DNA repair in competent cells of Bacillus subtilis.

A series of isogenic transformable strains of Bacillus subtilis carrying the uvr-19 or rec-43 mutation or both were constructed. Both mutations made competent cells defective in repairing UV-irradiated cellular or transforming DNA, and their effects were additive in a doubly deficient strain, suggesting that two repair processes, requiring uvr-19+ and rec-43+ gene products, are independently functional in competent cells of B. subtilis.     

Ability of Bacillus subtilis protoplasts to repair irradiated bacteriophage deoxyribonucleic acid via acquired and natural enzymatic systems.

A novel form of "enzyme therapy" was achieved by utilizing protoplasts of Bacillus subtilis. Photoreactivating enzyme of Escherichia coli was successfully inserted into the protoplasts of B. subtilis treated with polyethylene glycol. This enzyme was used to photoreactivate ultraviolet-damaged bacteriophage deoxyribonucleic acid (DNA). Furthermore, in polyethylene glycol-treated protoplasts, ultraviolet-irradiated transfecting bacteriophage DNA was shown to be a functional substrate for the host DNA excision repair system. Previous results (R. E. Yasbin, J. D. Fernwalt, and P. I. Fields, J. Bacteriol. 137:391-396, 1979) showed that ultraviolet-irradiated bacteriophage DNA could not be repaired via the excision repair system of competent cells. Therefore, the processing of bacteriophage DNA by protoplasts and by competent cells must be different. This sensitive protoplast assay can be used to identify and to isolate various types of DNA repair enzymes.     

Translesion synthesis is the main component of SOS repair in bacteriophage lambda DNA.

Agents that interfere with DNA replication in Escherichia coli induce physiological adaptations that increase the probability of survival after DNA damage and the frequency of mutants among the survivors (the SOS response). Such agents also increase the survival rate and mutation frequency of irradiated bacteriophage after infection of treated bacteria, a phenomenon known as Weigle reactivation. In UV-irradiated single-stranded DNA phage, Weigle reactivation is thought to occur via induced, error-prone replication through template lesions (translesion synthesis [P. Caillet-Fauquet, M: Defais, and M. Radman, J. Mol. Biol. 117:95-112, 1977]). Weigle reactivation occurs with higher efficiency in double-stranded DNA phages such as lambda, and we therefore asked if another process, recombination between partially replicated daughter molecules, plays a major role in this case. To distinguish between translesion synthesis and recombinational repair, we studied the early replication of UV-irradiated bacteriophage lambda in SOS-induced and uninduced bacteria. To avoid complications arising from excision of UV lesions, we used bacterial uvrA mutants, in which such excision does not occur. Our evidence suggests that translesion synthesis is the primary component of Weigle reactivation of lambda phage in the absence of excision repair. The greater efficiency in Weigle reactivation of double-stranded DNA phage could thus be attributed to some inducible excision repair unable to occur on single-stranded DNA. In addition, after irradiation, lambda phage replication seems to switch prematurely from the theta mode to the rolling circle mode.     

Effect of lysogeny on transfection and transfection enhancement in Bacillus subtilis.

Strains of Bacillus subtilis 168 lysogenic for bacteriophage phi105 transfer with deoxyribonucleic acid (DNA) isolated from bacteriophage SPO2 at a higher efficiency than non-lysogenic strains. This enhancement of transfection was not the result of recombination between bacteriophages SPO2 and phi105. Superinfection marker rescue increased transfection with DNA from bacteriophage phi105 occurred simultaneously with the addition of the transfecting DNA. Again, this enhancement of transfection was not the result of recombination but rather a protection of the transfecting DNA by the superinfecting bacteriophage. The ability of the superinfecting bacteriophage to protect the transfecting DNA from inactivation was maximal when the bacteria were just becoming competent. Bacteriophage phi1 cannot replicate after the transfection of competent bacteria lacking a functional DNA replication system, whereas bacteriophage phi1 was able to replicate after infection of competent bacteria grown under comparable conditions. These observations support the hypothesis that GAPase and an inducible repair system play an important role in the development of competence.     

UV-inducible DNA repair in the cyanobacteria Anabaena spp.

Strains of the filamentous cyanobacteria Anabaena spp. were capable of very efficient photoreactivation of UV irradiation-induced damage to DNA. Cells were resistant to several hundred joules of UV irradiation per square meter under conditions that allowed photoreactivation, and they also photoreactivated UV-damaged cyanophage efficiently. Reactivation of UV-irradiated cyanophage (Weigle reactivation) also occurred; UV irradiation of host cells greatly enhanced the plaque-forming ability of irradiated phage under nonphotoreactivating conditions. Postirradiation incubation of the host cells under conditions that allowed photoreactivation abolished the ability of the cells to perform Weigle reactivation of cyanophage N-1. Mitomycin C also induced Weigle reactivation of cyanophage N-1, but nalidixic acid did not. The inducible repair system (defined as the ability to perform Weigle reactivation of cyanophages) was relatively slow and inefficient compared with photoreactivation.     

Nucleotide excision repair rates in rat tissues.

We have determined and compared nucleotide excision repair capability of several rat tissues by a method based on restoration of the transformation activity of UV-irradiated pBlueScript by incubation in repair-competent protein extracts. After 3 h of incubation, plasmid DNA was isolated and used to transform competent Escherichia coli cells. Damaged plasmids showed low transformation efficiency prior to incubation in repair-competent extracts. After incubation the transformation efficiency was restored to different extents permitting calculation of the repair capacity of the extracts. Our results showed that rapidly proliferating tissues such as liver, kidney and testis showed higher nucleotide excision repair capacity than slowly proliferating tissues such as heart, muscle, lung and spleen. When liver and splenocytes were stimulated to proliferation by partial hepatectomy and mitogen stimulation, their repair capability increased in parallel with the respective proliferative rates.     

Repair of UV-irradiated plasmid DNA in mutants of Saccharomyces cerevisiae and Escherichia coli deficient in repair of pyrimidine dimers

The repair of in vitro UV-irradiated DNA of plasmid pBB29 was studied in excision defective yeast mutants rad1, rad2, rad3, rad4, rad10 and in Escherichia coli mutants uvr- and recA-, by measuring the cell transformation frequency. Rad2, rad3, rad4, and rad10 mutants could repair plasmid DNA despite their inability to repair nuclear DNA, whereas the reduced ability of rad1 mutant for plasmid DNA repair demonstrated alone the same dependence on the host functions that are needed for nuclear DNA repair. In E. coli the repair of UV-irradiated plasmid DNA is carried out only by the excision-repair system dependent on uvr genes. Treatment of UV-irradiated plasmid DNA with UV endonuclease from Micrococcus luteus greatly enhances the efficiency of transformation of E. coli uvr- mutants. Similar treatment with cell-free extracts of yeast rad1 mutant or wild-type strains as well as with nuclease BaL31, despite their ability for preferential cutting of UV damaged DNA, showed no influence on cell transformation.     

Dithiothreitol pretreatment and inducible repair in UV-irradiated Escherichia coli K12 cells

The UV radiation survival of several Escherichia coli K12 strains was measured after pretreatment of the cells with dithiothreitol (DTT). In DNA repair-competent cells (AB1157), UV survival was enhanced (ER = 1.2) after pretreating cells for 1.0 h using 10 mmol dm-3 DTT and then incubating the cells for 1.5 h in buffer before UV irradiation. Similar experiments using the excision repair mutant, AB1886uvrA6, or the recombination repair and SOS-deficient mutant, AB2462recA, strains did not show enhanced UV survival. None of the E. coli strains tested were protected against UV killing by simultaneous treatment with DTT (10 mmol dm-3). These results, and the fact that incubation in chloramphenicol removed the wild-type response in DTT-pretreated, UV-irradiated cells, suggest that the observed UV radioprotection was a result of inducible enzymatic repair processes such as recA-dependent repair. The proposed stimulus for inducible repair in these cells is DNA damage caused by intracellular hydroxyl radicals arising from thiol oxidation. The involvement of oxygen radicals in the induction pathway is supported by results that showed superoxide dismutase and catalase could inhibit a portion (one-third) of the inducible repair.     

Repair of UV-irradiated plasmid DNA in Saccharomyces cerevisiae. Inability to complement mutational defects in excision repair by in vitro treatment with Micrococcus luteus UV endonuclease

Excision repair defects of Saccharomyces cerevisiae rad1-1, rad4-4, rad7-1 and rad14 mutants were examined. As previously found, transformation of such cells with UV-irradiated plasmid DNA is poor compared to wild-type yeast. Treatment of UV-irradiated YRp12 plasmid DNA with crude preparations of Micrococcus luteus UV endonuclease before introducing it into rad1-1 cells increased transformation efficiency to wild-type levels. This is consistent with earlier reports of rad1-1 mutants being defective in the incision step of excision repair. However, with purified UV endonuclease little or no rescue occurred when the UV-irradiated plasmid was incised before transformation into rad1-1 or rad4-4 cells. Furthermore, the purified UV endonuclease reduced transformation of rad7-1 and rad14 mutants to levels seen in rad1-1 and rad4-4 cells. In contrast such treatment caused only a small decrease in the transforming ability of UV-irradiated DNA in wild-type cells. These results show that yeast can normally process pre-incised, UV-irradiated DNA and that this activity is absent in rad1-1, rad4-4, rad7-1 and rad14 mutants. Thus, in addition to their previously reported roles in incision, the RAD1, 4, 7 and 14 gene products are also required for repair to continue after the incision of DNA lesions.     

Inhibition of repair of UV-damaged DNA by caffeine and mutation induction in Chinese hamster cells

The effect of caffeine on UV-irradiated Chinese hamster cells in vitro was studied on the cellular and molecular levels. Caffeine (1 mM) was shown to decrease the colony-forming ability and the frequencies of spontaneous and UV-induced mutations in Chinese hamster cells. The effect of caffeine in reducing the frequency of UV-induced mutations was demonstrated only if caffeine was present in the culture medium during the first post-irradiation cell division. Using alkaline sucrose gradient centrifugation, both parental and newly synthesized DNA in UV-irradiated and unirradiated cells were studied in the presence and absence of caffeine. Caffeine affected the sedimentation profile of DNA synthesized in UV-irradiated cells but not in unirradiated cells. Caffeine had no apparent effect on the incorporation of [³H]-thymidine into DNA of control or UV-irradiated cells, nor on the small amount of excision of UV-induced pyrimidine dimers. These results may be interpreted by a hypothesis that caffeine inhibits a certain S-phase specific, post-replication, dark-repair mechanism. The hamster and perhaps other rodent cells exposed to low doses of UV are capable of DNA replication, by-passing the non-excised pyrimidine dimers. This postulated repair process probably involves de novo DNA synthesis to seal the gaps in the nascent strand. This repair may be also responsible for the enzymatic production of mutations.     

Mutants of Staphylococcus aureus with Increased Sensitivity to Ultraviolet Radiation

Nitrosoguanidine (NG) mutagenesis of Staphylococcus aureus resulted in the isolation of eight mutants exhibiting 3 to 28 times greater sensitivity to ultraviolet (UV) radiation. These mutants were further characterized by their ability to repair UV-irradiated bacteriophage, to act as recipients in the transduction of antibiotic resistance, and their sensitivity to NG. Based on the available data, six of these mutants are reduced in their ability to perform host-cell reactivation. One of the remaining two mutants may be deficient in post-replication repair.     

60 years of SOS repair

This review integrates 60 years of research on SOS repair and SOS mutagenesis in prokaryotes and eukaryotes, from Jean Weigle’s experiment in 1953 (mutagenesis of lambda bacteriophage in UV-irradiated bacteria) to the latest achievements in studying SOS mutagenesis on all living organisms, i.e., Eukarya, Archaea and Bacteria. A key role in establishing of a biochemical basis for SOS mutagenesis belongs to the finding in 1998–1999 that specific error-prone DNA polymerases (PolV and others) catalyzed translesion synthesis on damaged DNA. This review focuses on recent studies that address new models for SOS-induced mutagenesis in Escherichia coli and Homo sapiens cells.     

Lack of SOS repair in Streptococcus pneumoniae

Wild-type strains of Streptococcus pneumoniae were non-mutable by UV radiation and thymidine starvation. Moreover, UV-irradiated pneumococcal ω2 phages were not reactivated in an irradiated host. This suggests that, in pneumococcus, there is no efficient inducible repair process similar to the SOS repair described in detail for E. coli. We also report that mutations cannot be induced by a process thought to be linked to competence during transformation with isogenic wild-type DNA either on wild-type strains or in strains in which the hex function of excision and repair of mismatched bases in inactive.     

Inhibitors of repair DNA synthesis.

We have measured repair DNA synthesis in UV-irradiated normal human fibroblasts, grown to a defined state of quiescence in order to avoid the problem of discriminating repair from replicative DNA synthesis. We have assessed the effects of various DNA synthesis inhibitors on repair. Inhibition of repair synthesis by hydroxyurea, 1-beta-D-arabinofuranosylcytosine and aphidicolin is associated with the ability to accumulate DNA breaks due to enzymic incision at DNA damage sites; the inhibition by novobiocin is in accord with its known ability to block incision.     

Mechanisms for recF-dependent and recB-dependent pathways of postreplication repair in UV-irradiated Escherichia coli uvrB.

The molecular mechanisms for the recF-dependent and recB-dependent pathways of postreplication repair were studied by sedimentation analysis of DNA from UV-irradiated Escherichia coli cells. When the ability to repair DNA daughter strand gaps was compared, uvrB recF cells showed a gross deficiency, whereas uvrB recB cells showed only a small deficiency. Nevertheless, the uvrB recF cells were able to perform some limited repair of daughter strand gaps compared with a "repairless" uvrB recA strain. The introduction of a recB mutation into the uvrB recF strain greatly increased its UV radiation sensitivity, yet decreased only slightly its ability to repair daughter strand gaps. Kinetic studies of DNA repair with alkaline and neutral sucrose gradients indicated that the accumulation of unrepaired daughter strand gaps led to the formation of low-molecular-weight DNA duplexes (i.e., DNA double-strand breaks were formed). The uvrB recF cells were able to regenerate high-molecular-weight DNA from these low-molecular-weight DNA duplexes, whereas the uvrB recF recB and uvrB recA cells were not. A model for the recB-dependent pathway of postreplication repair is presented.     

Inducible reactivation and mutagenesis of UV-irradiated bacteriophage P22 in Salmonella typhimurium LT2 containing the plasmid pKM101.

The inducible (Weigle) reactivation of UV-irradiated bacteriophage P22 has been examined on strains of Salmonella typhimurium with and without the mutagenesis-enhancing plasmid pKM101. A large inducible reactivation was observed in the plasmid-containing strain, but only a small response was observed in the strain lacking the plasmid. An increased frequency of clear-plaque mutants was detected among the survivors. The efficiencies of the plasmid-mediated and cellular repair processes have been determined. The kinetics of induction of the phage reactivation have been investigated. The relationship of the observed results to the inducible reactivation of UV-irradiated lambda in Escherichia coli and to error-prone repair is discussed.     

Role of the radB gene in postreplication repair in UV-irradiated Escherichia coli uvrB

In UV-irradiated Escherichia coli, the radB101 mutation sensitized uvrB recF cells 4-fold and uvrB recB cells 1.2-fold, but did not sensitize uvrB recB recF cells. The radB mutation had very little effect (1.2-fold or less) on the repair of UV radiation-induced DNA daughter-strand gaps in uvrB cells, but it did cause about a 3-fold deficiency in the repair of the DNA double-strand breaks that arise in association with nonrepaired daughter-strand gaps in UV-irradiated uvrB recF cells. Thus, the radB gene does not appear to be involved in the recF-dependent or recF recB-independent processes for the repair of DNA daughter-strand gaps, but is involved in the recB-dependent postreplication repair of DNA double-strand breaks.     

Biochemical and cytogenetical characterization of Chinese hamster ovary X-ray-sensitive mutant cells xrs 5 and xrs 6. VI. The correlation between UV-induced DNA lesions and chromosomal aberrations, and their modulations with inhibitors of DNA repair synthesis

The role of UV-induced DNA lesions and their repair in the formation of chromosomal aberrations in the xrs mutant cell lines xrs 5 and xrs 6 and their wild-type counterpart, CHO-K1 cells, were studied. The extent of induction of DNA single-strand breaks (SSBs) and DNA double-strand breaks (DSBs) due to UV irradiation in the presence or absence of 1-beta-D-arabinofuranosylcytosine (ara-C) and hydroxyurea (HU) was determined using the alkaline and neutral elution methods. Results of these experiments were compared with the frequencies of induced chromosomal aberrations in UV-irradiated G1 cells treated under similar conditions. Xrs 6 cells showed a defect in their ability to perform the incision step of nucleotide repair after UV irradiation. Accumulation of breaks 2 h after UV irradiation in xrs 6 cells in the presence of HU and ara-C remained at the level of incision breaks estimated after 20 min, which was about 35% of that found in wild-type CHO-K1 cells. In UV-irradiated CHO-K1 and xrs 5 cells, more incision breaks were present after 2 h compared with 20 min post-treatment with ara-C, a further increase was evident when HU was added to the combined treatment. The level of incision breaks induced under these conditions in xrs 5 was about 80% of that observed in CHO-K1 cells. UV irradiation itself did not induce any detectable DNA strand breaks. Accumulation of SSBs in UV-irradiated cells post-treated with ara-C and HU coincides with the increase in the frequency of chromosomal aberrations. These data suggest that accumulated SSBs when converted to DSBs in G1 give rise to chromosome-type aberrations, whereas strand breaks persisting until S-phase result in chromatid-type aberrations. Xrs 6 appeared to be the first ionizing-radiation-sensitive mutant with a partial defect in the incision step of DNA repair of UV-induced damage.