Summary: Ten leucine-requiring mutants of were induced to revert with the alkylating agent, diethylsulphate (DES). The pattern of reversion was studied by following the appearance of leucine-independent colonies on minimal plates, and of turbid (revertant) tubes among tubes containing 10-10 treated bacteria in minimal medium. Five mutants gave high (0·6-5 x 10) frequencies of reversion. The other five gave lower (5 x 10-1·3 x 10) frequencies immediately after plating on minimal agar or dilution in minimal medium, but reversion continued to take place until a high frequency was attained. This delayed mutation was found to be independent of cell metabolism. The possibility is discussed that the different patterns of reversion reflect alkylation of different base pairs. It is suggested that mutants having guanine: cytosine (G:C) base pairs at the mutant site revert mainly by pairing errors made by alkylated guanine. Mutants with adenine: thymine (A:T) sites are assumed to revert through the production of apurinic gaps, formed by the hydrolysis of ethyladenine.


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