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7,8-Dihydro-8-oxoguanine (8-oxoG) is one of the most common oxidative DNA lesions. 8-oxoguanine DNA glycosylases (Oggs) detect and excise 8-oxoG through a multiple-step process. To better understand the basis for estranged base recognition, we have solved the crystal structures of MBOgg1, the 8-oxoguanine DNA glycosylase of Thermoanaerobacter tengcongensis, in complex with DNA containing a tetrahydrofuranyl site (THF, a stable abasic site analog) paired with an estranged cytosine (MBOgg1/DNA^THF:C) or thymine (MBOgg1/DNA^THF:T). Different states of THF (extrahelical or intrahelical) are observed in the two complexes of the ASU of MBOgg1/DNA^THF:C structure. Analyses of their different interaction modes reveal that variable contacts on the 5′ region flanking the THF abasic site are correlated with the states of the THF. Comparison of MBOgg1/DNA^THF:T with MBOgg1/DNA^THF:C indicates …