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

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Abstract
Summary 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.