Genetic Characterization of theKlebsiella pneumoniae waaGene Cluster, Involved in Core Lipopolysaccharide Biosynthesis

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Abstract
Mutator cells that lack the mismatch repair system (MMR−) occur at rates of 10−5 or less in laboratory populations started from wild-type cells. We show that after selection for recombinants in an interspecies mating betweenSalmonella enterica serovar Typhimurium andEscherichia coli, the percentage of MMR− cells rises to several percent of the recombinant population, and after a second successive mating and selection, greater than 95% of the recombinants are MMR−. Coupling a single cross and selection with either mutagenesis or selection for spontaneous mutants also results in a dramatic increase in MMR− cells. We discuss how horizontal transfer can result in mutator strains during adaptive evolution.