Previous studies of gene amplification in Escherichia coli have suggested that it occurs in two steps: duplication and expansion. Expansion is thought to result from homologous recombination between the repeated segments created by duplication. To explore the mechanism of expansion, a 7 kbp duplication in the chromosome containing a leaky mutant version of the lac operon was constructed, and its expansion into an amplified array was studied.
Under selection for lac function, colonies bearing multiple copies of the mutant lac operon appeared at a constant rate of approximately 4 to 5 per million cells plated per day, on days two through seven after plating. Expansion was not seen in a recA strain; null mutations in recBCD and ruvC reduced the rate 100- and 10-fold, respectively; a ruvC recG double mutant reduced the rate 1000-fold. Expansion occurred at an increased rate in cells lacking dam, polA, rnhA, or uvrD functions. Null mutations of various other cellular recombination, repair, and stress response genes had little effect upon expansion. The red recombination genes of phage lambda could substitute for recBCD in mediating expansion. In the red-substituted cells, expansion was only partially dependent upon recA function.
These observations are consistent with the idea that the expansion step of gene amplification is closely related, mechanistically, to interchromosomal homologous recombination events. They additionally provide support for recently described models of RecA-independent Red-mediated recombination at replication forks.