Amplification of lac cannot account for adaptive mutation to Lac+ in Escherichia coli

When the Lac- strain of Escherichia coli, FC40, is incubated with lactose as its sole carbon and energy source, Lac+ revertants arise at a constant rate, a phenomenon known as adaptive mutation. Two alternative models for adaptive mutation have been proposed: (i) recombination-dependent mutation, which specifies that recombination occurring in nongrowing cells stimulates error-prone DNA synthesis, and (ii) amplification-dependent mutation, which specifies that amplification of the lac region and growth of the amplifying cells creates enough DNA replication to produce mutations at the normal rate. Here, we examined several of the predictions of the amplification-dependent mutation model and found that they are not fulfilled. First, inhibition of adaptive mutation by a gene that is toxic when overexpressed does not depend on the proximity of the gene to lac. Second, mutation at a second locus during selection for Lac+ revertants is also independent of the proximity of the locus to lac. Third, mutation at a second locus on the episome occurs even when the lac allele under selection is on the chromosome. Our results support the hypothesis that most Lac+ mutants that appear during lactose selection are true revertants that arise in a single step from Lac- cells, not from a population of growing or amplifying precursor cells.