Gene conversion in Escherichia coli: the recF pathway for resolution of heteroduplex DNA.

The independent repair of mismatched nucleotides present in heteroduplex DNA has been used to explain gene conversion and map expansion after general genetic recombination. We have constructed and purified heteroduplex plasmid DNAs that contain heteroallelic 10-base-pair insertion-deletion mismatches. These DNA substrates are similar in structure to the heteroduplex DNA intermediates that have been proposed to be produced during the genetic recombination of plasmids. These DNA substrates were transformed into wild-type and mutant Escherichia coli strains, and the fate of the heteroduplex DNA was determined by both restriction mapping and genetic tests. Independent repair events that yielded a wild-type Tetr gene were observed at a frequency of approximately 1% in both wild-type and recB recC sbcB mutant E. coli strains. The independent repair of small insertion-deletion-type mismatches separated by 1,243 base pairs was found to be reduced by recF, recJ, and ssb single mutations in an otherwise wild-type genetic background and reduced by recF, recJ, and recO mutations in a recB recC sbcB genetic background (the ssb mutation was not tested in the latter background). Independent repair of small insertion-deletion-type mismatched nucleotides that were as close as 312 nucleotides apart was observed. There was no apparent bias in favor of the insertion or deletion of mutant sequences.