Evaluation of photo-genotoxicity using the umu test in strains with a high sensitivity to oxidative DNA damage.

Six compounds known to be photo-genotoxic were assayed with the photo-irradiated 96-well umu test system using the original Salmonella strain TA1535/pSK1002 as well as the newly developed strains NM8001 and NM8021. The latter two strains were obtained by introducing the pSK1002 plasmid into strains YG3001 and YG3021. These strains are highly sensitive to oxidative DNA damage owing to the deletion of the nucleotide excision repair enzyme uvrB and the base excision repair enzyme mutY and the nucleotide excision repair enzyme uvrB and the base excision repair enzymes mutY and mutM, respectively. Among the compounds tested under UVA irradiation, methylene blue, neutral red and dichlorobenzidine showed only a slight induction of β-galactosidase activity, whereas 8-methoxypsoralen, chloropromazine and 9,10-dimethylbenzanthracene showed a significant increase in the relative LacZ level as an indicator of genotoxicity. The activity of NM8001 induced by the photo-genotoxins was quite similar to that of NM8021, which indicated that the deficiency of mutY did not affect detection of the selected photo-genotoxins. With NM8001, under UVA light, riboflavin was able to induce the SOS response in bacterial cells. Moreover, neutral red was also found to exert photo-genotoxicity under fluorescent light. Phenalenone, a known atmospheric contaminant present in large amounts, showed positive response with NM8001 under UVA. These results indicate that the photo-irradiated 96-well version of the umu test can be used for rapid screening of the photo-genotoxicity of compounds.