Adaptive mutation and slow-growing revertants of an Escherichia coli lacZ amber mutant.

We have studied revertants, selected on lactose minimal agar medium, of the Escherichia coli lacZam strain that was first used by Cairns and his colleagues to demonstrate the phenomenon of "adaptive mutation." We have found, by performing appropriate reconstruction studies, that most of the late-arising Lac+ revertants of this lac amber strain (appearing as colonies in 3-5 days) are slow-growing ochre suppressor mutants that probably existed in the culture prior to plating and cannot, therefore, be classified as "adaptive." The appearance of a small number of fast-growing, late-arising Lac+ revertants may result from residual cell growth and turnover or from phenomena related to the fact that the lacZam mutation in strain SM195 is carried on an F' plasmid. Thus, the appearance of late-arising revertants in this lacZam system does not provide convincing evidence that selective conditions specifically increase the rate of occurrence of favorable mutations.