DNA repair defect in AT cells and their hypersensitivity to low-dose-rate radiation

Ataxia telangiectasia (AT) and normal cells immortalized with the human telomerase gene were irradiated in non-proliferative conditions with high- (2 Gy/min) or low-dose-rate (0.3 mGy/min) radiation. While normal cells showed a higher resistance after irradiation at a low dose rate than a high dose rate, AT cells showed virtually the same survival after low- and high-dose-rate irradiation. Although the frequency of micronuclei induced by low-dose-rate radiation was greatly reduced in normal cells, it was not reduced significantly in AT cells. The number of gamma-H2AX foci increased in proportion to the dose in both AT and normal cells after high-dose-rate irradiation. Although few gamma-H2AX foci were observed after low-dose-rate irradiation in normal cells, significant and dose-dependent numbers of gamma-H2AX foci were observed in AT cells even after low-dose-rate irradiation, indicating that DNA damage was not completely repaired during low-dose-rate irradiation. Significant phosphorylation of ATM proteins was detected in normal cells after low-dose-rate irradiation, suggesting that the activation of ATM plays an important role in the repair of DNA damage during low-dose-rate irradiation. In conclusion, AT cells may not be able to repair some fraction of DNA damage and are severely affected by low-dose-rate radiation.     

Thermal enhancement of the radiation damage in the mouse foot at different heat and radiation dose: influence of thermotolerance

We studied the reaction of the mouse foot after combined X-irradiation and heat treatment. Acute reactions after heat differ from those after irradiation, however, after healing of the lesions, the same symptoms of deformity of the mouse foot remain. Prior heat treatment, 30 min at 43 degrees C, of the foot led to thermotolerance and this thermotolerance resulted in resistance to combined irradiation-heat treatments and hence to a decreased thermal enhancement of radiation effects. Resistance could be observed up to 168 h after prior heat treatment. The development of resistance to combined treatment at higher irradiation dose (15 or 20 Gy) and less severe heating was slower than at lower irradiation dose (10 Gy) and more severe heating. Thermal enhancement was confirmed to be dependent on the sequence of, and the interval between irradiation and heat treatment. When the mouse foot was made thermotolerant by prior heat treatment, thermal enhancement was always reduced, regardless of the sequence, when the combined heat and radiation treatments were given with an interval of less than 12 h. Thermotolerance led to an apparent decrease in the effective temperature employed in a combined treatment equivalent to approximately 1.0 degrees C, at temperatures above 43 degrees C in a 1 h heat treatment.     

Activation of antioxidative enzymes induced by low-dose-rate whole-body gamma irradiation: adaptive response in terms of initial DNA damage

An adaptive response induced by long-term low-dose-rate irradiation in mice was evaluated in terms of the amount of DNA damage in the spleen analyzed by a comet assay. C57BL/ 6N female mice were irradiated with 0.5 Gy of (137)Cs gamma rays at 1.2 mGy/h; thereafter, a challenge dose (0.4, 0.8 or 1.6 Gy) at a high dose rate was given. Less DNA damage was observed in the spleen cells of preirradiated mice than in those of mice that received the challenge dose only; an adaptive response in terms of DNA damage was induced by long-term low-dose-rate irradiation in mice. The gene expression of catalase and Mn-SOD was significantly increased in the spleen after 23 days of the low-dose-rate radiation (0.5 Gy). In addition, the enzymatic activity of catalase corresponded to the gene expression level; the increase in the activity was observed at day 23 (0.5 Gy). These results suggested that an enhancement of the antioxidative capacities played an important role in the reduction of initial DNA damage by low-dose-rate radiation.     

Radiation sensitivity of <Emphasis Type="Italic">N. pharaonis</Emphasis> in comparison with <Emphasis Type="Italic">E. coli </Emphasis>K12 strains

To investigate the radiation sensitivity of the natronobacterium Natronomonas pharaonis in comparison with Escherichia coli strains (N. pharaonis DSM 2160T, E. coli strains AB1157 and K12 lambda s) were exposed to gamma-radiation (60Co-gamma-source, 100 Gy min-1) in the presence of oxygen (air) and under strongly reduced oxygen conditions (argon-saturated medium). After irradiation, the colony-forming ability (dose-survival curves) and the D37 dose were determined. The oxygen content of the solutions containing high NaCl concentrations was measured with an oxygen electrode (Clark electrode). It was found that N. pharaonis can tolerate a remarkably higher irradiation dose than the two E. coli strains (approx. 1.5-fold of K12 lambda s and approx. 4-fold of AB1157). The oxygen enhancement ratio (OER) is 2.8 for N. pharaonis and 2.6 for both E. coli strains. Therefore the higher radiation resistance of the N. pharaonis is not due to the low oxygen content of the cell solution (high salt concentration) but is probably related to the higher DNA repair ability of this archaebacteria strain.     

The response of murine stem spermatogonia to radiation combined with 3-aminobenzamide

The influence of 3-aminobenzamide (3-AB) on the radiation response of the stem spermatogonia of the CBA mouse has been investigated. Doses of 3-AB from 66 to 450 mg/kg, administered 1 h before irradiation, significantly enhanced stem-cell killing. Enhancement was observed when 3-AB (450 mg/kg) was given up to 5 h before, but not if administered after, irradiation. When radiation was delivered at a lower dose rate (5 cGy/min compared to 180 cGy/min) significant dose sparing was achieved for radiation alone. Pretreatment with 3-AB resulted in slightly less enhancement at the low dose rate than at the high. Split-dose studies (9 Gy total dose) with radiation alone resulted in a recovery ratio of 1.4-1.5. Administration of 3-AB before the first dose resulted in a similar recovery ratio, but if given immediately after the first dose the ratio was smaller. Pretreatment of mice with the radiosensitizer RSU-1069 indicated that at least some of the stem cells were radiobiologically hypoxic. We suggest therefore that the enhancement of spermatogonial stem-cell killing by 3-AB is not entirely due to inhibition of repair processes but may also involve modification of the oxygen status of the testis.     

Oxygen-dependent protection of radiation lung damage in mice by WR 2721

The modification of early and late radiation damage to the mouse lung by oxygen and WR 2721 has been studied by measurement of breathing rate, lethality, pleural fluid and hydroxyproline content. Protection by hypoxia and sensitization by hyperoxia of early radiation pneumonitis were demonstrated. There was a tendency for the protective effect of WR 2721 to decrease as the breathed oxygen concentration was raised above normal levels. WR 2721 protection of the late damage was higher (PF = 1.6-1.65) than was seen for early pneumonitis (PF = 1.3-1.35) when either breathing rate or lethality were used. Protection factors (PF) gained from measurements of pleural fluid at a year after treatment were similar to those for other endpoints of late damage (PF = 1.7). In contrast, the measurement of fibrosis through determination of lung hydroxyproline at 1 year gave a somewhat lower protection factor for WR 2721. In the same experiments the degree of epilation on the dorsal thorax was scored at 6 weeks. One hundred per cent oxygen gave enhancement (dose enhancement factor (DEF) = 1.2), 9 per cent oxygen reduced damage (DEF less than 0.7) and WR 2721 gave PF values in excess of 1.4 at all oxygen concentrations used. This showed that the radiation response of hair follicles was more sensitive to WR 2721 or to changes of oxygen than the lung. The results presented indicate a competitive interaction between WR 2721 and oxygen for the same injury site causing a shift in the oxygen K curve to higher oxygen concentrations. The validity of applying functional or survival measurements to assess the extent of pulmonary fibrosis is discussed.     

不同剂量的X-射线全身照射对小鼠健康状况及涎腺的影响

目的:通过直线加速器全身照射昆明小鼠建立辐射损伤模型,探索不同放射剂量对小鼠健康状况及涎腺功能和结构的影响。方法:选取八种不同剂量对昆明小鼠行体外全身照射,于照射后一个月内观察小鼠生长情况、体重变化;照射后一周、一个月检测各组小鼠血象的变化;测定放射半数致死剂量;照射后两个月,测定各组小鼠的唾液流量及唾液淀粉酶含量,并对下颌下腺组织切片行HE染色。结果:13Gy和15Gy照射组小鼠的体重逐渐下降,一周后死亡,其余组小鼠体重最终呈增加趋势。X-射线全身照射的半数致死量为10Gy。照射后一周,照射组小鼠的白细胞数目明显降低,与对照组比较有明显统计学差异(P0.01);在其他血象方面,除了7Gy组外,其他照射组与对照组比较也均有统计学差异(P0.05)。照射一个月后,各照射组小鼠的血象均恢复正常。照射后两个月,9Gy组和11Gy组小鼠的唾液流量及唾液淀粉酶含量均显著低于0Gy组,且11Gy组较9Gy组亦明显降低,差异均有统计学意义(P0.05)。随照射剂量的增加,小鼠的下颌下腺腺泡细胞数目逐步减少,结构排列紊乱,组织损伤逐渐加重。结论:X-射线全身照射引起小鼠健康状况受损,免疫功能减低,损伤程度与放射线强度呈剂量依赖性,小鼠半数致死量为10Gy,该剂量适合建立全身放射损伤模型。     

Inhibition of checkpoint kinase 1 sensitizes lung cancer brain metastases to radiotherapy

The most important therapeutic tool in brain metastasis is radiation therapy. However, resistance to radiation is a possible cause of recurrence or treatment failure. Recently, signal pathways about DNA damage checkpoints after irradiation have been noticed. We investigated the radiosensitivity can be enhanced with treatment of Chk1 inhibitor, AZD7762 in lung cancer cell lines and xenograft models of lung cancer brain metastasis. Clonogenic survival assays showed enhancement of radiosensitivity with AZD7762 after irradiation of various doses. AZD7762 increased ATR/ATM-mediated Chk1 phosphorylation and stabilized Cdc25A, suppressed cyclin A expression in lung cancer cell lines. In xenograft models of lung cancer (PC14PE6) brain metastasis, AZD7762 significantly prolonged the median survival time in response to radiation. Depletion of Chk1 using shRNA also showed an enhancement of sensitivity to radiation in PC14PE6 cells. The results of this study support that Chk1 can be a good target for enhancement of radiosensitivity.     

Exposure to low dose of gamma radiation enhances the excision repair in Saccharomyces cerevisiae

The effect of low doses of ionizing and nonionizing radiation on the radiation response of yeast Saccharomyces cerevisiae toward ionizing and nonionizing radiation was studied. The wild-type strain D273-10B on exposure to 54 Gy gamma radiation (resulting in about 10% cell killing) showed enhanced resistance to subsequent exposure to UV radiation. This induced UV resistance increased with the incubation time between the initial gamma radiation stress and the UV irradiation. Exposure to low doses of UV light on the other hand showed no change in gamma or UV radiation response of this strain. The strains carrying a mutation at rad52 behaved in a way similar to the wild type, but with slightly reduced induced response. In contrast to this, the rad3 mutants, defective in excision repair, showed no induced UV resistance. Removal of UV-induced pyrimidine dimers in wild-type yeast DNA after UV irradiation was examined by analyzing the sites recognized by UV endonuclease from Micrococcus luteus. The samples that were exposed to low doses of gamma radiation before UV irradiation were able to repair the pyrimidine dimers more efficiently than the samples in which low gamma irradiation was omitted. The nature of enhanced repair was studied by scoring the frequency of induced gene conversion and reverse mutation at trp and ilv loci respectively in strain D7, which showed similar enhanced UV resistance induced by low-dose gamma irradiation. The induced repair was found to be essentially error-free. These results suggest that irradiation of strain D273-10B with low doses of gamma radiation enhances its capability for excision repair of UV-induced pyrimidine dimers.     

An investigation of the reaction kinetics of luciferase and the effect of ionizing radiation on the reaction rate

The bioluminescence produced by luciferase, a firefly enzyme, requires three substrates: luciferin, ATP and oxygen. We find that ionizing radiation, in the form of a proton beam from a cyclotron, will eliminate dissolved oxygen prior to any damage to other substrates or to the protein. The dose constant for removal of oxygen is 70 ± 20 Gy, a much smaller dose than required to cause damage to protein. Removal of oxygen, which is initially in excess, leads to a sigmoidal response of bioluminescence to radiation dose, consistent with a Michaelis–Menten relationship to substrate concentration. When excess oxygen is exhausted, the response becomes exponential. Following the irradiation, bioluminescence recovers due to a slow leak of oxygen into the solution. This may also explain previous observations on the response of bioluminescent bacteria to radiation. We have studied the dependence of the reaction rate on enzyme and substrate concentration and propose a model for the reaction pathway consistent with this data. The light output from unirradiated samples decreases significantly with time due to product inhibition. We observe that this inhibition rate changes dramatically immediately after a sample is exposed to the beam. This sudden change of the inhibition rate is unexplained but shows that enzyme regulatory function responds to ionizing radiation at a dose level less than 0.6 Gy.     

Dose-rate effects of 8-methoxypsoralen plus 365-NM irradiation on cell killing in Saccharomyces cerevisiae.

Tow types of dose-rate effect that alter the survival response of haploid yeast cells to 8-methoxypsoralen (8-MOP) plus treatment with irradiation at 365 nm were studied. (1) When the concentration of 8-MOP was varied between 9.2 X 10(-5) and 2.3 X 10(-8) M and the dose rate of 365-nm irradiation kept constant, the efficiency of the irradiation for killing increased relatively to that of 8-MOP whe the concentration of 8-MOP decreased. This indicated that there was no strict reciprocity between radiation dose and concentration of drug. (2) When the dose rate of radiation was varied between 0.66 X 10(3) and 108 X 10(3) J m-2 h-1 and the concentration of 8-MOP was kept constant, the survival of wild-type cells increased strikingly at low dose rates of radiation as compared with high dose rates. Cells responded more to changes at low dose rates than to equal changes a high dose rates. The high resistance of wild-type cells to 8-MOP plus radiation delivered at low dose rates absent from rad 1-3 cells defective in excision-repair. This suggests that the dose-rate effect seen in wild-type cells depended at least in part on an active excision-repair function. At low dose rates of radiation, the shoulder of the survival curve for rad1-3 cells, i.e. the ability to accumulate sub-lethal damage, was increased by a factor of about 2 when compared with that seen at a high dose rate. Thus it is likely that at low dose rates a repair function other than excision-resynthesis may operate in rad1-3 cells.     

Early radiation effects on muscarinic receptor-induced secretory responsiveness of the parotid gland in the freely moving rat

Although the salivary glands have a low rate of cell turnover, they are relatively radiosensitive. To study the possible mechanism behind this inherent radiosensitivity, a rat model was developed in which saliva can be collected after local irradiation of the parotid gland without the use of anesthetics or stressful handling. Saliva secretion was induced by the partial muscarinic receptor agonist pilocarpine (0.03-3 mg/kg) with or without pretreatment with the beta-adrenoceptor antagonist propranolol (2.5 mg/kg), or the full muscarinic receptor agonist methacholine (0.16-16 mg/min), and measured during 5 min per drug dose before and 1, 3, 6 and 10 days after irradiation. The maximal secretory response induced by pilocarpine plus propranolol was increased compared to that with pilocarpine alone but did not reach the level of methacholine-induced secretion, which was about five times higher. One day after irradiation a decrease in maximal pilocarpine-induced secretion was observed (-22%) using the same dose of pilocarpine that induces 50% of the maximal response (ED(50)), in both the absence and presence of propranolol, indicating that the receptor-drug interaction was not affected by the radiation at this time. The secretory response to methacholine 1 day after irradiation, however, was normal. At day 3 after irradiation, the maximal methacholine-induced secretion was also affected, whereas pilocarpine (+/-propranolol)-induced maximal secretion decreased further. At day 6 after irradiation, maximal secretory responses had declined to approximately 50% regardless of the agonist used, whereas ED(50) values were still unaffected. No net acinar cell loss was observed within the first 10 days after irradiation, and this therefore could not account for the loss in function. The results indicate that radiation does not affect cell number or receptor-drug interaction, but rather signal transduction, which eventually leads to the impaired response. We hypothesize that the early radiation effect, within 3 days, may be membrane damage affecting the receptor-G-protein signaltransfer. Later critical damage, however, is probably of a different nature and may be located in the second-messenger signal transduction pathway downstream from the G protein, not necessarily involving cellular membranes.     

Neutron and cobalt-60 gamma irradiation produce similar changes in DNA supercoiling

The effects of 60Co gamma-ray and d(20 MeV)Be neutron irradiation on DNA supercoiling have been studied using a nucleoid rewinding technique. Irradiation of viable CHO AA8 cells on ice with from 4 to 25 Gy of either radiation produced a similar resistance to rewinding of nuclear supercoils after treatment with ethidium bromide. The restitution from the effects of 12 Gy of either radiation was also similar, leaving no detectable residual damage. The discrepancy between these data and the reduced ability of neutrons to produce DNA breaks, as defined by the alkaline elution assay, is explained by the discontinuous deposition of dose associated with neutron irradiation. It is suggested from a microdosimetric analysis that the neutron radiation interacts with DNA at sites on average 5-10 times further apart than the interactions with gamma rays. The long DNA sequences which results after neutron irradiation are consequently eluted inefficiently during alkaline elution, giving a reported RBE of approximately 0.3. Restrictions in the rewinding of individual supercoils are not dependent on the interionization distance and thus give rise to an RBE of approximately 1. Furthermore, the complete removal of DNA damage, as measured by this technique, supports the hypothesis that neutron toxicity is associated with incorrect, not incomplete, rejoining of the DNA molecule.     

The dynamics of the rats higher nervous activity disturbances after total influence of electrons and gamma-rays in doses 5-100 Gy

The dynamics of using of stabilized motor defensive conditioned reflex of active avoidance in "shuttle-box" in rats after total influence of high energy electrons and of gamma-rays in doses 5-100 Gy were investigated. The quality structure of higher nervous activity disturbances after the influence of these kinds of ionizing radiation was identical. Therefore the tendency to disturbances aggravating after the electron radiation influence in the periods of the initial depression and of relatively normalization was revealed, especially after the irradiation in dose 50 Gy. The effective compensation of the functional disturbances in the central nervous system at the first 5-10 min after irradiation was after influence of electron radiation in doses about 30 Gy and after the influence of gamma-radiation in doses about 50 Gy. The irradiation of rats in doses 10 Gy and 5 Gy caused qualitative different dynamics of radiation disturbances in rats higher nervous activity. The differences in rats higher nervous activity after influence of electron and of gamma-radiation in these doses did not manifest distinctly.     

Repair of ultraviolet light-induced damage in Micrococcus radiophilus, an extremely resistant microorganism.

Repair of ultraviolet radiation damage was examined in an extremely radioresistant organism, Micrococcus radiophilus. Measurement of the number of thymine-containing dimers formed as a function of ultraviolet dose suggests that the ability of this organism to withstand high doses of ultraviolet radiation (20,000 ergs/mm2) is not related to protective screening by pigments. M. radiophilus carries out a rapid excision of thymine dimers at doses of ultraviolet light up to 10,000 ergs/mm2. Synthesis of deoxyribonucleic acid is reduced after irradiation, but after removal of photodamage the rate approaches that in unirradiated cells. A comparison is drawn with Micrococcus luteus and M. radiodurans. We conclude that the extremely high resistance to ultraviolet irradiation in M. radiophilus is at least partly due to the presence of an efficient excision repair system.     

Photo-reactivation of gamma-radiation damage in Escherichia coli as evidence for the nature of the oxygen-enhancement effect.

The enhancement of gamma-radiation-induced damage in bacteria by the presence of oxygen during irradiation has been attributed to changes in the rate of formation, or alternatively in the repair, of single-strand breaks. This paper presents data which support the hypothesis that the observed effect of oxygen of modifying viability after irradiation is in part associated with lesions other than DNA single-strand breaks. In particular, the influence of oxygen during gamma-irradiation on the subsequent efficiency of photo-reactivation and excision repair is used to demonstrate that oxygen enhancement is due to a reduction in the excision repair of non-photo-reactivable damage.     

PLD repair in rat rhabdomyosarcoma tumor cells irradiated in vivo and in vitro with high-LET and low-LET radiation

Results are reported of studies to measure the extent of recovery of potentially lethal damage (PLD) in rat rhabdomyosarcoma tumor cells after irradiation both in vivo and in vitro with either high-LET or low-LET radiation. Stationary-phase cultures were found to exhibit repair of PLD following irradiation in vitro either with low-LET X rays or with high-LET neon ions in the extended-peak ionization region. Following a 9-Gy dose of 225-kVp X rays or a 3.5-Gy dose of peak neon ions, both of which reduced the initial cell survival to 6-8%, the maximum PLD recovery factors were 3.4 and 1.6, respectively. In contrast, the standard tumor excision assay procedure failed to reveal any recovery from PLD in tumors irradiated in situ with either X rays or peak neon ions. PLD repair by the in vivo tumor cells could be observed, however, when the excision assay procedure was altered by the addition of a known PLD repair inhibitor beta-arabinofuranosyladenine (beta-ara-A). When a noncytotoxic 50 microM concentration of beta-ara-A was added to the excised tumor cells immediately following a 14.5-Gy in situ dose of X rays, cell survival in the inhibitor-treated cells was lower than in the untreated cells (0.018 compared to 0.056), resulting in a PLD repair inhibition factor of 3.1. Delaying the addition of beta-ara-A for 1, 2, or 3 h following tumor excision reduced the PLD repair inhibition factor to 1.6, 1.5, and 0.9, respectively. Following tumor irradiation in situ with neon ions in the extended-peak ionization region (median LET = 145 keV/micron), less PLD repair was observed than after X irradiation. For 5.8 Gy of peak neon ions, the PLD repair inhibition factors were 2.1, 1.5, 1.3, and 1.1 at 0, 1, 2, and 3 h, respectively. We interpret the absence of measurable PLD repair using the standard tumor excision assay procedure as resulting from undetectable repair occurring during the long interval (about 2 h) required for the cell dissociation and plating procedures. We conclude that at least for our tumor system, PLD repair does occur after irradiation of tumors in situ, even though it is not detectable using the standard tumor excision assay procedure. Thus a failure to measure such repair by this assay in a given tumor system does not necessarily mean the cells are incapable of PLD repair.     

Suppression of E. multilocularis Hydatid Cysts after Ionizing Radiation Exposure

Background

Heavy-ion therapy has an advantage over conventional radiotherapy due to its superb biological effectiveness and dose conformity in cancer therapy. It could be a potential alternate approach for hydatid cyst treatment. However, there is no information currently available on the cellular and molecular basis for heavy-ion irradiation induced cell death in cystic echinococcosis.

Methododology/Principal Findings

LD50 was scored by protoscolex death. Cellular and ultrastructural changes within the parasite were studied by light and electron microscopy, mitochondrial DNA (mtDNA) damage and copy number were measured by QPCR, and apoptosis was determined by caspase 3 expression and caspase 3 activity. Ionizing radiation induced sparse cytoplasm, disorganized and clumped organelles, large vacuoles and devoid of villi. The initial mtDNA damage caused by ionizing radiation increased in a dose-dependent manner. The kinetic of DNA repair was slower after carbon-ion radiation than that after X-rays radiation. High dose carbon-ion radiation caused irreversible mtDNA degradation. Cysts apoptosis was pronounced after radiation. Carbon-ion radiation was more effective to suppress hydatid cysts than X-rays.

Conclusions

These studies provide a framework to the evaluation of attenuation effect of heavy-ion radiation on cystic echinococcosis in vitro. Carbon-ion radiation is more effective to suppress E. multilocularis than X-rays.     

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.     

Enhancement of postreplication repair in ultraviolet-light-irradiated Chinese hamster cells by irradiation in G2 or S-phase.

Postreplication repair in synchronous Chinese hamster cells was determined after split doses of ultraviolet (UV) radiation. Repair was enhanced by irradiation of cells in G2 or S-phase with a small dose of UV radiation at least 1.5 h before a three-fold larger dose of UV. There was significantly greater enhancement when the first dose was given in G2 than when it was given in the S-phase 0.5-1.5 h before the test dose. These data indicate that enhancement of postreplication repair does not require active DNA replication and qualitatively is independent of when in the cell cycle the cells are irradiated.