Human DNA polymerase ι incorporates dCTP opposite template G via a G.C plus hoogsteen base pair

Human DNA polymerase iota (hPol iota), a member of the Y family of DNA polymerases, differs in remarkable ways from other DNA polymerases, incorporating correct nucleotides opposite template purines with a much higher efficiency and fidelity than opposite template pyrimidines. We present here the crystal structure of hPol iota bound to template G and incoming dCTP which reveals a G.C+ Hoogsteen base pair in a DNA polymerase active site. We show that the hPol iota, active site has evolved to favor Hoogsteen base pairing, wherein the template sugar is fixed in a cavity that reduces the C1'-C1' distance across the nascent base pair from similar to 10.5 angstrom in other DNA polymerases to 8.6 angstrom in hPol iota. The rotation of G from anti to syn is then largely in response to this curtailed C1'-C1' distance. A G.C+ Hoogsteen base pair suggests a specific mechanism for hPoli's ability to bypass N2-adducted guanines that obstruct replication.