Evolutionary implications of the frequent horizontal transfer of mismatch repair genes

Mutation and subsequent recombination events create genetic diversity, whic h is subjected to natural selection. Bacterial mismatch repair (MMR) defici ent mutants, exhibiting high mutation and homeologous recombination rates, are frequently found in natural populations. Therefore, we have explored th e possibility that MMR deficiency emerging in nature has left some "imprint " in the sequence of bacterial genomes. Comparative molecular phylogeny of MMR genes from natural Escherichia coli isolates shows that, compared to ho usekeeping genes, individual functional MMR genes exhibit high sequence mos aicism derived from diverse phylogenetic lineages. This apparent horizontal gene transfer correlates with hyperrecombination phenotype of MMR-deficien t mutators. The sequence mosaicism of MMR genes may be a hallmark of a mech anism of adaptive evolution that involves modulation of mutation and recomb ination rates by recurrent losses and reacquisitions of MMR gene functions.