A rapid and simple method for the determination of base substitution and frameshift specificity of mutagens

Abstract

A rapid method for the determination of mutagenic specificity has been developed which makes use of the ochre mutation (TAA) in the his-4 gene of Escherichia coli. Reversion to His + may occur by suppressor mutation (Type I) or by mutation within the his-4 gene (Type II). The Type I mutations may be further subdivided with respect to the type of suppressor mutation by their ability to suppress nonsense mutants of bacteriophage T4, thus allowing the identification of the responsible base substitution (Kato et al., 1980). The system has the ability to identify mutagens which produce A:T → G:C transitions since only Type II mutants can arise through this base substitution; and in fact, the system confirms the A:T → G:C specificity of the mutagen, N 4-hydroxycytidine (Janion and Glickman, 1980) since only Type II mutants were induced by treatment with this base analogue. When this system was further tested with several additional mutagens, the results indicate that ethyl methanesulphonate, methyl nitrosourea and ethyl nitrosourea produce primarily Type I revertants which were primarily G:C → A:T transitions. UV-light, γ-rays, 4NQO and methyl methanesulphonate produced all types of base substitutions. The tester strain was further improved by introducing a series of sequenced trp − frameshift mutations, thus allowing the simultaneous monitoring of frameshift and base-substitution mutations.