DNA polymerase I-mediated ultraviolet repair synthesis in toluene-treated Escherichia coli

Repair response of Escherichia coli to hydrogen peroxide DNA damage

The repair response of Escherichia coli to hydrogen peroxide-induced DNA damage was investigated in intact and toluene-treated cells. Cellular DNA was cleaved after treatment by hydrogen peroxide as analyzed by alkaline sucrose sedimentation. The incision step did not require ATP or magnesium and was not inhibited by N-ethylmaleimide (NEM). An ATP-independent, magnesium-dependent incorporation of nucleotides was seen after the exposure of cells to hydrogen peroxide. This DNA repair synthesis was not inhibited by the addition of NEM or dithiothreitol. In dnaB(Ts) strain CRT266, which is thermolabile for DNA replication, normal levels of DNA synthesis were found at the restrictive temperature (43 degrees C), showing that DNA replication was not necessary for this DNA synthesis. Density gradient analysis also indicated that hydrogen peroxide inhibited DNA replication and stimulated repair synthesis. The subsequent reformation step required magnesium, did not require ATP, and was not inhibited by NEM, in agreement with the synthesis requirements. This suggests that DNA polymerase I was involved in the repair step. Furthermore, a strain defective in DNA polymerase I was unable to reform its DNA after peroxide treatment. Chemical cleavage of the DNA was shown by incision of supercoiled DNA with hydrogen peroxide in the presence of a low concentration of ferric chloride. These findings suggest that hydrogen peroxide directly incises DNA, causing damage which is repaired by an incision repair pathway that requires DNA polymerase I.

DNA polymerase III-dependent repair synthesis in response to bleomycin in toluene-treated Escherichia coli

Bleomycin (BLM) is an antitumor drug which interacts with and damages DNA. We have reported a repair response dependent on DNA polymerase I in toluene-treated Escherichia coli. We report here that DNA polymerase III can also catalyze a repair response in toluene-treated E. coli following exposure to BLM. Polymerase III-mediated synthesis differs because it is ATP-dependent, whereas polymerase I-mediated repair synthesis is not. Polymerase III repair synthesis is independent of replicative synthesis, as demonstrated in a polA-, dnaBts strain, or use of Novobiocin to inhibit replication, and replication persists in the presence of repair synthesis. It appears that ATP-dependent repair synthesis in response to BLM is also present in polA+ strains. Repair synthesis does not require the uvrA gene product.

Dimer excision in Escherichia coli in the presence of caffeine

The observation that polA1 and recL152 mutations result in both slow pyrimidine dimer excision and large repair patch size leads to the hypothesis that patch size is directly related to the rate of excision. In this study caffeine, a known inhibitor of excision repair, was used to examine the extent of correlation between excision rate and patch size by measuring patch size in the presence of several concentrations of caffeine. Both the rate of excision and the resistance to ultraviolet radiation were reduced with increasing concentrations of caffeine after irradiation. Caffeine also inhibited the rate at which incisions were made and prolonged the time required to rejoin the discontinuities. Patch size, however, was unaffected by caffeine treatment.

Mechanism of the ATP effect in the DNA repair synthesis of gamma-irradiated Escherichia coli cells

Gamma-irradiation of Escherichia coli cells made permeable to deoxynucleoside triphosphates (dNTP) by toluene induces a repair-type DNA synthesis. As previous studies have shown ATP stimulates this DNA synthesis; we studied the mechanism of the ATP effect by analyzing the kinetics of nucleotide incorporation at various dNTP concentrations. The V values of the DNA repair synthesis rise with increasing dose (0-50 Gy); nonirradiated cells showed a negligible nucleotide incorporation. The apparent Michaelis constant KM for dNTP in the assay was 83-143 microM and the value was much higher than for a DNA polymerase reaction in vitro. ATP stimulated the DNA synthesis with concomitant decrease of KM yet unchanged V values. Similar results were obtained with a rec BC strain. We propose that the ATP effect is due to a greater affinity of dNTPs to the DNA polymerase, possibly by a stabilisation of the structural integrity of the complex DNA with repair enzymes. Activation of exonucleases by ATP could be excluded. Addition of NAD to the reaction mixture inhibits the DNA synthesis possibly by activation of ligase which closes the nicks in the DNA strand.

Introduction of an active enzyme into permeable cells of Escherichia coli: acquisition of ultraviolet light resistance by uvr mutants on introduction of T4 endonuclease V

Plasmolysed cells of Escherichia coli N212 (uvr A recA) acquired ultraviolet resistance when the cells were exposed to high concentrations of T4 endonuclease V. With increasing concentrations of T4 enzyme, survivals of plasmolysed cells after ultraviolet irradiation increased while colony-forming ability of unirradiated plasmolysed cells was not significantly affected by the enzyme treatment. Under appropriate conditions more than 200 fold increase in survivals was observed. When plasmolysed cells were treated with a pre-heated enzyme preparation or enzyme fractions derived from T4v1 (endonuclease V-deficient mutant)-infected cells, only little or no reactivation took place. Permeabilization of cells prior to the enzyme treatment was essential for the effective reactivation. Treatment of intact cells with the T4 enzyme did not cause any reactivation. Cells treated with 20 mM EGTA or 50 mM CaCl2 in cold were reactivated to certain extents by the enzyme, but the extents of the reactivation were far less compared to those of plasmolysed cells. Plasmolysed cells of strains carrying a mutation in one of uvrA, uvrB and uvrC genes were reactivated by introduction of T4 endonuclease V, as was the uvrA recA double mutant. UvrD mutants were also reactivated, but rather slightly. However, wild type strain as well as strains having a mutation in recA or polA gene were not reactivated. From these results it was suggested that T4 endonuclease V, taken up into permeable cells, can function in vivo to replace defective functions, which are controlled by the uvr genes. The conditions established in the present study may be used for introduction of other proteins into viable bacterial cells.

uvrC gene function in excision repair in toluene-treated Escherichia coli

We have examined the role of the uvrC gene in UV excision repair by studying incision, excision, repair synthesis, and DNA strand reformation in Escherichia coli mutants made permeable to nucleoside triphosphates by toluene treatment. After irradiation, incisions occur normally in uvrC cells in the presence of nicotinamide mononucleotide (NMN), a ligase-blocking agent, but cannot be detected otherwise. We conclude that repair incisions are followed by a ligation event in uvrC mutants, masking incision. However, a uvrC polA12 mutant accumulates incisions only slightly less efficiently than a polA12 strain without NMN. Excision of pyrimidine dimers is defective in uvrC mutants (polA(+) or polA12) irrespective of the presence or absence of NMN. DNA polymerase I-dependent, NMN-stimulated repair synthesis, which is demonstrable in wild-type cells, is absent in uvrC polA(+) cells, but the uvrC polA12 mutant exhibits a UV-specific, ATP-dependent repair synthesis like parental polA12 strains. A DNA polymerase I-mediated reformation of high-molecular-weight DNA takes place efficiently in uvrC polA(+) mutants after incision accumulation, and the uvrC polA12 mutant shows more reformation than the polA12 strain after incision. These results indicate that normal incision occurs in uvrC mutants, but there appears to be a defect in the excision of pyrimidine dimers, allowing resealing via ligation at the site of the incision. The lack of NMN-stimulated repair synthesis in uvrC polA(+) cells indicates that incision is not the only requirement for repair synthesis.