Letter to the Editor

The Contribution of Bacterial Hypermutators to Mutation in Stationary Phase

Corresponding author: John Cairns, Wilcote, Charlbury, Oxon OX7 3EA, England., j.cairns{at}ctsu.ox.ac.uk (E-mail)

IN their discussion of mutation in stationary populations of Escherichia coli, BULL et al. 2000 conclude that all the mutations are arising in a small fraction of the population. They give as their reason the fact that they could fit the data of TORKELSON et al. 1997 on multiple mutations to a Poisson distribution simply by assuming that all mutations are confined to a subpopulation comprising 1 in 10,000–100,000 cells. Their argument is unsound and, furthermore, makes a prediction that is contradicted by their own results.

They monitored seven classes of mutation: reversion of a frameshift in the lac operon (Lac⁺), and forward mutations leading to 5-fluorocytosine resistance (5-FC^R), 5-fluorouracil resistance (5-FU^R), inability to ferment xylose (Xyl^-), maltose (Mal^-), and fructose (Fruc^-), and inability to grow on minimal medium at high temperatures [temperature sensitive (ts)]. Unfortunately, the target sizes for the seven classes of mutation are clearly not the same. In the wild-type strain, the frequency of Lac⁺ revertants in the total population (the hypermutators and the rest) is 10^-6 (BULL et al. 2000 , Figure 2A), which is an order of magnitude less than the frequency of each of the six forward mutations (TORKELSON et al. 1997 , middle column in Table II); and in cells that have become Lac⁺, where the numbers are larger, the frequencies of the six forward mutations were seen to differ among themselves by 50-fold (TORKELSON et al. 1997 , left column in Table II). Therefore the frequencies of multiple mutations among the mutating fraction of the population cannot fit any simple distribution.

Nevertheless, it is clear from everyone's results that multiple mutations are much commoner than would be expected by chance. As HALL 1991 pointed out a few years ago, this suggests that some mutations are arising in a subpopulation of cells with a raised mutation rate. As mentioned earlier, Bull et al. have gone further and have proposed that virtually all mutations arise in hypermutators. This contradicts the available evidence. Their data, and that of ROSCHE and FOSTER 1999 , show that any given class of mutation is 10 times less common in single (Lac⁺) mutants than in double mutants (Lac⁺ plus a second mutation). For example, Torkelson et al. found that the frequency of Mal^- mutations was 31 per 42,617 Lac⁺ mutants and 6 per 394 double mutants (Lac⁺ plus one or more of the other five classes of mutation); similarly, Rosche and Foster found that the frequency of mutations causing a defect in motility (Mot*) was 210 per 3168 Lac⁺ mutants and 8 per 13 Lac⁺ doubles. These results imply that most single (Lac⁺) mutants are arising in cells that have a lower mutation rate than the cells that produce double mutants. As Rosche and Foster pointed out, the most reasonable hypothesis is that all cells are undergoing mutation, but a small minority have a much higher mutation rate. In other words, when there is selection for just a single novel trait, most of the survivors will not be burdened with multiple changes in the rest of their genome.

LITERATURE CITED

BULL, H. J., G. J. MCKENZIE, P. J. HASTINGS, and S. M. ROSENBERG, 2000  Evidence that stationary-phase hypermutation in the Escherichia coli chromosome is promoted by recombination. Genetics 154:1427-1437[Abstract/Full Text].

HALL, B. G., 1991  Adaptive evolution that requires multiple spontaneous mutations: mutations involving base substitutions. Proc. Natl. Acad. Sci. USA 88:5882-5886[Abstract].

ROSCHE, W. A. and P. L. FOSTER, 1999  The role of transient hypermutators in adaptive evolution. Proc. Natl. Acad. Sci. USA 96:6862-6867[Abstract/Full Text].

TORKELSON, J., R. S. HARRIS, M.-J. LOMBARDO, J. NAGENDRAN, and C. THULIN et al., 1997  Genome-wide hypermutation in a subpopulation of stationary-phase cells underlies recombination-dependent adaptive mutation. EMBO J. 16:3303-3311[Abstract/Full Text].

This article has been cited by other articles:

				A. Maki, H. Kono, M. Gupta, M. Asakawa, T. Suzuki, M. Matsuda, H. Fujii, and I. Rusyn Predictive Power of Biomarkers of Oxidative Stress and Inflammation in Patients with Hepatitis C Virus-Associated Hepatocellular Carcinoma Ann. Surg. Oncol., March 1, 2007; 14(3): 1182 - 1190. [Abstract] [Full Text] [PDF]

				P. L. Foster Rebuttal: Adaptive Point Mutation (Rosenberg and Hastings) J. Bacteriol., August 1, 2004; 186(15): 4845 - 4845. [Full Text] [PDF]

				J. R. Roth and D. I. Andersson Adaptive Mutation: How Growth under Selection Stimulates Lac+ Reversion by Increasing Target Copy Number J. Bacteriol., August 1, 2004; 186(15): 4855 - 4860. [Full Text] [PDF]

				J. Cairns and P. L. Foster The Risk of Lethals for Hypermutating Bacteria in Stationary Phase Genetics, December 1, 2003; 165(4): 2317 - 2318. [Full Text] [PDF]

				E. S. Slechta, K. L. Bunny, E. Kugelberg, E. Kofoid, D. I. Andersson, and J. R. Roth Adaptive mutation: General mutagenesis is not a programmed response to stress but results from rare coamplification of dinB with lac PNAS, October 28, 2003; 100(22): 12847 - 12852. [Abstract] [Full Text] [PDF]

				H.-M. Sung and R. E. Yasbin Adaptive, or Stationary-Phase, Mutagenesis, a Component of Bacterial Differentiation in Bacillus subtilis J. Bacteriol., October 15, 2002; 184(20): 5641 - 5653. [Abstract] [Full Text] [PDF]

				H. J. Bull, M.-J. Lombardo, and S. M. Rosenberg Stationary-phase mutation in the bacterial chromosome: Recombination protein and DNA polymerase IV dependence PNAS, July 17, 2001; 98(15): 8334 - 8341. [Abstract] [Full Text] [PDF]