Role of neighbouring bases and assessment of strand specificity in ethylmethanesulphonate and N-methyl-N'-nitro-N-nitrosoguanidine mutagenesis in the SUP4-o gene of Saccharomyces cerevisiae.

A total of 318 forward mutations induced by ethylmethanesulphonate (EMS) and N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) in the SUP4-o gene of the yeast Saccharomyces cerevisiae was characterized by DNA sequence analysis. Only base-pair substitutions were detected among the mutations examined and, for both agents, the majority (greater than 96%) were G.C to A.T. transitions. The remaining changes included A.T to G.C transitions and transversions at G.C sites. For EMS, two of the transversions were accompanied by nearby G.C to A.T transitions. There was considerable overlap of the sites within the SUP4-o gene that were mutated by EMS and MNNG and of the sites that each agent failed to mutate. However, EMS and MNNG mutagenesis differed with respect to the frequencies at which mutations were recovered at G.C pairs where the guanine is flanked (5') by a purine or pyrimidine. EMS exhibited no preference for either type of site, whereas a G.C site was 12-fold or fivefold more likely to be mutated by MNNG if preceded by a 5' adenine or guanine, respectively, than if flanked by a 5' pyrimidine. Finally, neither EMS nor MNNG mutagenesis showed a preference for G.C sites having the guanine on the non-transcribed strand.