A method for estimating mutagenic specificity in Escherichia coli (argE3, hisG4, thr-1, supE44), based upon the isolation of Arg+ or His+ revertants and identification of tRNA suppressors, is described. The method gives an insight not only into mutagenic pathways but also into the functioning of tRNA. With N-methyl-N'-nitro-N-nitrosoguanidine, 98% of mutations are GC----AT transitions. With N4-hydroxycytidine, 100% are AT----GC transitions. With hydroxylamine, apart from GC----AT transitions, approximately 30% of Arg+ revertants are formed by GC (or AT)----TA transversions. When the chemistry of the mutagenic attack is known, the method allows us to discriminate whether mutations occur on the transcribed or non-transcribed strands of DNA. It has been found that reversion of argE3 to Arg+ is a better monitor of mutagenic pathways than reversion of hisG4 to His+.

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