Induction of cell killing, mutation and umu gene expression by 6-mercaptopurine or 2-thiouracil with UVA irradiation

When Escherichia coli cells were irradiated by UVA in the presence of 6-mercaptopurine (6-MP) or 2-thiouracil (S2Ura), two kinds of repair-deficient strains of recA- and uvrA- were killed more efficiently than the parental wild-type strain having normal repair capacities. In addition, these agents with UVA exposure greatly induced the incidence of mutations in the uvrA- strain as compared with the wild-type strain but not the recA- strain. Furthermore, the induction of expression of umuDC genes was investigated in two Salmonella typhimurium strains, TA1535 and TA1538, carrying a pSK1002 plasmid. In these systems, it is easy to measure beta-galactosidase activities for the induced activities of SOS responses. These agents with UVA exposure also induced expression of the umuDC genes. These results suggest that 6-MP and S2Ura with UVA induce DNA damage which is repairable by the excision repair mechanism.