RecA protein is essential for the very high level of resistance of Deinococcus radiodurans to DNA damage induced by ionizing radiation or other DNA-damaging agents. Since the mechanism(s) involved in the control of recA expression and the extent of RecA induction following DNA damage in this species are still unclear, we have performed a genetic analysis of the recA locus and quantified the basal and induced levels of RecA protein in wild type, recA, and lexA mutants. We found that the two genes upstream of recA in the predicted cinA ligT recA operon appear to have no role in the regulation of recA expression or function, despite the fact that the reading frames in the operon overlap. By using a translational fusion of recA to a lacZ reporter gene, we showed that induction began with no delay following exposure to gamma-radiation or treatment with mitomycin, and continued at a constant rate until it reached a plateau. The induction efficiency increased linearly with inducer dose, levelling off at a concentration fourfold above the background. The basal concentration of RecA protein measured by Western blotting corresponded to approximately 11,000 monomers per cell, and the induced concentration to around 44,000 monomers per cell. These levels remained unchanged upon disruption of the lexA gene, indicating that LexA does not plays a role in recA regulation. However, inactivation of lexA caused cells to aggregate, suggesting that LexA may control the activity or expression of as yet undefined membrane functions. Cells bearing the recA670 mutation showed an elevated constitutive expression of recA in the absence of DNA damage. This phenotype did not result from the defect in DNA repair associated with the RecA670 protein, since the increased basal level of recA expression was also found in recA670/ recA(+) diploid cells that are proficient in DNA repair. These results suggest that RecA may be involved in regulating its own expression, possibly by stimulating proteolytic modification of other regulatory proteins.

Hydrogen peroxide is an important reactive oxygen species (ROS) that arises either during the aerobic respiration process or as a by-product of water radiolysis after exposure to ionizing radiation. The reaction of hydrogen peroxide with transition metals imposes on cells an oxidative stress condition that can result in damage to cell components such as proteins, lipids and principally to DNA, leading to mutagenesis and cell death. Escherichia coli cells are able to deal with these adverse events via DNA repair mechanisms, which enable them to recover their genome integrity. These include base excision repair (BER), nucleotide excision repair (NER) and recombinational repair. Other important defense mechanisms present in Escherichia coli are OxyR and SosRS anti-oxidant inducible pathways, which are elicited by cells to avoid the introduction of oxidative lesions by hydrogen peroxide. This review summarizes the phenomena of lethal synergism between UV irradiation (254 nm) and H 2 O 2 , the cross-adaptive response between different classes of genotoxic agents and hydrogen peroxide, and the role of copper ions in the lethal response to H 2 O 2 under low-iron conditions. Key words: hydrogen peroxide, cross-adaptive response, lethal synergism, copper and iron. General AspectsThe appearance of aerobic forms of life was an important step in the evolutionary process, since oxygen consumption leads to the production of ten-fold more energy from glucose than does anaerobic metabolism (Meneghini, 1987). However, this process imposes constraints on cell viability, because of the generation of reactive oxygen species during respiration.The consecutive univalent reduction of molecular oxygen to water produces three active intermediates: superoxide anion (O 2 -• ), hydrogen peroxide (H 2 O 2 ) and hydroxyl radical (OH • ). These intermediates, collectively referred to as reactive oxygen species (ROS) are potent oxidants of lipids, proteins, and nucleic acids (Halliwell and Gutteridge, 1984;Mello-Filho and Meneghini, 1985;Meneghini, 1988). Among the oxidative DNA lesions, one of the major classes of DNA damage leads to modification in purine and pyrimidine bases, together with oligonucleotide strand breaks, DNA-protein cross-links and abasic sites. Increasing evidence suggests that the cumulative damage caused by ROS contributes to numerous degenerative diseases associated with aging, such as atherosclerosis, rheumatoid arthritis and cancer (Ames et al., 1993;Halliwell and Gutteridge, 1999).Living organisms have developed specific mechanisms to prevent the production and effects of ROS. The reduction of O 2 by cytochrome oxidase without yielding ROS, the superoxide dismutase catalysis of O 2 -• into H 2 O 2 through a dismutation reaction, the decomposition of H 2 O 2 by catalase and peroxidases, and the scavenging of ROS by some vitamins comprise part of the set of cellular antioxidant defenses (Halliwell and Gutteridge, 1999). Synthesis of the enzymes that catalyze these reactions is a part of the adaptive response tr...

Aptamers are oligonucleotide reagents with high affinity and specificity, which among other therapeutic and diagnostic applications have the capability of acting as delivery agents. Thus, aptamers are capable of carrying small molecules, nanoparticles, radiopharmaceuticals or fluorescent agents as well as nucleic acid therapeutics specifically to their target cells. In most cases, the molecules may possess interesting therapeutic properties, but their lack of specificity for a particular cell type, or ability to internalise in such a cell, hinders their clinical development, or cause unwanted side effects. Thus, chemotherapy or radiotherapy agents, famous for their side effects, can be coupled to aptamers for specific delivery. Equally, siRNA have great therapeutic potential and specificity, but one of their shortcomings remain the delivery and internalisation into cells. Various methodologies have been proposed to date, including aptamers, to resolve this problem. Therapeutic or imaging reagents benefit from the adaptability and ease of chemical manipulation of aptamers, their high affinity for the specific marker of a cell type, and their internalisation ability via cell mediated endocytosis. In this review paper, we explore the potential of the aptamers as delivery agents and offer an update on current status and latest advancements.

In Escherichia coli, the repair of lethal DNA damage induced by H 2 O 2 requires exonuclease III, the xthA gene product. Here, we report that both endonuclease IV (the nfo gene product) and exonuclease III can mediate the repair of lesions induced by H 2 O 2 under low-iron conditions. Neither the xthA nor the nfo mutants was sensitive to H 2 O 2 in the presence of iron chelators, while the xthA nfo double mutant was significantly sensitive to this treatment, suggesting that both exonuclease III and endonuclease IV can mediate the repair of DNA lesions formed under such conditions. Sedimentation studies in alkaline sucrose gradients also demonstrated that both xthA and nfo mutants, but not the xthA nfo double mutant, can carry out complete repair of DNA strand breaks and alkali-labile bonds generated by H 2 O 2 under low-iron conditions. We also found indications that the formation of substrates for exonuclease III and endonuclease IV is mediated by the Fpg DNA glycosylase, as suggested by experiments in which the fpg mutation increased the level of cell survival, as well as repair of DNA strand breaks, in an AP endonuclease-null background.