Nonadaptive mutations occur on the F' episome during adaptive mutation conditions in Escherichia coli

Abstract

One of the most studied examples of adaptive mutation is a strain of Escherichia coli, FC40, that cannot utilize lactose (Lac ؊ ) but that readily reverts to lactose utilization (Lac ؉ ) when lactose is its sole carbon source. Adaptive reversion to Lac ؉ occurs at a high rate when the Lac ؊ allele is on an F episome and conjugal functions are expressed. It was previously shown that nonselected mutations on the chromosome did not appear in the Lac ؊ population while episomal Lac ؉ mutations accumulated, but it remained possible that nonselected mutations might occur on the episome. To investigate this possibility, a second mutational target was created on the Lac ؊ episome by mutation of a Tn10 element, which encodes tetracycline resistance (Tet r ), to tetracycline sensitivity (Tet s ). Reversion rates to Tet r during normal growth and during lactose selection were measured. The results show that nonselected Tet r mutations do accumulate in Lac ؊ cells when those cells are under selection to become Lac ؉ . Thus, reversion to Lac ؉ in FC40 does not appear to be adaptive in the narrow sense of the word. In addition, the results suggest that during lactose selection, both Lac ؉ and Tet r mutations are created or preserved by the same recombination-dependent mechanism.Mutations occurring in nondividing cells have been called adaptive when they allow cells to escape from nonlethal selective pressure (5,7,20). A narrower definition of adaptive mutation is that only useful, not deleterious or neutral, mutations occur during selection (7,12,29). A number of possible examples of such specifically adaptive mutations have been reported (7,13,21,22,24,44). Most current theories for adaptive mutation propose that cells under selection produce genetic variants at random, but these variants are transient, or the cells producing them die, unless a useful mutation occurs and the cells start to grow (3,7,22,43). All of these models predict that nonselected mutations may appear in the successful cells (because all variants that exist when a cell begins to grow will be captured), but nonselected mutations should not appear in the unsuccessful cells (reviewed in reference 12).One of the most studied examples of adaptive mutation is a strain of Escherichia coli, FC40, that cannot utilize lactose (Lac Ϫ ) but that readily reverts to lactose utilization (Lac ϩ ) when lactose is its sole carbon source (5). This high level of adaptive reversion occurs when the Lac Ϫ allele is on an FЈ episome and conjugal functions are expressed (17,19,39). Although mutations in a chromosomal gene, rpoB, do not appear in the Lac Ϫ population while episomal Lac ϩ mutations accumulate (13), the question remained whether nonselected mutations might occur on the episome. Here, I report that, indeed, nonselected mutations at another site on FЈ accumulate in Lac Ϫ cells when those cells are under selection to become Lac ϩ . Thus, any given gene on the episome may be subjected to a highly mutagenic process in cells under nonlethal selective pressure. Although t...

Stress responses and genetic variation in bacteria

“…When a second revertible allele, a +1 frameshift in the tetA gene, was on the episome close to the Lac − allele, TetR revertants appeared at about the same rate as did Lac + mutations during lactose selection. In addition, TetR reversion had the same genetic requirements as mutation to Lac + [45]. Although the two mutational events were probably not independent [45], these results demonstrate that the mutational mechanism that gives rise to adaptive mutations is not directed to the gene under selection.…”

“…In addition, TetR reversion had the same genetic requirements as mutation to Lac + [45]. Although the two mutational events were probably not independent [45], these results demonstrate that the mutational mechanism that gives rise to adaptive mutations is not directed to the gene under selection.…”

Stationary-phase mutation in the bacterial chromosome: Recombination protein and DNA polymerase IV dependence

“…The library was screened for isolates with insertions in upp based on their resistance to 5-fluoro-cytosine and insertion into upp confirmed by linkage to xseA and gua genes. Second, the tet ϩ gene of the upp::Tn10dtet allele was replaced with the tetϩ1 frameshift allele from FC722 (39) [Tn10dtet ϩ 1 G at bp 331 (4G to 5G), hereafter referred to as Tn10dtetϩ1] by homologous recombination of the upp::Tn10dtet transduced into FC722, then selected for Tet S recombinant cells (45). The putative upp::Tn10dtetϩ1 allele was transduced into SMR2597, selecting Gua ϩ , to produce SMR4576, and the presence of tetϩ1 was confirmed by PCR and sequencing.…”

“…Most wild E. coli carry conjugative plasmids and about 15% carry F-like conjugative plasmids (38). As suggested previously (39), an FЈ-specific mutation mechanism could be important to bacterial evolution because bacterial DNA is exchanged via recombination between episomes and chromosomes. The recombination proteindependent mutation mechanism acting on the FЈ would appear to be more obviously relevant to bacterial evolution if chromosomal genes also were affected.…”

Hereditary stability and variation in evolution and development

“…The reversion of a particular lacZ frameshift during selection for lactose utilization (Cairns et al 1988) was originally proposed to be "adaptive" (i.e., the mutation's probability of occurrence is greater when it would be of benefit to the cell). This system has been reinterpreted and reversion of the lacZ frameshift allele appears to be part of a more generalized stress response (Foster 1997;Torkelson et al 1997), or perhaps no stress response at all (Andersson et al 1998).…”