Cloning and nucleotide sequence of the ada gene of Salmonella typhimurium encoding O6-methylguanine-DNA methyltransferase

Abstract

Mutations induced by N-methyl-N′-nitro-N-nitrosoguanidine (MNNG) were strongly enhanced in the presence of o-vanillin in E. coli B. The enhancement was also observed in uvrA, umuC, recA, polA, or alkB mutants. This effect was lower in an alkA mutant, but was restored in an alk A umuC double mutant. By contrast, the enhancing effect was almost blocked in an ada and ada umuC double mutant. It was necessary to add simultaneously MNNG and o-vanillin to the growth medium.Further investigations were conducted on the induction of ada and umuC genes using ada′-lacZ′ and umuC′-lacZ′ plasmids. o-Vanillin suppressed the induction of the ada gene by MNNG treatment, but not that of the umuC gene. In fact expression of the umuC gene was induced by lower concentrations of MNNG in the presence of o-vanillin.The results suggest that o-vanillin inhibits induction of the adaptive response, and consequently, the MNNG-induced mutation frequency is increased due to unrepaired O6-methylguanine.