Induction and repair of double- and single-strand DNA breaks in bacteriophage lambda superinfecting Escherichia coli

Induction and repair of double- and single-strand DNA breaks have been measured after decays of 125I and 3H incorporated into the DNA and after external irradiation with 4 MeV electrons. For the decay experiments, cells of wild type Escherichia coli K-12 were superinfected with bacteriophage lambda DNA labelled with 5'-(125I)iodo-2'-deoxyuridine or with (methyl-3H)thymidine and frozen in liquid nitrogen. Aliquots were thawed at intervals and lysed at neutral pH, and the phage DNA was assayed for double- and single-strand breakage by neutral sucrose gradient centrifugation. The gradients used allowed measurements of both kinds of breaks in the same gradient. Decays of 125I induced 0.39 single-strand breaks per double-strand break. No repair of either break type could be detected. Each 3H disintegration caused 0.20 single-strand breaks and very few double-strand breaks. The single-strand breaks were rapidly rejoined after the cells were thawed. For irradiation with 4 MeV electrons, cells of wild type E. coli K-12 were superinfected with phage lambda and suspended in growth medium. Irradiation induced 42 single-strand breaks per double-strand break. The rates of break induction were 6.75 x 10(-14) (double-strand breaks) and 2.82 x 10(-12) (single-strand breaks) per rad and per dalton. The single-strand breaks were rapidly repaired upon incubation whereas the double-strand breaks seemed to remain unrepaired. It is concluded that double-strand breaks in superinfecting bacteriophage lambda DNA are repaired to a very small extent, if at all.     

In vivo nicking and rejoining of nuclear DNA in ultraviolet-irradiated radiation-resistant and sensitive strains of Dictyostelium discoideum

Summary Some aspects of DNA repair in several radiation-resistant and radiation-sensitive strains of Dictyostelium discoideum were investigated by using alkaline sucrose gradients to analyze for the production and resealing of single-strand breaks following irradiation with 254 nm UV. All radiation-resistant strains and all mutants assayed that are sensitive to both UV and ⁶⁰Co gamma rays produced singlestrand breaks in their nuclear DNA after a UV fluence of 15 J/m². Mutants at the radC locus which are sensitive to UV but as resistant as their parental strains to ⁶⁰Co gamma rays produced many fewer single-strand breaks in their DNA after irradiation with UV. Thus, the radC mutations alter a repair pathway specific for UV-induced DNA damage and presumably affect the activity of a UV-damage-specific endonuclease involved in excision repair. All radiation-resistant strains and all of our mutants sensitive to gamma rays rejoined much of their DNA during a three-hour post-UV-irradiation incubation, suggesting that these strains have at least a partially intact excision repair system.Abbreviations used UV ultraviolet light - PBS phosphate buffered saline - cpm counts per minute     

A role for single-strand breaks in bacteriophage phi-X174 genetic recombination

Formation of genetic recombinants in bacteriophage φX174 is stimulated up to 50-fold in host cells carrying the recA⁺ allele by subjecting the virus particles to ultraviolet irradiation before infection, or by starving the host cell for thymine during infection; in recA host strains no such increases are observed.φX174 replicative form DNA molecules formed in vivo from ultraviolet-irradiated bacteriophage consist of an intact, circular full-length viral (+) strand and a partially complete complementary (?) strand extending from the point of origin of complementary strand DNA synthesis to an ultraviolet lesion. φX174 replicative form DNA molecules formed in thymine-deficient host strains during thymine starvation have nearly complete circular viral (+) and complementary (?) strands, which contain random single-strand nicks or gaps.Correlation of these structures with the observed increases in recombination suggests that single-strand “breaks” are aggressive intermediate structures in the formation of φX174 genetic recombinants mediated by the host recA⁺ gene product.     

Biological function for 6-methyladenine residues in the DNA of Escherichia coli K12

A strain of Escherichia coli K12 mutant at the dam² site contains 0.8 mole % 6-methyl adenine as compared to 0.50 mole % in the wild type, and the residual DNA methylation is not due to the K12 modification methylase specified by the hsp genes. The dam-3 mutant is more sensitive to ultraviolet irradiation and to mitomycin C than the wild type and also shows a higher mutability. DNA isolated from the dam-3 mutant contains single-stranded breaks that are amplified in dam-3 polA12 and dam-3 lig-7 double mutants. A function of dam-specified 6-methyl adenine residues in DNA would, therefore, appear to be the protection of DNA from a nuclease(s) that causes the development of breaks. Combination of dam-3 with polA, recA, recB and recC is lethal.     

DNA crosslinks,single-strand breaks and effects on bacteriophage T4 survival from tritium decay of [2-3H]adenine, [8-3H]adenine and [8-3H]guanine

Escherichia coli and bacteriophage T4 DNA containing [2-³H]adenine accumulated crosslinks between the complementary strands. For T4 DNA stored in frozen solution there were 0.41 to 0.54 crosslinks formed per tritium decay. The crosslinks were demonstrated both by an increased DNA sedimentation rate in alkaline sucrose gradients and by an increasing amount of DNA that renatured quickly after denaturation by heat or alkali. Single-strand breaks were also formed with an efficiency of 0.08 to 0.50 breaks per tritium decay. DNA containing both [8-³H]adenine and [8-³H]guanine showed no crosslinking but did undergo single-strand breaks at a rate of 0.08 per tritium decay. T4 bacteriophage containing [2-³H]adenine lost plaque-forming ability when stored at 4 °C, with 0.34 lethal hits per tritium decay, whereas the same phage labeled with a mixture of [8-³H]adenine and [8-³H]guanine sustained only 0.12 lethal hits per tritium decay. The loss of plaque-forming ability in the latter case is probably due to a radiation effect from the emitted beta particle; the high lethal efficiency for tritium decay at 2-adenine is probably caused either by crosslinks between complementary strands or from some undetected lesion produced in the DNA.     

Repairable and unrepairable DNA strand breaks induced by decay of3H and125I incorporated into DNA of mammalian cells

Summary Chinese hamster cells (Cl : 1) were labelled with³H-thymidine or¹²⁵Iododeoxyuridine for 18 h and after 3 h in non-radioactive medium they were stored at 0° C up to 6 h. The number of DNA strand breaks observed after the labelling period (37° C) or after treatment at 0° C was determined using the DNA-unwinding technique.¹²⁵I-decays in DNA were significantly more efficient than³H-decays in introducing unrepairable DNA strand breaks during the labelling period. 32% of¹²⁵I-induced and 3% of³H-induced DNA strand breaks were unrepaired after 21 h at 37° C. Comparison between the effects of¹²⁵I- or 3H-disintegrations in DNA in three different ways shows 7–12 times more pronounced effects for¹²⁵I-decays. For¹²⁵I-labelled cells 3–4 DNA strand breaks were found per decay and the corresponding value for³H- labelled cells was 2.     

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.     

The Mechanism of Inactivation of T4 Bacteriophage by Tritium Decay

Coliphage T4 was used as a model system to study the mechanism of biological inactivation produced by tritium decay. Experimentally, tritiated precursors were incorporated into phage DNA (thymidine-³H) or into phage protein (³H-amino acids). The ratio of killing efficiencies for decays originating in phage DNA to those originating in phage protein was 2.6. Inactivation by decays from labeled amino acids was assumed to occur exclusively from β-particle irradiation of phage DNA. If decays originating in DNA are due solely to irradiation of DNA, then the killing efficiencies reflect the energy transfer paths in phage DNA for decays originating in phage DNA and in the protein coat. The energy transfer paths were determined for the two cases with the help of a computer and found to be very nearly equal to the experimentally determined ratio (2.6). The killing efficiencies for decays originating in phage DNA were 0.12 and for decays originating in protein 0.046.     

Effects of x irradiation on a temperate bacteriophage of Haemophilus influenzae.

The inactivation of bacteriophage HP1c1 by X rays in a complex medium was found to be exponential, with a D0 (the X-ray exposure necessary to reduce the survival of the phage to 37%) of approximately 90 kR. Analysis of results of sucrose sedimentation of DNA from X-irradiated whole phage showed that the D0 for intactness of single strands was about 105kR, and for intactness of double strands, it was much higher. The D0 for attachment of X-irradiated phage to the host was roughly estimated as about 1,100 kR. Loss of DNA from the phage occurred and was probably due to lysis of the phage by X irradiation, but the significance of the damage is not clear. The production of single-strand breaks approaches the rate of survival loss after X irradiation. However, single-strand breaks produced by UV irradiation, in the presence of H2O2, equivalent to 215 kR of X rays, showed no lethal effect on the phage. Although UV-sensitive mutants of the host cell, Haemophilus influenzae, have been shown to reactivate UV-irradiated phage less than does the wild-type host cell, X-irradiated phage survive equally well on the mutants as on the wild type, a fact suggesting that other repair systems are involved in X-ray repair.     

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.     

Mutants partially defective in excision repair at five autosomal loci in Drosophila melanogaster

Primary cell cultures derived from mutants in seventeen different genes were analyzed for their ability to excise pyrimidine dimers from DNA. Five of these mutagen-sensitive mutants [mus(2)205^A1, mus(3)302^D1, mus(3) 304^D3, mus(3)306^D1, mus(3)308^D2] display a significantly reduced excision capacity relative to control cultures. In addition, two of the five [mus(3)306^D1, mus(3)308^D2] are defective in the accumulation of single-strand breaks normally seen after ultraviolet irradiation. This study, therefore, brings the total number of Drosophila mutants known to be defective in excision repair to seven. The results are discussed relative to other genetic and biochemical properties of these mutants. This work is dedicated to Professor W. Beermann whose own contributions were instrumental in focusing a modern analysis of the eukaryotic genome on the diptera. Those of us who benefitted so much from his personal guidance recognize that we did so as a result of some sacrifice on his part. One of Boyd's contemporaries in Tübingen once remarked: “It's terrifying to think what Professor Beermann could do if he were in the lab full time.”     

Repair Replication in Escherichia coli As Measured by the Photolysis of Bromodeoxyuridine

The ability to selectively photolyze bromouracil-(BrUra-)containing repaired regions in cellular DNA has allowed us to estimate the average size of repaired regions in ultraviolet (UV) light-irradiated Escherichia coli. Cells were labeled with thymidine-³H, irradiated at 254 nm, and incubated in nonradioactive bromodeoxyuridine (BrdUrd). After incubation the cells were exposed to 10⁶ ergs·mm^-2 at 313 nm, lysed, and sedimented in alkaline sucrose gradients so as to measure the average molecular weight of single DNA strands. In strains that had excised ~45 cyclobutane pyrimidine dimers/10⁸ daltons, the 313 nm treatment resulted in ~6 single-strand breaks/10⁸ daltons. In an excisionless strain, the same treatment resulted in only 1.5 breaks/10⁸ daltons. From the determination of the sensitivities of fully substituted DNAs to 313 nm light, we calculate that the repaired regions in excising strains of E. coli contain an average of 4-6 BrUra residues. Photoreactivation experiments indicate that the excision of pyrimidine dimers in the presence of BrdUrd is the primary source of repaired regions selectively photolyzed by 313 nm radiation.     

Incisions in ultraviolet irradiated circular bacteriophage lambda DNA molecules in excision proficient and deficient lysogens of E. coli

Summary The breakage of closed covalent DNA molecules in lysogenic host cells after ultravilet irradiation was investigated. In repair proficient host cells incisions are introduced immediately following irradiation. A steady-state of strand interruptions is observed within 20–50 seconds after irradiation, where the number of broken molecules is dose dependent for doses up to 600 ergs/mm². No ultraviolet promoted strand breaks were observed in uvrA or uvrB lysogens, in accordance with previous results obtained by Shimada, Ogawa & Tomizawa [Molec. gen. Genet. 101, 245 (1968)]. In contrast, uvrC mutants have the ability to form breaks in superinfecting DNA molecules after ultraviolet irradiation. The ultraviolet specific endonucleolytic activity observed in uvrC host cells differs from that observed in uvr ⁺ host cells in that, (i) the first break is introduced at least 15 times slower, (ii) for doses below 300 ergs/mm² the number of strand breaks is higher, (iii) the dose dependence terminates at a lower dose. The possible function of the uvrC gene product in the repair is discussed.     

Comet assay evaluation of DNA single- and double-strand breaks induction and repair in C3H10T1/2 cells

Ionizing radiation is a potent inducer of DNA damage because it causes single- and double-strand breaks, alkali-labile sites, base damage, and crosslinks. The interest in ionizing radiation is due to its environmental and clinical implications. Single-strand breaks, which are the initial damage induced by a genotoxic agent, can be used as a biomarker of exposure, whereas the more biologically relevant double-strand breaks can be analyzed to quantify the extent of damage. In the present study the effects of ¹³⁷Cs γ-radiation at doses of 1, 5, and 10 Gray on DNA and subsequent repair by C3H10T1/2 cells (mouse embryo fibroblasts) were investigated. Two versions of the comet assay, a sensitive method for evaluating DNA damage, were implemented: the alkaline one to detect single-strand breaks, and the neutral one to identify double-strand breaks. The results show a good linear relation between DNA damage and radiation dose, for both single-strand and double-strand breaks. A statistically significant difference with respect to controls was found at the lowest dose of 1 Gy. Heterogeneity in DNA damage within the cell population was observed as a function of radiation dose. Repair kinetics showed that most of the damage was repaired within 2 h after irradiation, and that the highest rejoining rate occurred with the highest dose (10 Gy). Single-strand breaks were completely repaired 24 h after irradiation, whereas residual double-strand breaks were still present. This finding needs further investigation. This revised version was published online in July 2006 with corrections to the Cover Date.     

Unwinding of DNA induced by fluorescein mercuric acetate

Fluorescein mercuric acetate causes the unwinding of DNA and binds to the separated bases. This unwinding process can be followed by measuring absorption changes of this reagent. For untreated calf thymus DNA, the initial rate was very slow, and the shape of the kinetic curve was sigmoidal. When double-strand breaks of DNA were produced by DNase II treatment or sonication, the initial rate increased and the sigmoidal character disappeared. The initial rate was shown to be proportional to the concentration of helix ends. From this relation the rate of unwinding was estimated to be 2.0 base pairs/sec at 1.0 × 10^?5M fluorescein mercuric acetate and 25°C. DNase I treatment, which produces single-strand breaks and a smaller number of double-strand breaks, also increased the initial rate. However, this increase was due only to the double-strand breaks, that is, single-strand breaks had no significant effect on the initial rate. Also, uv irradiation increased the initial rate linearly with uv dose, at least up to 2 × 10⁵ erg/mm², suggesting that this increase is due to photoproducts other than usual pyrimidine dimers. We discuss the usefulness of this kinetic method in structural studies of DNA.     

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     

Single-strand breakage on binding of DNA to cells in the genetic transformation of Diplococcus pneumoniae.

Mutants of Diplococcus pneumoniae that lack a membrane-localized DNAase are defective in transformation because entry of DNA into the cell is blocked. Such mutants still bind DNA on the outside of the cell. The bound DNA is double-stranded and its double-stranded molecular weight is unchanged. Its sedimentation behavior in alkali, however, shows that it has undergone single-strand breakage. The breaks are located randomly in both strands of the bound DNA at a mean separation of 2 × 10⁶ daltons of single-stranded DNA. Both binding and single-strand breakage occur in the presence of EDTA. Single-strand breaks are similarly formed on binding of DNA to normally transformable cells in the presence of EDTA. The single-strand breaks appear to be a consequence of attachment. DNA may be bound to the cell surface at the point of breakage.A mutant that is partially blocked in entry also binds DNA mainly on the outside of the cell. In the presence of EDTA, DNA bound by this mutant undergoes only single-strand breaks. In the absence of EDTA, however, double-strand breaks occur, apparently as a result of the initiation of entry. It is possible that the double-strand breaks arise from additional single-strand breaks opposite those that occurred on binding. The double-strand breaks presumably result from action of the membrane DNAase as it begins to release oligonucleotides from one strand segment while drawing the complementary strand segment into the cell.     

Properties of Mitomycin C-sensitive Mutants of Escherichia coli K-12

Strains hypersensitive to mitomycin C (MC) were isolated from Escherichia coli K-12 after treatment with nitrosoguanidine. Of 43 MC-sensitive strains tested for their ultraviolet light (UV) sensitivity and for their ability to reactivate UV-inactivated λ phage, 38 were found to be insensitive to UV irradiation and to be able to reactivate UV-irradiated bacteriophage λ. Some properties of the MC-sensitive, uvr⁺ mutants were analyzed. Synthesis of deoxyribonucleic acid (DNA) in MC-sensitive, uvr⁺ mutants was inhibited at a lower concentration of MC than in the wild-type strain. Mutant cells, labeled with ³H-thymidine and then exposed to MC, released radioactivity as low molecular weight compounds. The amount of radioactivity released was the same as that from the wild-type strain. MC-sensitive, uvr⁺ mutants, as well as the corresponding wild-type strain, were equally susceptible to induction of prophage 80 by UV irradiation. However, MC induction of prophage was achieved in MC-sensitive, uvr⁺ mutants at a lower concentration of the antibiotic than in the wild-type strain. Genetic experiments indicated that a gene controlling MC sensitivity is located close to that determining lactose fermentation of E. coli. It is situated on episome F′13, and the wild type is dominant to the MC-sensitive allele.     

In vitro complementation of Tdp1 deficiency indicates a stabilized enzyme-DNA adduct from tyrosyl but not glycolate lesions as a consequence of the SCAN1 mutation

A homozygous H493R mutation in the active site of tyrosyl-DNA phosphodiesterase (TDP1) has been implicated in hereditary spinocerebellar ataxia with axonal neuropathy (SCAN1), an autosomal recessive neurodegenerative disease. However, it is uncertain how the H493R mutation elicits the specific pathologies of SCAN1. To address this question, and to further elucidate the role of TDP1 in repair of DNA end modifications and general physiology, we generated a Tdp1 knockout mouse and carried out detailed behavioral analyses as well as characterization of repair deficiencies in extracts of embryo fibroblasts from these animals. While Tdp1^?/? mice appear phenotypically normal, extracts from Tdp1^?/? fibroblasts exhibited deficiencies in processing 3′-phosphotyrosyl single-strand breaks and 3′-phosphoglycolate double-strand breaks (DSBs), but not 3′-phosphoglycolate single-strand breaks. Supplementing Tdp1^?/? extracts with H493R TDP1 partially restored processing of 3′-phosphotyrosyl single-strand breaks, but with evidence of persistent covalent adducts between TDP1 and DNA, consistent with a proposed intermediate-stabilization effect of the SCAN1 mutation. However, H493R TDP1 supplementation had no effect on phosphoglycolate (PG) termini on 3′ overhangs of double-strand breaks; these remained completely unprocessed. Altogether, these results suggest that for 3′-phosphoglycolate overhang lesions, the SCAN1 mutation confers loss of function, while for 3′-phosphotyrosyl lesions, the mutation uniquely stabilizes a reaction intermediate.     

Radiation-induced DNA single-strand scission and its rejoining in spermatogonia and spermatozoa of mouse

Gamma-ray-induced DNA single-strand scissions and the ability to repair the scissions in spermatogonia from young mice and in spermatozoa from adult mice were studied quantitatively by an alkaline sucrose density-gradient centrifugation method. The average size of DNAs in non-irradiated spermatogonia was 2.6–3.0 × 10⁸ daltons, similar to those of a spermatid-rich population, and the size of DNA in non-irradiated spermatozoa was 1.2 × 10⁸ daltons.In spermatogonia, the radiosensitivity of DNA was 0.42 single-strand breaks/ 10¹² daltons of DNA/rad in oxic conditions and only 0.24 under anoxic conditions. In spermatozoa the break efficiency of DNA was 0.22 single-strand breaks/10¹² daltons of DNA/rad under oxic conditions and altered little under anoxic irradiation. The DNA scissions were efficiently repaired in spermatogonia within 10 min, whereas the breaks in spermatozoa were not rejoined at all even after two days of post-irradiation time.The radiosensitivities of DNA, repair capability and non- and/or slow-reparable DNA scissions were compared in spermatogonium-rich, spermatid-rich and spermatozoan-rich populations.