Identification of proteins of Escherichia coli and Saccharomyces cerevisiae that specifically bind to C/C mismatches in DNA

The pathways leading to G:C-->C:G transversions and their repair mechanisms remain uncertain. CIC and GIG mismatches arising during DNA replication ar e a potential source of G:C-->C:G transversions. The Escherichia coli mutHL S mismatch repair pathway efficiently corrects G/G mismatches, whereas CIC mismatches are a poor substrate. Escherichia coli must have a more specific repair pathway to correct CIC mismatches. In this study, we performed gel- shift assays to identify CIC mismatch-binding proteins in cell extracts of E,coli. By testing heteroduplex DNA (34mers) containing CIC mismatches, two specific band shifts were generated in the gels The band shifts were due t o mismatch-specific binding of proteins present in the extracts. Cell extra cts of a mutant strain defective in MutM protein did not produce a low-mobi lity complex, Purified MutM protein bound efficiently to the C/C mismatch-c ontaining heteroduplex to produce the low-mobility complex. The second prot ein, which produced a high-mobility complex with the CIC mismatches, was pu rified to homogeneity, and the amino acid sequence revealed that this prote in was the FabA protein of E.coli. The high-mobility complex was not formed in cell extracts of a fabA mutant. From these results it is possible that MutM and FabA proteins are components of repair pathways for C/C mismatches in E,coli, Furthermore, we found that Saccharomyces cerevisiae OGG1 protei n, a functional homolog of E.coli MutM protein, could specifically bind to the CIC mismatches in DNA.