Effect of inorganic arsenics on cytotoxicity and mutagenicity of ultraviolet light on Escherichia coli and the mechanism involved.

The genetic effect of arsenite (AsO-2) and arsenate (HAsO4(2-)) on the cells of E. coliB (argF-) tester strains was investigated. Both the arsenics, which did not show mutagenic effect on the tester strains, reduced UV-induced mutation frequency of H/r30R (wild-type; Exc+Rec+) cells, while they did not change that of Hs30R (uvrA-; Exc-Rec+) cells. To elucidate the mechanism of this antimutagenicity, modifying effect of the arsenics on UV-induced cytotoxicity for Hs30R and NG30 (recA-; Exc+Rec-) cells was examined. In a nutrient medium containing the arsenics, the survived cell fraction of NG30 after ultraviolet (UV) irradiation was markedly increased, whereas that of Hs30R was not altered. When the UV-exposed NG30 cells were subjected to recovery incubation in a nutrient medium containing the arsenics, the survived cell fraction was remarkably increased before the cell proliferation in a similar manner as liquid-holding recovery observed in an arsenic-free and non-nutrient medium in which DNA replication was suppressed. The arsenics were found to cause a delay of DNA replication in UV-irradiated NG30 cells. These results indicate that arsenite and arsenate enhance the error-free excision repair of UV-damaged DNA by retarding the DNA replication and thus by prolonging the period for excision repair. This may lead to the reduction in UV-induced mutation.