Crystal structure of a thwarted mismatch glycosylase DNA repair complex

The bacterial mismatch-specific uracil-DNA glycosylase (MUG) and eukaryotic thymine-DNA glycosylase (TDG) enzymes form a homologous family of DNA glyc osylases that initiate base-excision repair of G:U/T mismatches. Despite lo w sequence homology, the MUG/TDG enzymes are structurally related to the ur acil-DNA glycosylase enzymes, but have a very different mechanism for subst rate recognition. We have now determined the crystal structure of the Esche richia coli R-IUG enzyme complexed with an oligonucleotide containing a non -hydrolysable deoxyuridine analogue mismatched with guanine, providing the first structure of an intact substrate-nucleotide productively bound to a h ydrolytic DNA glycosylase. The structure of this complex explains the prefe rence for G:U over G:T mispairs, and reveals an essentially non-specific py rimidine-binding pocket that allows MUG/TDG enzymes to excise the alkylated base, 3,N-4-ethenocytosine, Together with structures for the free enzyme a nd for an abasic-DNA product complex, the MUG-substrate analogue complex re veals the conformational changes accompanying the catalytic cycle of substr ate binding, base excision and product release.