Phylogenetic evidence for horizontal transfer of mutS alleles among naturally occurring Escherichia coli strains

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 betwee n disparate strains, While much is known regarding the mutS phenotype, the evolutionary structure of the mutS(+) gene in Escherichia coli remains uncl ear. The physical proximity of mutS to an adjacent polymorphic region of th e chromosome suggests that this gene itself may be subject to horizontal tr ansfer and recombination events. To test this notion, a phylogenetic approa ch 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 sequen ces) revealed striking levels of phylogenetic discordance among mutS allele s and their respective strains. We interpret these incongruences as signatu res of horizontal exchange among mutS alleles. Examination of additional si tes surrounding mutS also revealed incongruous distributions compared to E. coli strain phylogeny, This suggests that other regional sequences are equ ally subject to horizontal transfer, supporting the hypothesis that the 61. 5-min mutS-rpoS region is a recombinational hot spot within the E. coli chr omosome. Furthermore, these data are consistent with a mechanism for stabil izing adaptive changes promoted by mutS mutators through rescue of defectiv e mutS alleles with wild-type sequences.