ENHANCEMENT OF FRAME-SHIFT MUTATION BY THE OVERPRODUCTION OF MSDNA INESCHERICHIA-COLI

A minor population of wild Escherichia coli strains contain retroeleme nts called retrons, which produce a peculiar satellite DNA, multicopy single-stranded DNA (msDNA). It has been reported that mismatched base pairs in the secondary structure formed in msDNA are mutagenic in E. coli [Maas et al. (1994) Mel. Microbiol. 14, 437-441; Maas et al. (199 6) Mel. Microbiol. 19, 505-509]. We reexamined this proposal by conver ting mismatched base pairs to matched base pairs using a single msDNA species, msDNA-Ec86, or by deleting mismatched regions using msDNA-Ec7 3. We also examined the effect of reverse transcriptases (RT) without msDNA production on mutagenesis. All the constructs are under the lpp/ lac promoter-operator control so that their mutagenic effects can be t ested in the absence and the presence of a lac inducer. It was found t hat when the production of msDNA-Ec86 or Ec73 was induced, reversion f requencies from Lac(-) to Lac(+) significantly increased in the case o f a Lac(-) mutation caused by a frame-shift mutation, but much less by a substitution mutation. The removal of mismatched base pairs elimina ted the high mutation frequencies, and the inducible expression of RT alone was not mutagenic. These results are consistent with the hypothe sis of Maas and his associates that mismatched base pairs in msDNA seq uester a cellular mismatch repair system, resulting in the increase of frame-shift mutations.