These results suggest that ROS generation might be the cause of cytotoxicity, which seems to be related to initial genetic damage rather than to lipid peroxidation. HeLa cells showed to be more sensitive than normal cells.
Ionizing radiation causes several types of DNA lesions, mainly single- or double-strand breaks and base damage. By means of the chromotest, an assay that allows the level of the SOS response to be monitored via beta-galactosidase enzymatic activity, the roles of several repair (uvrA, recN and oxyR) and recombination (recB, recJ and recO) genes in the response of Escherichia coli to gamma-radiation were studied. The results indicate that all the repair- and recombination-deficient strains were more sensitive to the lethal effects of ionizing radiation. However, the SOS activation pattern was somewhat different. The minimal inducing dose in uvrA and recN mutants was lower than in the wild-type, whereas their SOS response was higher at all doses. Conversely, in the strains lacking an active recB, recJ or recO gene, the doubling dose was almost the same as in the wild-type but the level of induction remained stable over a wide dose range. These findings suggest that neither single- nor double-strand breaks are in themselves direct SOS inducers and that while uvrA, recN and oxyR take part in different repair or protective pathways, apparently recB, recJ and recO participate in damage processing leading to SOS induction, as well as in recombination repair.
Ionizing radiation causes different types of genetic damage, ranging from base modifications to single- and double-stranded DNA breaks, which may be deleterious or even lethal to the cell. There are different repair or tolerance mechanisms to counteract the damage. Among them is the Escherichia coli SOS system: a set of genes that becomes activated upon DNA damage to confer better opportunities for cell survival. However, since this response is triggered by single-stranded DNA regions, most lesions have to be processed or modified prior to SOS activation. Several genes such as recO, recB and recJ that seem to be required to induce the response have already been reported. The results of this work indicate that the four known E.coli single-strand exonucleases take part in processing gamma radiation damage, though RecJ and ExoI proved to be more important than ExoVII or ExoX. In addition, ExoV as well as glycosylases such as Nth and, to a lesser extent, Fpg are also required. A model intended to explain the role of all these genes in damage processing is presented.
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