MutS recognition of exocyclic DNA adducts that are endogenous products of lipid oxidation

The ability of the methyl-directed mismatch repair system to recognize and repair the exocyclic adducts propanodeoxyguanosine (PdG) and pyrimido[1,2-a lpha] purin-10(3H)-one (M(1)G), the major adduct derived from the endogenou s mutagen malondialdehyde, has been assessed both in vivo and in vitro. Bot h adducts were site-specifically incorporated into M13MB102 DNA, and the ad ducted genomes were electroporated into wild-type or mutS-deficient Escheri chia coli strains. A decrease in mutations caused by both adducts was obser ved in mutS-deficient strains, suggesting that MutS was binding to the addu cts and blocking repair by nucleotide excision repair, This hypothesis was supported by the differences in mutation frequency observed when hemimethyl ated genomes containing PdG on the (-)-strand were electroporated into a uv rA(-) strain. The ability of purified MutS to bind to PdG- or M(1)G-contain ing 31-mer duplexes in vitro was assessed using both surface plasmon resona nce and gel shift assays. MutS bound to M(1)G: T-containing duplexes with s imilar affinity to a G:T mismatch but less strongly to M(1)G:C- and PdG-con taining duplexes, Dissociation from each of the adduct containing duplexes occurred at a faster rate than from a G:T mismatch. The present results ind icate that MutS can bind to exocyclic adducts resulting from endogenous DNA damage and trigger their removal by mismatch repair or protect them from r emoval by nucleotide excision repair.