Alkylation and oxidative damages to DNA: constitutive and inducible repair systems

African swine fever virus AP endonuclease is a redox-sensitive enzyme that repairs alkylating and oxidative damage to DNA

African swine fever virus (ASFV) encodes an AP endonuclease (pE296R) which is essential for virus growth in swine macrophages. We show here that the DNA repair functions of pE296R (AP endonucleolytic, 3′ → 5′ exonuclease, 3′-diesterase and nucleotide incision repair (NIR) activities) and DNA binding are inhibited by reducing agents. Protein pE296R contains one intramolecular disulfide bond, whose disruption by reducing agents might perturb the interaction of the viral AP endonuclease with the DNA substrate. The characterization of the 3′ → 5′ exonuclease and 3′-repair diesterase activities of pE296R indicates that it has strong preference for mispaired and oxidative base lesions at the 3′-termini of single-strand breaks. Finally, the viral protein protects against DNA damaging agents in both prokaryotic and eukaryotic cells, emphasizing its importance in vivo. The biochemical and genetic properties of ASFV AP endonuclease are consistent with the repair of DNA damage generated by the genotoxic intracellular environment of the host macrophage.

Differential induction of adaptive responses by paraquat and hydrogen peroxide against the genotoxicity of methyl mercuric chloride, maleic hydrazide and ethyl methane sulfonate in plant cells in vivo

Induction of adaptive response by conditioning doses of paraquat (PQ) and hydrogen peroxide (H2O2) in embryonic shoot cells of Hordeum vulgare and root meristem cells of Allium cepa was tested against the genotoxicity of challenge doses of methyl mercuric chloride (MMCl), maleic hydrazide (MH) or ethylmethane sulfonate (EMS). Plant tissue fixed at different recovery hours following the challenge treatments was analysed for cells with genotoxicity markers that include spindle or chromosome aberrations and micronuclei. The results provided clear-cut evidence that whereas H2O2 induced adaptive response for the chromosome damage caused by MMCl and MH, PQ induced the same for MMCl and EMS, but not for damage caused by MH. The findings pointed to the differences in the underlying mechanisms of oxidative responses induced by H2O2 and O2-.

Transcriptional and posttranscriptional regulation of manganese superoxide dismutase biosynthesis in Escherichia coli, studied with operon and protein fusions

Protein and operon fusions between the manganese superoxide dismutase (MnSOD) gene, sodA, and genes of the lactose operon were constructed in an attempt to explore the effects of various factors on MnSOD expression and the level at which they operate. In sodA-lacZ protein fusions, induction of beta-galactosidase perfectly mimicked MnSOD induction (i.e., beta-galactosidase was not expressed in anaerobiosis and was induced by oxygen, redox-cycling compounds in aerobiosis, and iron chelators in anaerobiosis). In tac-sodA operon fusions, MnSOD induction was monitored only by the lactose operon inducer isopropyl-beta-D-thiogalactopyranoside. Various plasmids carrying part or all of the sodA regulatory and structural region inhibited aerobic beta-galactosidase induction in sodA-lacZ fusions. This included plasmids carrying only the transcription start and upstream region and also plasmids which did not contain this region and in which MnSOD was under foreign transcriptional control. The role of metal ions was also investigated. Addition of Mn(II) enhanced MnSOD activity but did not affect induction. The anaerobic expression of MnSOD from the oxygen-insensitive tac promoter was enhanced threefold by iron-chelating agents, implying a posttranscriptional or most likely a posttranslational modulation of enzyme activity via metal ions. To accommodate all these data, multiregulation of MnSOD is proposed.

Effects of oxygen stress on membrane functions in Escherichia coli: role of HPI catalase

Different conditions of oxidative stress were used to study their effects on membrane transport in Escherichia coli K-12. The oxidizing conditions included H2O2, plumbagin (a redox cycling compound that generates superoxide radicals [O2-]), and increased partial pressure of oxygen. Both superoxide radical-generating conditions and H2O2 treatments were found to cause a rapid decrease in proton motive force-dependent and -independent transport. H2O2-pretreated cells had the ability to rapidly recover both proton motive force-dependent and -independent transport. The induction required transcription and translation and was dependent on oxyR+ and katG+, providing evidence that these genes play crucial roles in the rapid recovery of transport. The effects of oxidatively induced loss of proton motive force on cell growth and macromolecular synthesis were also investigated.

Glutathione in Escherichia coli is dispensable for resistance to H2O2 and gamma radiation

Escherichia coli devoid of glutathione (because of transposon insertions in the gshA gene) has normal resistance to H2O2, cumene hydroperoxide, heat, or ionizing radiation. Intracellular glutathione thus does not protect E. coli from such lethal oxidative damage. The use of gshA::Tn10 mutants also revealed a glutathione-independent, H2O2-inducible resistance to N-ethylmaleimide.