Previously, we showed that mutations to methyl-directed mismatch repair proteins of Escherichia coli reduce the occurrence of large deletions from long (CTG.CAG) repeats contained on plasmids (Jaworski et al., 1995). By contrast, mutations to the mismatch repair proteins increase the frequency of small length changes (insertions and deletions) to DNA triplet repeats (Schumacher et al., 1998; Wells et al., 1998). Using plasmids with a variety of lengths and purity of (CTG.CAG) repeats, we have resolved these apparently conflicting observations. We show that all lengths of (CTG.CAG) repeats are subject to small length changes (<8 triplet repeats) upon inactivation of the mismatch repair pathway. However, large deletions (>8 triplet repeats) in (CTG.CAG)n occur more readily in cells with active mismatch repair. The incidence of large deletions is proportional to the tract length and they become prominent in tracts greater than 80 repeats. Interruptions to repeat purity enhance the occurrence of largedeletions. In addition, we observed a high frequency of deletions in (CTG.CAG) repeats for cultures passing repeatedly through stationary phase during long-term growth experiments of all strains (i.e. with active or inactive mismatch repair). Transcription into (CTG.CAG)175 can increase the observed frequency of largedeletions in the repeat (Bowater et al., 1997) and we observed that this occurs independently of the mismatch repair pathway. These results fit with current theories of mismatch repair acting on DNA slippage events that occur within the DNA triplet repeats.

References

Bowater, Jaworski, Larson, Parniewski, and Wells (1997) Nucl. Acids Res. 25, 2861-2868.
Jaworski, et al. (1995) Proc. Natl. Acad. Sci. USA 92, 11019-11023.
Schumacher, Fuchs, and Bichara (1998) J. Mol. Biol. 279, 1101-1110.
Wells, et al. (1998) J. Biol. Chem. 273, 19532-19541.