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Journal of Bacteriology, March 2001, p. 1631-1644, Vol. 183, No. 5
0021-9193/01/$04.00+0   DOI: 10.1128/JB.183.5.1631-1644.2001

Phylogenetic Evidence for Horizontal Transfer of mutS Alleles among Naturally Occurring Escherichia coli Strains

Eric W. Brown, J. Eugene LeClerc, Baoguang Li, William L. Payne, and Thomas A. Cebula*

Molecular Biology Branch, Center for Food Safety & Applied Nutrition, Food and Drug Administration, Washington, D.C. 20204

Received 3 October 2000/Accepted 30 November 2000

mutS mutators accelerate the bacterial mutation rate 100- to 1,000-fold and relax the barriers that normally restrict homeologous recombination. These mutators thus afford the opportunity for horizontal exchange of DNA between disparate strains. While much is known regarding the mutS phenotype, the evolutionary structure of the mutS+ gene in Escherichia coli remains unclear. The physical proximity of mutS to an adjacent polymorphic region of the chromosome suggests that this gene itself may be subject to horizontal transfer and recombination events. To test this notion, a phylogenetic approach was employed that compared gene phylogeny to strain phylogeny, making it possible to identify E. coli strains in which mutS alleles have recombined. Comparison of mutS phylogeny against predicted E. coli "whole-chromosome" phylogenies (derived from multilocus enzyme electrophoresis and mdh sequences) revealed striking levels of phylogenetic discordance among mutS alleles and their respective strains. We interpret these incongruences as signatures of horizontal exchange among mutS alleles. Examination of additional sites surrounding mutS also revealed incongruous distributions compared to E. coli strain phylogeny. This suggests that other regional sequences are equally subject to horizontal transfer, supporting the hypothesis that the 61.5-min mutS-rpoS region is a recombinational hot spot within the E. coli chromosome. Furthermore, these data are consistent with a mechanism for stabilizing adaptive changes promoted by mutS mutators through rescue of defective mutS alleles with wild-type sequences.

* Corresponding author. Mailing address: Division of Molecular Biology Research and Evaluation (HFS-235), Center for Food Safety & Applied Nutrition, US Food & Drug Administration, 200 C Street SW, Washington, DC 20204. Phone: (202) 205-4217. Fax: (202) 401-1105. E-mail: tcebula{at}cfsan.fda.gov, tac{at}cfsan.fda.gov.

Journal of Bacteriology, March 2001, p. 1631-1644, Vol. 183, No. 5
0021-9193/01/$04.00+0   DOI: 10.1128/JB.183.5.1631-1644.2001


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