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Journal of Bacteriology, March 2001, p. 1631-1644, Vol. 183, No. 5
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.
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
*
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.
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