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Chapter 21 : Darwin in My Lab
The author’s methods for direct visualization of individual mutation and recombination events in living Escherichia coli bacteria are based only on the shrewd use of a natural DNA modification (methylation of A in the GATC sequences) and of two fluorescent proteins. A special protein involved in the process of DNA error correction (called mismatch repair) binds exclusively, extensively, and stably to uncorrected DNA copy errors, forming a fluorescent focus in the living cell. In recombination, a fluorescent version of the E. coli SeqA protein, which binds exclusively the hemimethylated DNA (a DNA duplex with only one strand methylated), allows us to monitor the integrity of permanently hemimethylated DNA over an unlimited number of generations. The molecular phylogenies of preexisting mutations from genomes of many sequenced natural isolates of E. coli suggest that the acquisition of a mutation at any locus in the genome is about 100 times more likely to occur by recombination than by de novo mutation. The results of a study conducted by the author, appeared as expected from the E. coli– Salmonella enterica serovar Typhimurium crosses: when both partners had mutator histories, genetic recombination dropped 10-fold in mut+ cells but not in cells with mut defects; when only one partner had mutator history, recombination dropped only 3- to 4-fold; but when neither had a mutator history, there was no effect on recombination.