THE VSR GENE-PRODUCT OF ESCHERICHIA-COLI K-12 IS A STRAND-SPECIFIC AND SEQUENCE-SPECIFIC DNA MISMATCH ENDONUCLEASE

IN Escherichia coli K-12, the Dcm methyltransferase catalyses methylation of the inner cytosine residue in the sequence CC(A)/(T)GG 1,2. Hydrolytic deamination of 5-methylcytosine bases in DNA leads to thymine residues, and hence to T/G mismatches, pre-mutagenic DNA lesions consisting of two natural DNA constituents and thus devoid of an obvious marker of the damaged DNA strand. These mismatches are corrected by the VSP repair pathway, which is characterized by very short patches of DNA repair synthesis 3,4. It depends on genes vsr 5 and polA 6 and is strongly stimulated by mutL and mutS 7-9. The vsr gene product (Vsr; M(r) 18,000) was purified and characterized as a DNA mismatch endonuclease, a unique and hitherto unknown type of enzyme. Vsr endonuclease nicks double-stranded DNA within the sequence CT(A)/(T)GN or NT(A)/(T)GG next to the underlined thymidine residue, which is mismatched to 2'-deoxyguanosine. The incision is mismatch-dependent and strand-specific. These results illustrate how Vsr endonuclease initiates VSP mismatch repair.