Oxaliplatin, together with cisplatin, is among the most important drugs used in cancer chemotherapy. Oxaliplatin, which contains a bulky diaminocyclohexane (DACH) moiety, kills cancer cells mainly by producing (DACH)Pt–GpG intrastrand cross-links that impede transcription. The Pt–GpG tolerance by translesion DNA synthesis (TLS) polymerases contributes to the resistance of tumors to platinum-based chemotherapy. In particular, human DNA polymerase η (Polη) readily bypasses Pt–GpG adducts. While many structural studies have addressed how TLS polymerases interact with cisplatin–DNA adducts, a structure of DNA polymerase in complex with oxaliplatin–DNA adducts has not been reported, limiting our understanding of bypass of the bulky (DACH)Pt–GpG lesion by TLS polymerases. Herein, we report the first structure of DNA polymerase bound to oxaliplatinated DNA. We determined a crystal structure of Polη incorporating dCTP opposite the 3′G of the (DACH)Pt–GpG, which provides insights into accurate, efficient bypass of the oxaliplatin–GpG adducts by TLS polymerases. In the catalytic site of Polη, the 3′G of the (DACH)Pt–GpG formed three Watson–Crick hydrogen bonds with incoming dCTP and the primer terminus 3′-OH was optimally positioned for nucleotidyl transfer. To accommodate the bulky (DACH)Pt–GpG lesion, the Val59–Trp64 loop in the finger domain of Polη shifted from the positions observed in the corresponding Polη–cisplatin–GpG and undamaged structures, suggesting that the flexibility of the Val59–Trp64 loop allows the enzyme's bypass of the (DACH)Pt–GpG adducts. Overall, the Polη–oxaliplatin–GpG structure provides a structural basis for TLS-mediated bypass of the major oxaliplatin–DNA adducts and insights into resistance to platinum-based chemotherapy in humans.

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