Apparent gene conversion in an Escherichia coli rec+ strain is explained by multiple rounds of reciprocal crossing-over.

Gene conversion, the non-reciprocal transfer of sequence information between homologous DNA sequences, has been reported in lower eukaryotes, mammals and in Escherichia coli. In an E. coli rec+ strain, we established a plasmid carrying two different deleted neo genes (neoDL and neoDR) in an inverted orientation and then selected for homologous recombination events that had reconstructed an intact neo+ gene. We found some plasmids that had apparently experienced intramolecular gene conversion. Further evidence, however, suggests that they are products of multiple rounds of reciprocal crossing-over, apparently involving two plasmid molecules. First, most of the Neo+ clones contained multiple types of Neo+ plasmids, although the frequency of producing the neo+ clones was low. Second, all the neo+ clones also contained, as a minority, one particular form of dimer, which can be formed by reciprocal crossing-over between neoDL of one plasmid molecule and neoDR of another plasmid molecule. Third, in reconstruction experiments, we cloned and purified this dimer and transferred it back into the rec+ cells. The dimer gave rise to clones containing multiple types of neo+ recombinant monomers, including those apparent gene conversion types, and containing only few molecules of this dimer plasmid.