A DNA repair process in Escherichia coli corrects U:G and T:G mismatches to C:G at sites of cytosine methylation.

Escherichia coli contains a base mismatch correction system called VSP repair that is known to correct T:G mismatches to C:G when they occur in certain sequence contexts. The preferred sequence context for this process is the site for methylation by the E. coli DNA cytosine methylase (Dcm). For this reason, VSP repair is thought to counteract potential mutagenic effects of deamination of 5-methylcytosine to thymine. We have developed a genetic reversion assay that quantitates the frequency of C to T mutations at Dcm sites and the removal of such mutations by DNA repair processes. Using this assay, we have studied the repair of U:G mismatches in DNA to C:G and have found that VSP repair is capable of correcting these mismatches. Although VSP repair substantially affects the reversion frequency, it may not be as efficient at correcting U:G mismatches as the uracil DNA glycosylase-mediated repair process.