The classical experiments of Luria and Delbrück showed convincingly that mutations exist before selection and do not contribute to the creation of mutations when selection is lethal. In contrast, when nonlethal selections are used, measuring mutation rates and separating the effects of mutation and selection are difficult and require methods to fully exclude growth after selection has been applied. Although many claims of stress-induced mutagenesis have been made, it is difficult to exclude the influence of growth under nonlethal selection conditions in accounting for the observed increases in mutant frequency. Instead, for many of the studied experimental systems the increase in mutant frequency can be explained better by the ability of selection to detect small differences in growth rate caused by common small effect mutations. A very common mutant class, found in response to many different types of selective regimens in which increased gene dosage can resolve the problem, is gene amplification. In the well-studied lac system of Cairns and Foster, the apparent increase in Lac+ revertants can be explained by high-level amplification of the lac operon and the increased probability for a reversion mutation to occur in any one of the amplified copies. The associated increase in general mutation rate observed in revertant cells in that system is an artifact caused by the coincidental co-amplification of the nearby dinB gene (encoding the error-prone DNA polymerase IV) on the particular plasmid used for these experiments. Apart from the lac system, similar gene amplification processes have been described for adaptation to toxic drugs, growth in host cells, and various nutrient limitations.
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