8-Oxo-7, 8-dihydrodeoxyguanosine (8-oxo-dG), one of the representative oxidative DNA lesions, frequently mispairs with the incoming dAMP during mammalian DNA replication. Mispaired dA is removed by post-replicative base excision repair (BER) initiated by adenine DNA glycosylase, MYH, creating an apurinic (AP) site. The subsequent mechanism ensuring a dC:8-oxo-dG pair, a substrate for 8-oxo-guanine DNA glycosylase (OGG1), remains to be elucidated. At the nucleotide insertion step, none of the mammalian DNA polymerases examined exclusively inserted dC opposite 8-oxo-dG that was located in a gap. AP endonuclease 1, which possesses 3′→5′ exonuclease activity and potentially serves as a proofreader, did not discriminate dA from dC that was located opposite 8-oxo-dG. However, human DNA ligases I and III joined 3′-dA terminus much more efficiently than 3AC terminus when paired to 8-oxo-dG. In reconstituted short-patch BER, repair products contained only dA opposite 8-oxo-dG. These results indicate that human DNA ligases discriminate dC from dA and that MYH-initiated short-patch BER is futile and hence this BER must proceed to long-patch repair, even if it is initiated as short-patch repair, through strand displacement synthesis from the ligation-resistant dC terminus to generate the OGG1 substrate, dC:8-oxo-dG pair.
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