AUTHOR=Tesfahun Almaz Nigatu , Alexeeva Marina , Tomkuvienė Miglė , Arshad Aysha , Guragain Prashanna , Klungland Arne , Klimašauskas Saulius , Ruoff Peter , Bjelland Svein 

TITLE=Alleviation of C⋅C Mismatches in DNA by the Escherichia coli Fpg Protein

JOURNAL=Frontiers in Microbiology

VOLUME=12

YEAR=2021

URL=https://www.frontiersin.org/journals/microbiology/articles/10.3389/fmicb.2021.608839

DOI=10.3389/fmicb.2021.608839

ISSN=1664-302X

ABSTRACT=<p>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 <italic>Escherichia coli</italic>, and its repair mechanism remains elusive. We present here <italic>in vitro</italic> evidence that C⋅C mismatch can be processed by base excision repair initiated by the <italic>E. coli</italic> formamidopyrimidine-DNA glycosylase (Fpg) protein. The <italic>k</italic><sub>cat</sub> for C⋅C is, however, 2.5 to 10 times lower than for its primary substrate 8-oxoguanine (oxo<sup>8</sup>G)⋅C, but approaches those for 5,6-dihydrothymine (dHT)⋅C and thymine glycol (Tg)⋅C. The <italic>K</italic><sub>M</sub> 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 <italic>N</italic><sup>4</sup>- 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.</p>