Characteristics of DNA repair of a UV-sensitive mutant (radC) of Dictyostelium discoideum

Summary A radiation-sensitive mutant, TW8(radC), of Dictyostelium discoideum is more sensitive to ultraviolet light (UV) killing than the parental wild strain NC4(RAD ⁺), but is resistant to 4-nitroquinoline 1-oxide (4NQO) at almost the same level as NC4. In TW8 amoebae, single-strand breaks of DNA molecules were hardly detectable immediately after UV irradiation, and the removal of pyrimidine dimers was depressed during the postirradiation incubation when compared with that of NC4 amoebae. After treatment with 4NQO, however, single-strand breaks were detected in TW8 amoebae. The almost complete rejoining of these breaks was also detected after the removal of 4HAQO-adducts. The TW8 amoebae have an efficient repair capacity against DNA damage caused by 4NQO, MMS, MMC and MNNG but not UV.Abbreviations 4NQO 4-nitroquinoline 1-oxide - MMS methyl methanesulphonate - MMC mitomycin C - MNNG N-methyl-N-nitro-N-nitrosoguanidine     

Sedimentation of DNA of Dictyostelium discoideum lysed on alkaline sucrose gradients: role of single-strand breaks in gamma ray lethality of sensitive and resistant strains

The nuclear and mitochondrial DNA of the amoebae of the cellular slime mold Dictyostelium discoideum have been labeled with [methyl-³H]thymidine by allowing them to grow on Escherichia coli 15T^? containing this label in its DNA. Neutral CsCl gradients were used to identify the labeled molecules. Alkaline sucrose sedimentation profiles of cells lysed directly on the gradients revealed two high molecular weight species, one of about 90 S (single-strand mol wt = 1.4 × 10⁸) identified by alkaline CsCl rebanding as nuclear DNA, and another of 43 S (single-strand mol wt = 2.3 × 10⁷), identified as mitochondrial DNA. These alkaline sucrose gradients were used to study the production of single-strand breaks and their rejoining in DNA of a gamma ray-resistant strain (NC-4; 10% survival dose for cell proliferation, D₁₀ = 300 krad) and in two radiation-sensitive daughter mutants (γs-18, D₁₀ = 75 krad; γs-13, D₁₀ = 4 krad). With ⁶⁰Co gamma rays, breaks were produced in nuclear and mitochondrial DNA at an efficiency of one break per 33 eV in all three strains. At doses up to about 100 krad, these single-strand breaks were closed equally well during post-irradiation incubation of NC-4, γs-18 and γs-13, even though their survivals were widely different, indicating no apparent correlation between parental strand rejoining and survival in the sensitive strains. At higher doses, post-irradiation treatment with 1 mg caffeine/ml sensitized NC-4 and retarded strand-rejoining, suggesting that lethality in this resistant strain may be related to strand breaks. It is concluded that single-strand rejoining is a necessary, but not sufficient, condition for radiation survival in this organism. The nature of the apparently unrepaired lesions leading to lethality in the sensitive strains is not known.     

Photoreactivation, Excision, and Strand-Rejoining Repair in R Factor-Containing Minicells of Escherchia coli K-12

Chromosomeless “minicells” are formed by misplaced cell fissions near the polar extremities of an Escherichia coli K-12 mutant strain. Resistance (R)-factor deoxyribonucleic acid (DNA) can be introduced into minicells by segregation from an R⁺ (R64-11) derivative of the original mutant. We have assessed the ability of R⁺ minicells to correct defects produced in their plasmid DNA by ultraviolet (UV) and gamma radiations. Minicells harboring plasmid DNA, in comparison with their repair-proficient minicell-producing parents, possess (i) an equal competence to rejoin single-strand breaks induced in DNA by gamma rays, (ii) a reduced capacity for the photoenzymatic repair of UV-induced pyrimidine dimers, and (iii) a total inability to excise dimers, apparently owing to a deficiency in UV-specific endonuclease activity responsible for mediating the initial incision step in excision repair. Assuming that the DNA repair properties of R⁺ minicells reflect the concentration of repair enzymes located in the plasmid-containing polar caps of entire cells, these findings suggest that: (i) the enzymes responsible for rejoining single-strand breaks are distributed throughout the cell; (ii) photoreactivating enzyme molecules tend to be concentrated near bacterial DNA and to a lesser extent near plasmid DNA; and (iii) UV-specific endonuclease molecules are primarily confined to the central region of the E. coli cell and, thus, seldom segregate with R-factor DNA into minicells.     

Repair of deoxyribonucleic acid in ultraviolet light-sensitive and -resistant Dictyostelium discoideum strains.

Some responses of the cellular slime mold Dictyostelium discoideum to ultraviolet light (UV) irradiation were investigated by analyzing two aspects of deoxyribonucleic acid (DNA) excision repair in the vegetative cells: (i) the fate of thymine-containing dimers and (ii) the production and rejoining of single-strand breaks. Experiments were done with the parental, radiation-resistant NC-4 strain and with the radiation-sensitive gammas-13 strain. The majority (greater than 85%) of the thymine-containing dimers produced in both strains by an energy fluence of 100/Jm2 were removed from the acid-insoluble DNA fraction within the first 3 to 4 h of reincubation in the dark. Moreover, as measured by alkaline sucrose gradients, single-strand breaks appeared in the DNA of both NC-4 and gammas-13 irradiated cells very rapidly and at low temperatures. This was presumed to be a result of the incision (nicking) step of excision repair as performed by UV-specific endonuclease(s). In NC-4 the time required for dimer excision correlated with the sealing of breaks as well as with the UV-induced division delays. In gammas-13 the single-strand breaks were closed at a slower rate than in NC-4. However, this was not accompanied by more extensive division delays.     

Induction and repair of DNA damage in gamma-irradiated human lymphoblasts: irradiation in the presence and absence of misonidazole

The effects of oxygen and misonidazole on the induction of DNA lesions were examined in human TK6 lymphoblasts irradiated with 60Co gamma rays. We have investigated both the formation and subsequent repair of two classes of DNA damage, single-strand breaks and lesions recognized by the gamma endonuclease activity in a cell-free extract of Micrococcus luteus. Relative to irradiation under hypoxia, single-strand break yields were increased by the presence of either oxygen or misonidazole at the time of irradiation. In contrast, M. luteus enzyme-sensitive site yields were unaffected by the presence of either oxygen or misonidazole. No significant differences in single-strand break or enzyme-sensitive site repair kinetics were observed for lesions induced under any of the irradiation conditions employed. These results confirm the sensitizing effects of oxygen and oxygen-mimetic drugs on the induction of single-strand breaks but provide no support for their ability to enhance the induction of enzyme-sensitive sites.     

Repair of Ultraviolet Radiation Damage in Sensitive Mutants of Micrococcus radiodurans

Various aspects of the repair of ultraviolet (UV) radiation-induced damage were compared in wild-type Micrococcus radiodurans and two UV-sensitive mutants. Unlike the wild type, the mutants are more sensitive to radiation at 265 nm than at 280 nm. The delay in deoxyribonucleic acid (DNA) synthesis following exposure to UV is about seven times as long in the mutants as in the wild type. All three strains excise UV-induced pyrimidine dimers from their DNA, although the rate at which cytosine-thymine dimers are excised is slower in the mutants. The three strains also mend the single-strand breaks that appear in the irradiated DNA as a result of dimer excision, although the process is less efficient in the mutants. It is suggested that the increased sensitivity of the mutants to UV radiation may be caused by a partial defect in the second step of dimer excision.     

Dose-dependent increase in repair of 1-β-d-arabinofuranosylcytosinedetectable DNA lesions in UV-treated xeroderma pigmentosum (group A) fibroblasts

The extent of DNA-excision repair was determined in human fibroblast strains from clinically normal and xeroderma pigmentosum complementation group A (XP-A) donors after irradiation with 254-nm ultraviolet (UV) light. Repair was monitored by the use of 1-β-d-arabinofuranosylcytosine (araC), a potent inhibitor of DNA synthesis, and alkaline sucrose velocity sedimentation to quantitate DNA single-strand breaks. In this approach, the number of araC-accumulated breaks in post-UV incubated cultures becomes a measure of the efficiency of a particular strain to perform long-patch excision repair. The maximal rate of removal of araC-detectable DNA lesions equalled ∼ 1.8 sites/10⁸ dalton/h in the normal strains (GM38, GM43), while it was more than 10-fold lower in both XP-A strains (XP4LO, XP12BE) examined. In normal fibroblasts the number of lesions removed during the first 4 h after irradiation saturated at ∼ 10 J/m². In contrast, the residual amount of repair in the excision-deficient cells increased as a linear function of UV fluence over a range 5–120 J/m². Thus we conclude that the repair of araC-detectable UV photoproducts in XP group A fibroblasts is limited by availability of damaged regions in the genome to repair complexes.     

Properties of uvrE mutants of Escherichia coli K12. I. Effects of UV irradiation on DNA metabolism

Escherichia coli K12 uvrE is a mutator strain which is highly sensitive to ultraviolet (UV) radiation.In an attempt to determine the underlying molecular basis for the UV sensitivity, we have compared a mutant and an isogenic wild type strain with regard to several metabolic responses to 254-nm radiation. The introduction of single-strand breaks into intracellular DNA after irradiation is normal. However, the rate of excision of pyrimidine dimers as well as of DNA degradation and final rejoining of the strand breaks is lower in the mutant as compared to the repair proficient strain.These data suggest that the uvrE gene product may be involved in a reaction between the incision and excision steps in the excision repair process.     

Survival, mutation and capacity to repair single-strand DNA breaks after gamma irradiation in different Exr? strains of Escherichia coli

Summary Strains of Escherichia coli K-12 and B/r made by transduction of the exrA allele from a B_s-2 derivative have been compared with Exr(W)^– strains derived from B_s-1 and B_s-2 by mutation (E.M. Witkin, 1967). Both transduced exrA and Exr(W)^– strains were almost unmutable by gamma radiation, but the former class were as sensitive to gamma radiation as recA strains and, like them, were unable to repair single-strand DNA breaks as detected by the McGrath-Williams technique. In contrast the Exr(W)^–strains were as resistant to gammaradiation as Exr(W)⁺ strains derived from them and were equally efficient in repairing single-strand breaks. The existence of Exr(W)^–strains suggests that the mutagenicity of single-strand breaks may depend entirely on the way in which they are repaired. The properties of the (Exr(W)^–strains cannot be ascribed solely to the transducable exrA allele.A large effect of diffuse daylight in lowering the molecular weight of DNA on alkaline sucrose gradients is described which, unless prevented, may lead to erroneous results in such experiments.     

Molecular Mechanisms of Pyrimidine Dimer Excision in Saccharomyces cerevisiae: Incision of Ultraviolet-Irradiated Deoxyribonucleic Acid In Vivo

A group of genetically related ultraviolet (UV)-sensitive mutants of Saccharomyces cerevisiae has been examined in terms of their survival after exposure to UV radiation, their ability to carry out excision repair of pyrimidine dimers as measured by the loss of sites (pyrimidine dimers) sensitive to a dimer-specific enzyme probe, and in terms of their ability to effect incision of their deoxyribonucleic acid (DNA) during post-UV incubation in vivo (as measured by the detection of single-strand breaks in nuclear DNA). In addition to a haploid RAD⁺ strain (S288C), 11 different mutants representing six RAD loci (RAD1, RAD2, RAD3, RAD4, RAD14, and RAD18) were examined. Quantitative analysis of excision repair capacity, as determined by the loss of sites in DNA sensitive to an enzyme preparation from M. luteus which is specific for pyrimidine dimers, revealed a profound defect in this parameter in all but three of the strains examined. The rad14-1 mutant showed reduced but significant residual capacity to remove enzyme-sensitive sites as did the rad2-4 mutant. The latter was the only one of three different rad2 alleles examined which was leaky in this respect. The UV-sensitive strain carrying the mutant allele rad18-1 exhibited normal loss of enzyme-sensitive sites consistent with its assignment to the RAD6 rather than the RAD3 epistatic group. All strains having mutant alleles of the RAD1, RAD2, RAD3, RAD4, and RAD14 loci showed no detectable incubation-dependent strand breaks in nuclear DNA after exposure to UV radiation. These experiments suggest that the RAD1, RAD2, RAD3, RAD4 (and probably RAD14) genes are all required for the incision of UV-irradiated DNA during pyrimidine dimer excision in vivo.     

Studies on radiation-sensitive mutants of E. coli. II. Breakage and repair of ultraviolet irradiated intracellular DNA of phage lambda

Summary It has been shown that linear DNA molecules of phage are converted to the twisted circular structure (species I) by covalent closure of the both strands at the cohesive ends after infection to the immune bacteria and that the twisted circular molecules are transformed to the circular form (species II) by a single-strand break in one of the strands of their DNA. This system offers a very sensitive method to study on the strand breaks or their repair. For characterization of the defects of ultraviolet sensitive strains, the structural changes of ultraviolet irradiated DNA in these strains were studied.Ultraviolet irradiation to phage greatly reduced the extent of conversion of the molecules to the species I in the uvrD mutant while the irradiation showed little effect on the conversion in the uvrA, B and C mutants. When infected bacteria carrying species I molecules were irradiated, the species I molecules in the uvrD mutant were disrupted while most of the molecules in the uvrA, B and C mutants kept the structure. These results indicate that in the irradiated DNA strand breaks are rarely introduced or, if introduced, repaired rapidly in the uvrA, B and C mutants and they are introduced in the uvrD mutant leading to the degradation of the DNA. These results provide a firm evidence that the defect of the uvrD mutant is different from other Her^- mutants and in the process of repair synthesis.Ultraviolet irradiation to the uvrD mutants promote the formation of the species I molecules from the infected irradiated -DNA.Such effect was not observed with the uvrA mutant. Since the uvrD mutant has UV reactivation capacity and the uvrA mutant has not, the above phenomenon is probably caused by UV reactivation and may provide a more direct method to study the mechanisms of UV reactivation than the plaque assay.Abbreviations used UV Ultraviolet light - UVr Ultraviolet light reactivation This work was aided in part by a research grant GM 08384 from the United States Public Health Service.     

Rejoining kinetics of DNA single- and double-strand breaks in normal and DNA ligase-deficient cells after exposure to ultraviolet C and gamma radiation: an evaluation of ligating activities involved in different DNA repair processes

The repair kinetics for rejoining of DNA single- and double-strand breaks after exposure to UVC or gamma radiation was measured in cells with deficiencies in DNA ligase activities and in their normal counterparts. Human 46BR cells were deficient in DNA ligase I. Hamster EM9 and EM-C11 cells were deficient in DNA ligase III activity as a consequence of mutations in the XRCC1 gene. Hamster XR-1 cells had mutation in the XRCC4 gene, whose product stimulates DNA ligase IV activity. DNA single- and double-strand breaks were assessed by the comet assay in alkaline conditions and by the technique of graded-field gel electrophoresis in neutral conditions, respectively. 46BR cells, which are known to re-ligate at a reduced rate the DNA single-strand breaks incurred during processing of damage induced by UVC but not gamma radiation, were shown to have a normal repair of radiation-induced DNA double-strand breaks. EM9 cells exhibited a reduced rate of rejoining of DNA single-strand breaks after exposure to ionizing radiation, as reported previously, as well as UVC radiation. EM-C11 cells were deficient in the repair of radiation-induced-DNA single-strand breaks but, in contrast to EM9 cells, demonstrated the same kinetics as the parental cell line in the resealing of DNA breaks resulting from exposure to UVC radiation. Both EM9 and EM-C11 cells displayed a significant defect in rejoining of radiation-induced-DNA double-strand breaks. XR-1 cells were confirmed to be highly deficient in the repair of radiation-induced DNA double-strand breaks but appeared to rejoin DNA single-strand breaks after UVC and gamma irradiation at rates close to normal. Taken together these results indicate that: (1) DNA ligase I is involved only in nucleotide excision repair; (2) DNA ligase IV plays an important role only in repair of DNA double-strand breaks; and (3) DNA ligase III is implicated in base excision repair and in repair of DNA double-strand breaks, but probably not in nucleotide excision repair.     

Theory of survival of bacteria exposed to ionizing radiation. I. X and gamma rays

A new model for the survival of bacteria exposed to ionizing radiation is constructed in the framework of a target theory based on microdosimetric concepts, where single- and double-strand breaks of DNA and their repair in vivo can be described consistently in terms of the microdosimetric quantity j (number of effective primary events per track per target). In this model, the ability of cells to repair DNA damage is taken into consideration in terms of the repair capacities for single- and double-strand breaks of DNA, xi 1 and xi 2 (0 less than or equal to xi 1, xi 2 less than or equal to 1). To apply this model to Escherichia coli K-12 strains with different repair abilities, values of the repair capacity for single-strand breaks, xi 1, were derived from experimental survival curves. The theoretical survival curves for 60Co gamma rays were found to be effectively insensitive to the value of xi 2. Experimental survival curves for the wild-type, uvr, and rec strains of E. coli K-12 were well reproduced in this model. From these results, it is concluded that the theoretical formulation for the survival fraction of bacteria can afford a quantitative method for analysis of the repair process for radiation-induced single-strand breaks in DNA in vivo.     

X-ray Sensitivity and Repair Capacity of a polA1 exrA Strain of Escherichia coli K-12

A polA1 exrA strain of Escherichia coli K-12 was constructed. It was found to be more sensitive to aerobic or anoxic X irradiation than were mutants containing either polA1 or exrA alone. The ability of polA1 exrA and related strains to repair X-ray-induced single-strand breaks in deoxyribonucleic acid DNA was examined. The polA1 strain was deficient in type II (buffer) repair but not in type III (growth medium-dependent) repair. The exrA strain was not deficient in type II repair but was deficient in type III repair (similar to rec strains). The double mutant polA1 exrA was deficient in both type II and type III repair. Thus, the increased X-ray sensitivity of the polA1 exrA double mutant was correlated with its decreased ability to repair X-ray-induced single-strand breaks in DNA. We have tested the hypothesis that polA rec double mutants are not viable because they lack the types II and III systems for the repair of DNA single-strand breaks. Since the polA1 exrA strain is viable and is deficient in both of these repair processes, this hypothesis seems not to be correct.     

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.     

Effect of a recD mutation on DNA damage resistance and transformation in Deinococcus radiodurans

The bacterium Deinococcus radiodurans is resistant to extremely high levels of DNA-damaging agents such as UV light, ionizing radiation, and chemicals such as hydrogen peroxide and mitomycin C. The organism is able to repair large numbers of double-strand breaks caused by ionizing radiation, in spite of the lack of the RecBCD enzyme, which is essential for double-strand DNA break repair in Escherichia coli and many other bacteria. The D. radiodurans genome sequence indicates that the organism lacks recB and recC genes, but there is a gene encoding a protein with significant similarity to the RecD protein of E. coli and other bacteria. We have generated D. radiodurans strains with a disruption or deletion of the recD gene. The recD mutants are more sensitive than wild-type cells to irradiation with gamma rays and UV light and to treatment with hydrogen peroxide, but they are not sensitive to treatment with mitomycin C and methyl methanesulfonate. The recD mutants also show greater efficiency of transformation by exogenous homologous DNA. These results are the first indication that the D. radiodurans RecD protein has a role in DNA damage repair and/or homologous recombination in the organism.     

Double-strand Scissions in DNA of Gamma-irradiated Micrococcus radiodurans and their Repair during Postirradiation Incubation

Strand scissions in DNA of M, radiodurans after in vivo irradiation with ⁶⁰Co gamma rays were investigated by the sedimentation analysis using neutral sucrose gradients. Double-strand scission in DNA was estimated to occur at the rate of one double cut per 800 eV. This rate is in a good agreement of the value reported for mammalian cells. The rejoining of these double-strand scissions was observed during the repair process of the post-irradiation incubation and the mean rejoining time, i.e., the time reducing the remaining fraction of the double-strand scission to 0.37, was found to be 52 min. This rejoining repair was inhibited by adding chloramphenicol, tetracycline or actinomycin D to the postirradiation incubation medium. It is suggested that the high resistance character of M. radiodurans to gamma rays may be due to the efficient capacity of this rejoining repair.     

Repair of Single-Strand Deoxyribonucleic Acid Breaks in Ultraviolet Light-Irradiated Haemophilus influenzae

The wild-type strain and mutants of Haemophilus influenzae, sensitive or resistant to ultraviolet light (UV) as defined by colony-forming ability, were examined for their ability to perform the incision and rejoining steps of the deoxyribonucleic acid (DNA) dark repair process. Although UV-induced pyrimidine dimers are excised by the wild-type Rd and a resistant mutant BC200, the expected single-strand DNA breaks could not be detected on alkaline sucrose gradients. Repair of the gap resulting from excision must be rapid when experimental conditions described by us are employed. Single-strand DNA breaks were not detected in a UV-irradiated sensitive mutant (BC100) incapable of excising pyrimidine dimers, indicating that this mutant may be defective in a dimer-recognizing endonuclease. No single-strand DNA breaks were detected in a lysogen BC100(HP1c1) irradiated with a UV dose large enough to induce phage development in 80% of the cells.     

Effects of sodium arsenite on single-strand DNA break formation and post-replication repair in E. coli following UV irradiation

A non-lethal dose of sodium arsenite is found to inhibit the formation of single-strand DNA breaks in Escherichia coli WP2 wild-type and WP6 polA strains after UV irradiation. Inhibition of single-strand breakage follows a dose-dependent relationship with respect to increasing sodium arsenite concentration. ATP level in WP2 cells is decreased in the presence of sodium arsenite and therefore the inhibition of DNA break formation may be mediated through lowered ATP levels in the irradiated cells. In the presence of a non-lethal dose of sodium aresenite, post-replication repair in WP2 uvrA strains after UV irradiation is also inhibited.