Amplification of lac Cannot Account for Adaptive Mutation to Lac ⁺ in Escherichia coli

Abstract

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) amplificationdependent 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.

The SMC-Like Protein Complex SbcCD Enhances DNA Polymerase IV-Dependent Spontaneous Mutation inEscherichia coli

“…The frameshift is in a run of repeated G ⅐ C base pairs, a mutational target very similar to that in the lacI33-lacZ allele used for adaptive mutation (16). Pol IV also appears to preferentially make mutations on the F episome (40,73), and so the mutant tetA allele in FC722 is carried on the FЈ episome, where it reverts at a rate that is 5-fold higher than when it is carried on the chromosome (66,73). We have hypothesized that this high level of mutation on the episome is due to a high frequency of DSBs, which allow Pol IV to gain access to DNA termini (73) (discussed further below).…”

Role of Polyphosphates in Microbial Adaptation to Extreme Environments

“…When cells are subjected to environmental stresses, the Pol6 polymerases (members of the Y-family polymerases), which have mutagenic and error bypass activities, are activated, resulting in the induction of Pol Lac ϩ and UV-dependent mutations (86). However, the mutagenic and lesion bypass activities of these polymerases are lost if the cells are unable to synthesize polyP (21,87).…”

“…Defense and adaptive mechanisms used by H. arsenicoxydans include the activation of catalases, DNA repair protein, and DNA polymerase Pol4 (56), each of which has expression regulated by polyP (3,21,76,86,87). Another strategy used by bacteria is sequestration of heavy metals such as Cr and Pb by biofilm adsorption (31,90).…”

Impact of a stress-inducible switch to mutagenic repair of DNA breaks on mutation in Escherichia coli

“…Second, because dinB is near lac in this specific F′, when Lac + mutants are selected, spontaneous amplifications of the weakly functional lac gene can be selected for increased lac function and might include and overexpress the nearby dinB. The mutagenesis was argued to be independent of stress and merely a byproduct of growth of a cell carrying the selected gene amplification (16,19), and although other studies failed to repeat (22) and con-tradicted (e.g., refs. 22 and 23) key predictions of the amplification growth-and-mutation model in the Lac assay, the model persists as an explanation for DSB-dependent SIM (16,19).…”