed to revert the E. coli auxotrophs under these conditions, all five reverted S. acidocaldarius auxotrophs.

Archaeal genome organization and stress responses: implications for the origin and evolution of cellular life

For DNA to be used as an informational molecule it must exist in the cell on the edge of stability because all genomic processes require local controlled melting. This presents mechanistic opportunities and problems for genomic DNA from hyperthermophilic organisms, whose unpackaged DNA could melt at optimal temperatures for growth. Hyperthermophiles are suggested to employ the novel positively supercoiling topoisomerase enzyme reverse gyrase (RG) to form positively supercoiled DNA that is intrinsically resistant to thermal denaturation. RG is presently the only archaeal gene that is uniquely found in hyperthermophiles and therefore is central to hypotheses suggesting a hypothermophilic origin of life. However, the suggestion that RG has evolved by the fusion of two pre-existing enzymes has led to hypotheses for a lower temperature for the origin of life. In addition to the action of topoisomerases, DNA packaging and the intracellular ionic environment can also manipulate DNA topology significantly. In the Euryarchaeota, nucleosomes containing minimal histones can adopt two alternate DNA topologies in a salt-dependent manner. From this we hypothesize that since internal salt concentrations are increased following an increase in temperature, the genomic effects of temperature fluctuations could also be accommodated by changes in nucleosome organization. In addition, stress-induced changes in the nucleoid proteins could also play a role in maintaining the genome in the optimal topological state in changing environments. The function of these systems could therefore be central to temperature adaptation and thus be implicated in origin of life scenarios involving hyperthermophiles.