2624 Biochemistry, Vol. 49, No. 12, 2010 M1dG, PdG, and 1,N2-εdG lesions are inserted into the DNA duplex. In all three duplexes, the resonance frequencies for exocyclic protons shift upfield as compared to their chemical shifts in the single strands or monomers, consistent with the shielding of these exocyclic rings from neighboring base pairs and accommodation of 1,N2-εdG, M1dG, and PdG in intrahelical orientations. Structural Comparison with Other Duplexes Containing Unpaired Nucleotides. Previous reports have concluded that DNA accommodates unpaired nucleobases in both intra- and extrahelical environments (60), the orientations being dependent both upon the identity of the unpaired nucleobase and upon the DNA sequence flanking the site (61). For example, unpaired purines generally adopt an intrahelical conformation (62, 63), although an extrahelical orientation has been reported for an unpaired dA (64). For unpaired pyrimidines, both intra- helical (65, 66) and extrahelical conformations are observed (61, 67-69). Bypass of the 1,N2-εdG Lesion by Other Polymerases. When bypassed by E. coli polymerases I exo- and II exo-, T7 polymerase exo-, HIV-1 reverse transcriptase, and rat poly- merase β, the 1,N2-εdG lesion also produces misincorporation errors (70). The replicative human polymerase pol δ was blocked by 1,N2-εdG (71). Human polymerase η conducted error-prone replication past 1,N2-εdG, preferentially incorporating dGTP, irrespective of the identity of the base 50 to 1,N2-εdG in the template (71). Human polymerases ι and κ exhibited similar rates of incorporation of dTTP and dCTP (71). Conclusions. Characterization of the 1-BD duplex containing 1,N2-εdG opposite a one-base deletion in the 50-TXG-30 sequence reveals that the stability is increased as compared to that of the complementary duplex containing 1,N2-εdG opposite dC and the unmodified 1-BD duplex. 1,N2-εdG adopts the anti conforma- tion about the glycosyl bond and is in an intrahelical orientation. Watson-Crick base pairing is conserved for both the 50 and 30 neighboring base pairs. The increased stability of the 1,N2-εdG lesion in the absence of the complementary dC correlates with the observed extension of the one-base deletion product during the bypass of the 1,N2-εdG lesion by the Dpo4 polymerase (30), suggesting that the thermal stabilization of this bulged inter- mediate may be significant with regard to the biological proces- sing of the lesion. 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