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MINI-REVIEW

Rebuttal: Adaptive Point Mutation (Rosenberg and Hastings)

Microbiology Section, University of California, Davis, Davis, California 95616,¹ Department of Bacteriology, Swedish Institute for Infectious Disease Control, S-171 82 Solna,² Microbiology and Tumour Biology Center, Karolinska Institute, S-171 77, Solna, Sweden³

The Rosenberg-Hastings paper (4) defends a model much like that of Foster (1) and argues that the observed mutagenesis contradicts predictions of the amplification model. On the contrary, amplification indirectly causes weak mutagenesis and allows that mutagenesis to have a detectable effect on lac reversion. However, this mutagenesis is a minor side effect that is neither sufficient nor necessary to explain reversion. Mutagenesis has distracted attention from the main message of this system—a target number increase during growth under selection.

SELECTION INCREASES REVERTANT YIELD 10⁴-FOLD

Seen in the light of the amplification mutagenesis (AM) model, 100 lac⁺ revertant colonies arise from 10⁶ plated cells that carry a lac duplication and can therefore grow under selection. This revertant frequency (10^–4) is 10⁴-fold higher than that seen without selection (10^–8). Selection increases both target number and (indirectly) mutation rate. The whole effect of selection is the product of these two factors, because the mutation rate affects all lac copies. Target number increase (amplification and growth) provides a factor of more that 10³; mutagenesis provides a factor of 4 or 5.

FIVEFOLD EFFECT OF MUTAGENESIS

Elimination of dinB (and general mutagenesis) reduces revertant yield less than fivefold. Without mutagenesis, selection still causes a RecA-dependent 25-fold increase in revertant yield (based on revertant colony number). This importance of RecA in the absence of mutagenesis suggests that recombination (i.e., amplification) is central (7).

MUTAGENESIS CAUSES 80% OF REVERTANTS

To be responsible for 80% of point mutations, DinB must increase the basal mutation rate only fivefold (6). This small increase would produce five lac revertants if applied to 10⁸ nongrowing cells as suggested by the HM model. However, it produces 100 revertants if applied to 10⁷ growing cells (within colonies), each cell with 200 lac copies (2).

INDUCTION OF DinB

We suggest that the mutation rate increases fivefold when one dinB gene is induced (SOS) by single-stranded DNA produced by the F' replication origin aided by DNA fragments released by amplification segregation (6). This small increase explains revertant number, if applied to a pool of target lac copies enlarged by growth and amplification, but is too low to be detectable (by the methods used) as an increase in frequency of associated mutations.

ORIGINS OF ASSOCIATED MUTATIONS

A detectable level of associated unselected mutations forms in about 10% of lac revertants (3)—the subset that coamplifies dinB with lac. In these clones, SOS induces many dinB⁺ copies and thereby increases the mutation rate several hundred fold.

GROWTH—WITH OR WITHOUT SELECTION

This system showcases effects of selection on mutation in growing cells. With selection, cells reach the goal (Lac⁺) by a succession of genetic events, each allowing a clonal expansion. Each event is made more frequent by increases in target number provided by the previous expansion. Without selection (or without growth), the same goal can be reached only by rare single-step events. A diagram of this process is in reference 5.

	FOOTNOTES

 
* Corresponding author. Mailing address: University of California, Davis, Microbiology Section, One Shields Ave., Davis, CA 95616. Phone: (530) 752-6679. Fax: (530) 752-7663. E-mail: jrroth{at}ucdavis.edu.

REFERENCES

1. Foster, P. L. 2004. Adaptive mutation in Escherichia coli. J. Bacteriol. 186:4846-4852.[Free Full Text]
2. Hendrickson, H., E. S. Slechta, U. Bergthorsson, D. I. Andersson, and J. R. Roth. 2002. Amplification mutagenesis: evidence that growth with a selected gene amplification causes adaptive mutation and hyper-mutability. Proc. Natl. Acad. Sci. USA 99:2164-2169.[Abstract/Free Full Text]
3. Rosche, W. A., and P. L. Foster. 1999. The role of transient hypermutators in adaptive mutation in Escherichia coli. Proc. Natl. Acad. Sci. USA 96:6862-6867.[Abstract/Free Full Text]
4. Rosenberg, S. M., and P. J. Hastings. 2004. Adaptive point mutation and adaptive amplification pathways in the Escherichia coli Lac system: stress responses producing genetic change. J. Bacteriol. 186:4838-4843.[Free Full Text]
5. Roth, J. R., and D. I. Andersson. 2004. Amplification-mutagenesis—how growth under selection contributes to the origin of genetic diversity and explains the phenomenon of adaptive mutation. Res. Microbiol. 155:342-351.
6. Slechta, E. S., K. L. Bunny, E. Kugelberg, E. Kofoid, D. I. Andersson, and J. R. Roth. 2003. Adaptive mutation: general mutagenesis is not a programmed response to stress, but results from rare co-amplification of dinB with lac. Proc. Natl. Acad. Sci. USA 100:12847-12852.[Abstract/Free Full Text]
7. Slechta, S., J. Liu, D. I. Andersson, and J. R. Roth. 2002. Evidence that selected amplification of a bacterial lac frameshift allele stimulates Lac(+) reversion (adaptive mutation) with or without general hypermutability. Genetics 161:945-956.[Abstract/Free Full Text]

This article has been cited by other articles:

• Roth, J. R., Andersson, D. I. (2004). Rebuttal: Adaptive Mutation in Escherichia coli (Foster). J. Bacteriol. 186: 4854-4854 [Full Text]  

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