Analysis by pulsed-field gel electrophoresis of DNA double-strand breakage and repair in Deinococcus radiodurans and a radiosensitive mutant

Constitutive hyperactivity of histone deacetylases enhances radioresistance in Lepidopteran Sf9 insect cells

BACKGROUND

Lepidopteran insect cells withstand multifold higher radiation doses and suffer far less DNA damage despite carrying numerous structural/functional homologies with mammalian cells. Since DNA-histone interactions significantly influence radiation-induced DNA damage, we investigated the role of histones in insect cell radioresistance.

METHODS

Modified comet assay was used to assess the γ-radiation-induced DNA damage following serial histone depletion by varied salt concentrations. Acid-Urea-Triton (AUT) gel analysis combined with in silico predictions was used to compare mammalian and insect histones and acetylation status while HDAC activity was assessed/modified for studying the latter's role in radioresistance. Cell death was measured by morphological analysis and flow cytometry.

RESULTS

High-salt extraction pattern from Sf9 nuclei suggested stronger DNA-histone affinity as the two core histones H2A/H2B could be extracted at much higher (2M) concentration as compared to 1.2M NaCl in mammalian (AA8) cells. Electrophoretic mobility of unirradiated Sf9 cells remained unaltered at all salt concentrations (0.14M-2M NaCl), and radiation-induced DNA damage increased only by 2M-NaCl pre-treatment. In silico analysis confirmed excellent conservation of Lepidopteran H2A/H2B sequence with human histones including comparable N-terminal lysine residues, yet these had ~60% lower acetylation. Importantly, insect cells showed ~70% higher histone deacetylase activity whose inhibition by Trichostatin-A reversed hypo-acetylation state and caused significant radiosensitization, thereby confirming the protective contribution of reduced acetylation.

CONCLUSION

Our study reveals that the hypo-acetylated state of well-conserved core histones, maintained by considerable HDAC activity, contributes significantly in Lepidopteran radioresistance.

GENERAL SIGNIFICANCE

This investigation shows constitutively high activity of HDACs as a potential radioprotective mechanism existing in insect cells.

Escherichia coli genes and pathways involved in surviving extreme exposure to ionizing radiation

To further an improved understanding of the mechanisms used by bacterial cells to survive extreme exposure to ionizing radiation (IR), we broadly screened nonessential Escherichia coli genes for those involved in IR resistance by using transposon-directed insertion sequencing (TraDIS). Forty-six genes were identified, most of which become essential upon heavy IR exposure. Most of these were subjected to direct validation. The results reinforced the notion that survival after high doses of ionizing radiation does not depend on a single mechanism or process, but instead is multifaceted. Many identified genes affect either DNA repair or the cellular response to oxidative damage. However, contributions by genes involved in cell wall structure/function, cell division, and intermediary metabolism were also evident. About half of the identified genes have not previously been associated with IR resistance or recovery from IR exposure, including eight genes of unknown function.

The role of Deinococcus radiodurans RecFOR proteins in homologous recombination

Deinococcus radiodurans exhibits extraordinary resistance to the lethal effect of DNA-damaging agents, a characteristic attributed to its highly proficient DNA repair capacity. Although the D. radiodurans genome is clearly devoid of recBC and addAB counterparts as RecA mediators, the genome possesses all genes associated with the RecFOR pathway. In an effort to gain insights into the role of D. radiodurans RecFOR proteins in homologous recombination, we generated recF, recO and recR disruptant strains and characterized the disruption effects. All the disruptant strains exhibited delayed growth relative to the wild-type, indicating that the RecF, RecO and RecR proteins play an important role in cell growth under normal growth conditions. A slight reduction in transformation efficiency was observed in the recF and recO disruptant strains compared to the wild-type strain. Interestingly, disruption of recR resulted in severe reduction of the transformation efficiency. On the other hand, the recF disruptant strain was the most sensitive phenotype to γ rays, UV irradiation and mitomycin C among the three disruptants. In the recF disruptant strain, the intracellular level of the LexA1 protein did not decrease following γ irradiation, suggesting that a large amount of the RecA protein remains inactive despite being induced. These results demonstrate that the RecF protein plays a crucial role in the homologous recombination repair process by facilitating RecA activation in D. radiodurans. Thus, the RecF and RecR proteins are involved in the RecA activation and the stability of incoming DNA, respectively, during RecA-mediated homologous recombination processes that initiated the ESDSA pathway in D. radiodurans. Possible mechanisms that involve the RecFOR complex in homologous intermolecular recombination and homologous recombination repair processes are also discussed.

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.

Radiation desiccation response motif-like sequences are involved in transcriptional activation of the Deinococcal ssb gene by ionizing radiation but not by desiccation

Single-stranded-DNA binding protein (SSB) levels during poststress recovery of Deinococcus radiodurans were significantly enhanced by (60)Co gamma rays or mitomycin C treatment but not by exposure to UV rays, hydrogen peroxide (H₂O₂), or desiccation. Addition of rifampin prior to postirradiation recovery blocked such induction. In silico analysis of the ssb promoter region revealed a 17-bp palindromic radiation/desiccation response motif (RDRM1) at bp -114 to -98 and a somewhat similar sequence (RDRM2) at bp -213 to -197, upstream of the ssb open reading frame. Involvement of these cis elements in radiation-responsive ssb gene expression was assessed by constructing transcriptional fusions of edited versions of the ssb promoter region with a nonspecific acid phosphatase encoding reporter gene, phoN. Recombinant D. radiodurans strains carrying such constructs clearly revealed (i) transcriptional induction of the ssb promoter upon irradiation and mitomycin C treatment but not upon UV or H₂O₂ treatment and (ii) involvement of both RDRM-like sequences in such activation of SSB expression, in an additive manner.

The stable, functional core of DdrA from Deinococcus radiodurans R1 does not restore radioresistance in vivo

DdrA protein binds to and protects 3' DNA ends and is essential for preserving the genome integrity of Deinococcus radiodurans following treatment by gamma radiation in an environment lacking nutrients. Limited proteolysis was used to identify a stable and functional protein core, designated DdrA157, consisting of the first 157 residues of the protein. In vitro, the biochemical differences between wild-type and mutant proteins were modest. DdrA exhibits a strong bias in binding DNA with 3' extensions but not with 5' extensions. The mutant DdrA157 exhibited a greater affinity for 5' DNA ends but still bound to 3' ends more readily. However, when we replaced the wild-type ddrA gene with the mutant gene for ddrA157, the resulting D. radiodurans strain became almost as sensitive to gamma radiation as the ddrA knockout strain. These results suggest that while the stable protein core DdrA157 is functional for DNA binding and protection assays in vitro, the carboxyl terminus is required for important functions in vivo. The C terminus may therefore be required for protein or DNA interactions or possibly as a regulatory region for DNA binding or activities not yet identified.

Down-regulation of radioresistance by LexA2 in Deinococcus radiodurans

The extremely radioresistant bacterium Deinococcus radiodurans contains two LexA homologues (LexA1 and LexA2) that are possible transcriptional regulators associated with the DNA damage response. In this study, resequencing revealed that there was an additional cytosine nucleotide (nucleotide position 612) in the D. radiodurans lexA2 gene. Purified LexA2 possessed proteolytic activity that could be stimulated by RecA. In an effort to gain an insight into the role of LexA2 in the radiation response mechanism, recA, lexA1 and lexA2 disruptant strains were generated and investigated. The intracellular level of RecA increased in lexA1 and lexA2 disruptant strains following gamma-irradiation as in the wild-type strain. These results indicated that the two LexA homologues did not possess functional overlap regarding the induction of RecA. The lexA2 disruptant strains exhibited a much higher resistance to gamma-rays than the wild-type strain. Furthermore, a luciferase assay showed that pprA promoter activation was enhanced in the lexA2 disruptant strain following gamma-irradiation. The pprA gene encoding the novel radiation-inducible protein PprA plays a critical role in the radioresistance of D. radiodurans. The increase in radioresistance of the lexA2 disruptant strain is explained in part by the enhancement of pprA promoter activation.

Method for detecting DNA strand breaks in mammalian cells using the Deinococcus radiodurans PprA protein

In a previous study, we identified the novel protein PprA that plays a critical role in the radiation resistance of Deinococcus radiodurans. In this study, we focussed on the ability of PprA protein to recognize and bind to double-stranded DNA carrying strand breaks, and attempted to visualize radiation-induced DNA strand breaks in mammalian cultured cells by employing PprA protein using an immunofluorescence technique. Increased PprA protein binding to CHO-K1 nuclei immediately following irradiation suggests the protein is binding to DNA strand breaks. By altering the cell permeabilization conditions, PprA protein binding to CHO-K1 mitochondria, which is probably resulted from DNA strand break immediately following irradiation, was also detected. The method developed and detailed in this study will be useful in evaluating DNA damage responses in cultured cells, and could also be applicable to genotoxic tests in the environmental and pharmaceutical fields.

The radiation responsive promoter of the Deinococcus radiodurans pprA gene

In a previous study, we identified a novel radiation-inducible protein PprA that plays a critical role in the radiation resistance of Deinococcus radiodurans [Narumi, I., Satoh, K., Cui, S., Funayama, T., Kitayama, S., Watanabe, H., 2004. PprA: a novel protein from Deinococcus radiodurans that stimulates DNA ligation. Mol. Microbiol. 54, 278-285.]. Despite the interest in mechanisms underlying radiation responses in D. radiodurans, little is known about the radiation responsive promoter for radiation-inducible proteins. In this study, three transcriptional start points for pprA mRNA were identified by primer extension analysis, located at positions -156, -154 and -22 upstream from the pprA translation initiation site. The amount of the three extended products increased in cells exposed to 2 kGy followed by a 0.5-h post-incubation. This suggested the existence of at least two radiation responsive promoters for pprA expression. Functional characterization of the upstream region of the pprA gene using a luciferase reporter assay revealed that the distal promoter is located between positions -208 and -156 from the translation initiation site, while the proximal promoter is located between positions -57 and -22. The region located between positions -57 and -38 was indispensable for proximal promoter activity. Site-directed mutagenesis of a thymine positioned at -33 resulted in severe impairment of promoter activity, and suggested that the thymine functions as a master base for the proximal radiation responsive promoter. The product of the D. radiodurans pprI gene is thought to be a general switch in the radiation response [Hua, Y., Narumi, I., Gao, G., Tian, B., Satoh, K., Kitayama, S., Shen, B., 2003. PprI: a general switch responsible for extreme radioresistance of Deinococcus radiodurans. Biochem. Biophys. Res. Commun. 306, 354-360.]. We examined the effect of pprI disruption on pprA promoter activity. The results suggested that up-regulation of pprA expression by the pprI gene product is triggered at the promoter level.

A ring-like nucleoid is not necessary for radioresistance in the Deinococcaceae

Background

Transmission electron microscopy images of Deinococcus radiodurans R1 suggest that the nucleoid of this species exists as a "ring-like" body, and have led to speculation that this structure contributes to the radioresistance of the species. Since extreme radioresistance is characteristic of six other species of Deinococcus, we have attempted to correlate nucleoid morphology and radioresistance by determining whether the genomic DNA of each of these species exhibit similar structures.

Results

The nucleoid morphologies of seven recognized species of Deinococcus, the radioresistant bacterium Rubrobacter radiotolerans, and the more radiosensitive deinococcal relative Thermus aquaticus were evaluated using epifluorescence and deconvolution techniques. Although the nucleoids of Deinococcus murrayi, Deinococcus proteolyticus, Deinococcus radiophilus, and Deinococcus grandis have structures similar to D. radiodurans, the majority of nucleoids found in Deinococcus radiopugnans and Deinococcus geothermalis lack any specific organization. The nucleoid of R. radiotolerans consists of multiple highly condensed spheres of DNA scattered throughout the cell. The genomic DNA of Thermus aquaticus is uniformly distributed throughout the cell.

Conclusion

There is no obvious relationship between the shape of a species' nucleoid and extreme radioresistance. However, the genomes of all extremely radioresistance species examined are highly condensed relative to more radiosensitive species. Whether DNA in this tightly packed configuration contributes to the radioresistance of these bacteria remains unknown, but this common structural feature appears to limit diffusion of fragments generated post-irradiation even in cells incapable of repairing strand breaks.

Preserving genome integrity: the DdrA protein of Deinococcus radiodurans R1

The bacterium Deinococcus radiodurans can withstand extraordinary levels of ionizing radiation, reflecting an equally extraordinary capacity for DNA repair. The hypothetical gene product DR0423 has been implicated in the recovery of this organism from DNA damage, indicating that this protein is a novel component of the D. radiodurans DNA repair system. DR0423 is a homologue of the eukaryotic Rad52 protein. Following exposure to ionizing radiation, DR0423 expression is induced relative to an untreated control, and strains carrying a deletion of the DR0423 gene exhibit increased sensitivity to ionizing radiation. When recovering from ionizing-radiation-induced DNA damage in the absence of nutrients, wild-type D. radiodurans reassembles its genome while the mutant lacking DR0423 function does not. In vitro, the purified DR0423 protein binds to single-stranded DNA with an apparent affinity for 3′ ends, and protects those ends from nuclease degradation. We propose that DR0423 is part of a DNA end-protection system that helps to preserve genome integrity following exposure to ionizing radiation. We designate the DR0423 protein as DNA damage response A protein.

Deinococcus radiodurans is able to repair radiation induced genome damage by virtue of a homologue of the eukaryotic Rad52 protein, which preserves genome integrity by protecting DNA ends

The IrrE protein of Deinococcus radiodurans R1 is a novel regulator of recA expression

IRS24 is a DNA damage-sensitive strain of Deinococcus radiodurans strain 302 carrying a mutation in an uncharacterized locus designated irrE. Five overlapping cosmids capable of restoring ionizing radiation resistance to IRS24 were isolated from a genomic library. The ends of each cloned insert were sequenced, and these sequences were used to localize irrE to a 970-bp region on chromosome I of D. radiodurans R1. The irrE gene corresponds to coding sequence DR0167 in the R1 genome. The irrE gene encodes a 35,000-Da protein that has no similarity to any previously characterized peptide. The irrE locus of R1 was also inactivated by transposon mutagenesis, and this strain was sensitive to ionizing radiation, UV light, and mitomycin C. Preliminary findings indicate that IrrE is a novel regulatory protein that stimulates transcription of the recA gene following exposure to ionizing radiation.

The diverse and dynamic structure of bacterial genomes

Bacterial genome sizes, which range from 500 to 10,000 kbp, are within the current scope of operation of large-scale nucleotide sequence determination facilities. To date, 8 complete bacterial genomes have been sequenced, and at least 40 more will be completed in the near future. Such projects give wonderfully detailed information concerning the structure of the organism's genes and the overall organization of the sequenced genomes. It will be very important to put this incredible wealth of detail into a larger biological picture: How does this information apply to the genomes of related genera, related species, or even other individuals from the same species? Recent advances in pulsed-field gel electrophoretic technology have facilitated the construction of complete and accurate physical maps of bacterial chromosomes, and the many maps constructed in the past decade have revealed unexpected and substantial differences in genome size and organization even among closely related bacteria. This review focuses on this recently appreciated plasticity in structure of bacterial genomes, and diversity in genome size, replicon geometry, and chromosome number are discussed at inter- and intraspecies levels.

Against all odds: the survival strategies of Deinococcus radiodurans

Bacteria of the genus Deinococcus exhibit an extraordinary ability to withstand the lethal and mutagenic effects of DNA damaging agents-particularly the effects of ionizing radiation. These bacteria are the most DNA damage-tolerant organisms ever identified. Relatively little is known about the biochemical basis for this phenomenon; however, available evidence indicates that efficient repair of DNA damage is, in large part, responsible for the deinococci's radioresistance. Obviously, an explanation of the deinococci's DNA damage tolerance cannot be developed solely on the basis of the DNA repair strategies of more radiosensitive organisms. The deinococci's capacity to survive DNA damage suggests that (a) they employ repair mechanisms that are fundamentally different from other prokaryotes, or that (b) they have the ability to potentiate the effectiveness of the conventional complement of DNA repair proteins. An argument is made for the latter alternative.

Isolation and characterization of a novel Deinococcus radiodurans mutant abnormally susceptible to mutation induction by UV, gamma-ray, and mitomycin C

We isolated and characterized a novel, radiation 'hypermutable' mutant of Deinococcus radiodurans. Compared with the wild-type strain D. radiodurans IR, this mutator strain, designated S101, exhibited sensitivity to UV light, gamma-ray, mitomycin C, and N-methyl-N'-nitro-N-nitrosoguanidine. Spontaneous revertants of S101 that restored wild-type phenotype (non-mutability and resistance to these DNA-damaging agents) were also isolated. Furthermore, the increased susceptibility to DNA-damaging agents and mutability observed in S101 could be mimicked by treating D. radiodurans IR with Mn(II) ions. Our results suggest a putative new pathway of DNA repair in the extremely radioresistant bacterium D. radiodurans.

Radioresistance of Deinococcus radiodurans: functions necessary to survive ionizing radiation are also necessary to survive prolonged desiccation

Forty-one ionizing radiation-sensitive strains of Deinococcus radiodurans were evaluated for their ability to survive 6 weeks of desiccation. All exhibited a substantial loss of viability upon rehydration compared with wild-type D. radiodurans. Examination of chromosomal DNA from desiccated cultures revealed a time-dependent increase in DNA damage, as measured by an increase in DNA double-strand breaks. The evidence presented suggests that D. radiodurans' ionizing radiation resistance is incidental, a consequence of this organism's adaptation to a common physiological stress, dehydration.

Interplasmidic recombination following irradiation of the radioresistant bacterium Deinococcus radiodurans

Deinococcus radiodurans R1 and other members of the eubacterial family Deinococcaceae are extremely resistant to ionizing radiation and many other agents that damage DNA. For example, after irradiation, D. radiodurans can repair > 100 DNA double-strand breaks per chromosome without lethality or mutagenesis, while most other organisms can survive no more than 2 or 3 double-strand breaks. The unusual resistance of D. radiodurans is recA dependent, but the repair pathway(s) is not understood. Recently, we described how a plasmid present in D. radiodurans (plasmid copy number, approximately 6 per cell; chromosome copy number, approximately 4 per cell) during high-dose irradiation undergoes extreme damage like the chromosome and is retained by the cell without selection and fully repaired with the same efficiency as the chromosome. In the current work, we have investigated the repair of two similar plasmids within the same cell. These two plasmids were designed to provide both restriction fragment polymorphisms and a drug selection indicator of recombination. This study presents a novel system of analysis of in vivo damage and recombinational repair, exploiting the unique ability of D. radiodurans to survive extraordinarily high levels of DNA damage. We report that homologous recombination among plasmids following irradiation is extensive. For example, 2% of Tcs plasmids become Tcr as a result of productive recombination within a 929-bp region of the plasmids after repair. Our results suggest that each plasmid may participate in as many as 6.7 recombinational events during repair, a value that extrapolates to > 700 events per chromosome undergoing repair simultaneously. These results indicate that the study of plasmid recombination within D. radiodurans may serve as an accurate model system for simultaneously occurring repair in the chromosome.

Effects of heavy ions on inactivation and DNA double strand breaks in Deinococcus radiodurans R1

Inactivation and double strand break (dsb) induction after heavy ion irradiation were studied in stationary phase cells of the highly radiation resistant bacterium Deinococcus radiodurans R1. There is evidence that the radiation sensitivity of this bacterium is nearly independent on energy in the range of up to 15 MeV/u for lighter ions (Ar). The responses to dsb induction for charged particles show direct relationship between increasing radiation dose and residual intact DNA.

DNA repair in the extremely radioresistant bacterium Deinococcus radiodurans

Deinococcus radiodurans and other members of the same genus share extraordinary resistance to the lethal and mutagenic effects of ionizing and u.v. radiation and to many other agents that damage DNA. While it is known that this resistance is due to exceedingly efficient DNA repair, the molecular mechanisms responsible remain poorly understood. Following very high exposures to u.v. irradiation (e.g. 500 J m-2, which is non-lethal to D. radiodurans), this organism carries out extremely efficient excision repair accomplished by two separate nucleotide excision repair pathways acting simultaneously. One pathway requires the uvrA gene and appears similar to the UvrABC excinuclease pathway defined in Escherichia coli. The other excision repair pathway is specific for u.v. dimeric photoproducts, but is not mediated by a pyrimidine dimer DNA glycosylase. Instead, it is initiated by a second bona fide endonuclease that may recognize both pyrimidine dimers and pyrimidine-(6-4)pyrimidones. After high doses of ionizing-radiation (e.g. 1.5 Mrad), D. radiodurans can mend > 100 double-strand breaks (dsb) per chromosome without lethality or mutagenesis. Both dsb mending and survival are recA-dependent, indicating that efficient dsb mending proceeds via homologous recombination. D. radiodurans contains multiple chromosomes per cell, and it is proposed that dsb mending requires extensive recombination amongst these chromosomes, a novel phenomenon in bacteria. Thus, D. radiodurans may serve as an easily accessible model system for the double-strand-break-initiated interchromosomal recombination that occurs in eukaryotic cells during mitosis and meiosis.

In vivo damage and recA-dependent repair of plasmid and chromosomal DNA in the radiation-resistant bacterium Deinococcus radiodurans

Deinococcus radiodurans R1 and other members of this genus share extraordinary resistance to the lethal and mutagenic effects of ionizing radiation. We have recently identified a RecA homolog in strain R1 and have shown that mutation of the corresponding gene causes marked radiosensitivity. We show here that following high-level exposure to gamma irradiation (1.75 megarads, the dose required to yield 37% of CFU for plateau-phase wild-type R1), the wild-type strain repairs > 150 double-strand breaks per chromosome, whereas a recA-defective mutant (rec30) repairs very few or none. A heterologous Escherichia coli-D. radiodurans shuttle plasmid (pMD68) was constructed and found to be retained in surviving D. radiodurans R1 and rec30 following any radiation exposure up to the highest dose tested, 3 megarads. Plasmid repair was monitored in vivo following irradiation with 1.75 megarads in both R1/pMD68 and rec30/pMD68. Immediately after irradiation, plasmids from both strains contained numerous breaks and failed to transform E. coli. While irradiation with 1.75 megarads was lethal to rec30 cultures, a small amount of supercoiled plasmid was regenerated, but it lacked the ability to transform E. coli. In contrast, wild-type cultures showed a cell division arrest of about 10 h, followed by exponential growth. Supercoiled plasmid was regenerated at normal levels, and it readily transformed E. coli. These studies show that D. radiodurans retains a heterologous plasmid following irradiation and repairs it with the same high efficiency as its chromosomal DNA, while the repair defect in rec30 prevents repair of the plasmid. Taken together, the results of this study suggest that plasmid DNA damaged in vivo in D. radiodurans is repaired by recA-dependent mechanisms similar to those employed in the repair of chromosomal DNA.

Sequencing, targeted mutagenesis and expression of a recA gene required for the extreme radioresistance of Deinococcus radiodurans

Deinococcus radiodurans and other members of the same genus share extreme resistance to ionizing radiation and many other agents that damage DNA. A DNA damage-sensitive and natural transformation-deficient strain generated by chemical mutagenesis (strain rec30) was found to be defective in a gene that has extended homology with recA of Escherichia coli. Upon transformation with a chromosomal DNA fragment that contained this deinococcal recA gene from wild-type (wt) D. radiodurans both DNA damage resistance and full transformation competence were restored in the rec30 mutant. Targeted insertional mutagenesis of the deinococcal recA gene was used to construct a mutant isogenic with the wt. The insertional mutant was phenotypically indistinguishable from strain rec30, indicating that the recA defect alone was responsible for observed phenotypic alterations. For example, in the case of ionizing radiation, the D37 of the wt was about 1.75 Mrad, while the D37 of rec30 and the insertional mutant were both 25 krad, a 70-fold decrease. Evidence is presented that expression of the deinococcal recA gene in E. coli is lethal, suggesting that the mode of interaction of the deinococcal RecA protein with nucleic acids or other cellular proteins differs at least in part from RecA of E. coli.

Asymmetric fusion between protoplasts of tomato (Lycopersicon esculentum Mill.) and gamma-irradiated protoplasts of potato (Solanum tuberosum L.): the effects of gamma irradiation

This paper describes the aggregation of nuclei in heterokaryons of tomato and unirradiated or irradiated potato protoplasts and the effects of gamma irradiation of potato and tomato protoplasts on single- and double-stranded DNA fragmentation, DNA repair and DNA synthesis as revealed by alkaline and pulsed field gel electrophoresis and an immunocytochemical technique. The prospects for obtaining highly asymmetric somatic hybrids of tomato and gamma-irradiated potato are discussed.

Restoration of the DNA damage resistance of Deinococcus radiodurans DNA polymerase mutants by Escherichia coli DNA polymerase I and Klenow fragment

Deinococcus radiodurans and other species of this genus share extreme resistance to ionizing radiation and many other agents that damage DNA. D. radiodurans mutant strains defective in a deinococcal DNA polymerase that is homologous with E. coli DNA polymerase I are highly sensitive to DNA damage. In the current work we have inquired whether E. coli DNA Pol I can substitute for D. radiodurans Pol in partially or fully restoring to pol- D. radiodurans mutants the extreme DNA damage-resistance typical of this organism. The E. coli polA gene or a 5'-truncated polA gene that encodes the Klenow fragment were introduced and expressed in two different D. radiodurans pol- mutants: Strain 303, which is a chemically mutagenized derivative, and strain 6R1A, which is isogenic with wild-type D. radiodurans except for an insertional mutation within the pol gene. Expression of E. coli polA in both of these mutants fully restored wild-type resistance to ionizing- and UV254-radiation and mitomycin-C exposure. Expression of the Klenow fragment-encoding gene restored wild-type resistance to D. radiodurans strain 303, but only partial resistance to strain 6R1A. The observation that E. coli DNA Pol I is as effective as D. radiodurans Pol in restoring damage resistance, indicates that D. radiodurans DNA Pol per se does not have special properties that are essential or prerequisite for expression of the extreme resistance of D. radiodurans.

Mechanisms of induction and repair of DNA double-strand breaks by ionizing radiation: some contradictions

The various aspects of formation and repair of radiation-induced double-strand breaks (DSB) are summarized. Concerning the structure of DSB found in irradiated cells, enzymatic and microdosimetric analysis hints at complex damage of the DNA structure at the position of a DSB. With increasing LET, the DSB damage may be more complex than that induced by low-LET irradiation. Most of the DSB are repaired in the irradiated cell; apparently the kinetics of DSB repair and the fraction of unrejoined DSB determine cell survival or cell death. We do not know the details of the complex machinery of DSB repair; certainly recombination processes are involved, but there are still contradictions between our current knowledge about the mechanisms of recombinational DSB repair and the observed kinetics.

Identification, sequencing, and targeted mutagenesis of a DNA polymerase gene required for the extreme radioresistance of Deinococcus radiodurans

Deinococcus radiodurans and other species of the same genus share extreme resistance to ionizing radiation and many other agents that damage DNA. Two different DNA damage-sensitive strains generated by chemical mutagenesis were found to be defective in a gene that has extended DNA and protein sequence homology with polA of Escherichia coli. Both mutant strains lacked DNA polymerase, as measured in activity gels. Transformation of this gene from wild-type D. radiodurans restored to the mutants both polymerase activity and DNA damage resistance. A technique for targeted insertional mutagenesis in D. radiodurans is presented. This technique was employed to construct a pol mutant isogenic with the wild type (the first example of targeted mutagenesis in this eubacterial family). This insertional mutant lacked DNA polymerase activity and was even more sensitive to DNA damage than the mutants derived by chemical mutagenesis. In the case of ionizing radiation, the survival of the wild type after receiving 1 Mrad was 100% while survival of the insertional mutant extrapolated to 10(-24). These results demonstrate that the gene described here encodes a DNA polymerase and that defects in this pol gene cause a dramatic loss of resistance of D. radiodurans to DNA damage.