Singlet oxygen as an ultimately reactive species in Salmonella typhimurium DNA damage induced by methylene blue/visible light.

The specific recognition of various DNA modifications by repair enzymes is exploited for the analysis of DNA damage induced by visible light in the presence of methylene blue in Salmonella typhimurium. The relative frequencies of various endonuclease-sensitive sites and strand breaks are determined in the plasmid pAQ1 of the treated bacteria and are compared with those observed after exposure of isolated DNA to various conditions. This comparison of damage profiles indicates that the cellular DNA damage by illumination in the presence of methylene blue is caused predominantly by the direct action of singlet oxygen. Indirect mechanisms, e.g. involving a generation of superoxide and hydroxyl radicals or the activation of cellular nucleases, do not contribute very much. The damage is dominated by base modifications. These are subject to an efficient repair that is not mediated by uvrABC proteins and therefore most probably involves recognition by specific glycosylases. Revertant frequencies observed under these conditions in the strains TA1535, TA100, TA2638 and TA104 indicate a pronounced mutagenicity of the lesions induced. On the other hand, the DNA damage does not contribute significantly to the cytotoxicity caused by the treatment as an excision repair deficiency (uvrB) has no influence on cell killing.