Multicopy single-stranded DNA of Escherichia coli enhances mutation and recombination frequencies by titrating MutS protein.

Multicopy single-stranded DNA (msDNA) molecules consist of single-stranded DNA covalently linked to RNA. In Escherichia coli, such molecules are encoded by genetic elements called retrons. The DNA moieties of msDNAs have characteristic stem-loop structures, and most of these structures contain mismatched base pairs. Previously, we showed that retrons encoding msDNAs with mismatched base pairs are mutagenic when present in multicopy plasmids. In this study we show that such msDNAs, in a similar manner to genetic defects in mismatch repair, increase the frequency of interspecies recombination in matings between Salmonella typhimurium and E. coli. To demonstrate interference with mismatch repair by msDNA, we show that the addition of a plasmid containing the gene for MutS protein suppresses the mutagenic and recombinogenic effects of msDNAs. We also show that in mutS mutants, msDNA does not increase the frequency of either mutations or interspecies recombination. We conclude from these findings that the mutagenic and recombinogenic effects of msDNAs are due to titrating out MutS protein.