Escherichia coli MutS, -L, -H, and Dam proteins are the principal components of the DNA postreplicative mismatch repair system (MMRS). We have recently observed that an increased level of MutS, -L or -H in E. coli dam cells results in a more active MMRS. In E. coli dam cells the MMRS is non-directed, so it can result in double-strand breaks and DNA degradation. Using a plasmid carrying a mutated copy of a gene that confers resistance to tetracycline, we observed that E. coli dam cells generate less than 10% of the tetracycline resistant colonies that can be obtained with E. coli MutS or MutL-deficient strains. This result shows that the endogenous MutS/L/H proteins are able to act on plasmid DNA eliminating most of the copies in which a mutation occurred. We then analyzed the effect of normal and plasmid overexpressed MutS and/or MutL on plasmid stability in dam cells. After 5 days of successive culture in reach media without antibiotic one of the plasmid analyzed (pET) was found in approximately 80% of the dam cells or the dam cells with increased levels of MutS and/or MutL. The other plasmid analyzed (p5) was found in 100% of the dam cells but in less than 40% of the dam cells with increased levels of MutS and/or MutL. These results show that extra-chromosomal DNA mutability/stability in the E. coli dam cells can be affected by MutS and/or MutL level.
