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Open Access Research article

Multiple genetic switches spontaneously modulating bacterial mutability

Fang Chen12, Wei-Qiao Liu13, Abraham Eisenstark4, Randal N Johnston5, Gui-Rong Liu1* and Shu-Lin Liu123*

Author Affiliations

1 Genomics Research Center (one of The State-Province Key Laboratories of Biomedicine-Pharmaceutics of China), Harbin Medical University, Harbin, China

2 Department of Microbiology, Peking University Health Science Center, Beijing, China

3 Department of Microbiology and Infectious Diseases, University of Calgary, Calgary, Canada

4 Cancer Research Center and University of Missouri, Columbia, Missouri, USA

5 Department of Biochemistry and Molecular Biology, University of Calgary, Calgary, Canada

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BMC Evolutionary Biology 2010, 10:277  doi:10.1186/1471-2148-10-277

Published: 13 September 2010

Abstract

Background

All life forms need both high genetic stability to survive as species and a degree of mutability to evolve for adaptation, but little is known about how the organisms balance the two seemingly conflicting aspects of life: genetic stability and mutability. The DNA mismatch repair (MMR) system is essential for maintaining genetic stability and defects in MMR lead to high mutability. Evolution is driven by genetic novelty, such as point mutation and lateral gene transfer, both of which require genetic mutability. However, normally a functional MMR system would strongly inhibit such genomic changes. Our previous work indicated that MMR gene allele conversion between functional and non-functional states through copy number changes of small tandem repeats could occur spontaneously via slipped-strand mis-pairing during DNA replication and therefore may play a role of genetic switches to modulate the bacterial mutability at the population level. The open question was: when the conversion from functional to defective MMR is prohibited, will bacteria still be able to evolve by accepting laterally transferred DNA or accumulating mutations?

Results

To prohibit allele conversion, we "locked" the MMR genes through nucleotide replacements. We then scored changes in bacterial mutability and found that Salmonella strains with MMR locked at the functional state had significantly decreased mutability. To determine the generalizability of this kind of mutability 'switching' among a wider range of bacteria, we examined the distribution of tandem repeats within MMR genes in over 100 bacterial species and found that multiple genetic switches might exist in these bacteria and may spontaneously modulate bacterial mutability during evolution.

Conclusions

MMR allele conversion through repeats-mediated slipped-strand mis-pairing may function as a spontaneous mechanism to switch between high genetic stability and mutability during bacterial evolution.