Classifying mutagens as to their specificity in causing the six possible transitions and transversions: a simple analysis using the Salmonella mutagenicity assay.

The standard Salmonella tester strains used to detect base substitution mutations carry the hisG428 ochre mutation (TA102 and TA104) and the hisG46 missense mutation (TA100). These mutations can be reverted by base changes at their mutant his loci or at extragenic suppressor loci. The base changes resulting in each class of revertants of these mutations have been identified, and simple phenotypic screens have been developed to distinguish among them. Revertants at extragenic suppressor loci are distinguished from those at the his loci by their sensitivity to inhibitory histidine analogs. The four ochre suppressor loci of hisG428 are distinguished by their ability to support growth of nonsense mutants of phage P22. These screens are the basis for a rapid and simple system for determining the base substitution specificity of mutagens using hisG428- and hisG46-containing tester strains. Diagnostic mutagens specific for each of the six possible base changes (transitions and transversions) have been identified. Using these diagnostic mutagens, two additional strains, each specifically reverted by a single base substitution mutation, have been developed to provide a minimum of two loci at which to detect each type of base change. The ability of this system to provide detailed information about mutational specificity in a variety of DNA repair backgrounds will allow further elucidation of the mechanisms of mutagenesis and DNA repair.