Alleviation Of C.C Mismatches In Dna By The Escherichia Coli Fpg Protein

DNA polymerase III mis-insertion may, where not corrected by its 3 '-> 5 ' exonuclease or the mismatch repair (MMR) function, result in all possible non-cognate base pairs in DNA generating base substitutions. The most thermodynamically unstable base pair, the cytosine (C).C mismatch, destabilizes adjacent base pairs, is resistant to correction by MMR in Escherichia coli, and its repair mechanism remains elusive. We present here in vitro evidence that C.C mismatch can be processed by base excision repair initiated by the E. coli formamidopyrimidine-DNA glycosylase (Fpg) protein. The k(cat) for C.C is, however, 2.5 to 10 times lower than for its primary substrate 8-oxoguanine (oxo(8)G).C, but approaches those for 5,6-dihydrothymine (dHT).C and thymine glycol (Tg).C. The K-M values are all in the same range, which indicates efficient recognition of C.C mismatches in DNA. Fpg activity was also exhibited for the thymine (T).T mismatch and for N-4- and/or 5-methylated C opposite C or T, Fpg activity being enabled on a broad spectrum of DNA lesions and mismatches by the flexibility of the active site loop. We hypothesize that Fpg plays a role in resolving C.C in particular, but also other pyrimidine.pyrimidine mismatches, which increases survival at the cost of some mutagenesis.