The SOS DNA repair system is induced in bacteria treated with 4-quinolones. However, whether the response exacerbates or repairs the damage caused by these drugs is still unclear. The and the mutations impair recombination repair and render bacteria unable to induce the SOS response when treated with nalidixic acid or other agents that affect DNA synthesis. However, UV treatment induces the SOS response in mutants but not in mutants. Both these mutants are hypersensitive to nalidixic acid and, therefore, either recombination repair or SOS repair would appear to repair DNA damage caused by the drug. However, since the mutation (which also renders bacteria incapable of inducing the SOS response without affecting recombination repair) had no effect on the susceptibility of bacteria to nalidixic acid, the SOS response neither contributes to nor repairs DNA damage caused by the drug. Consequently, it would seem that the hypersensitivity of the and mutants to nalidixic acid is due to their deficiency in recombination repair. This view was confirmed by testing a mutant that is recombination-repair proficient but SOS repair-deficient and finding it to be no more sensitive to nalidixic acid than its parent. Thus it would appear that, although induced by nalidixic acid treatment, the SOS DNA repair system does not play any role in bacterial responses to the damage caused by the drug. In contrast, the recombination repair system does repair damage caused by nalidixic acid.


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