An incoming nucleotide imposes an anti to syn conformational change on the templating purine in the human DNA polymerase-ι active site

Substrate-induced conformational change of the protein is the linchpin of enzymatic reactions. Replicative DNA polymerases, for example, convert from an open to a closed conformation in response to dNTP binding. Human DNA polymerase-iota (hPol iota), a member of the Y family of DNA polymerases, differs strikingly from other polymerases in its much higher proficiency and fidelity for nucleotide incorporation opposite template purines than opposite template pyrimidines. We present here a crystallographic analysis of hPol iota binary complexes, which together with the ternary complexes show that, contrary to replicative DNA polymerases, the DNA, and not the polymerase, undergoes the primary substrate-induced conformational change. The incoming dNTP pushes templates A and G from the anti to the syn conformation dictated by a rigid hPol iota active site. Together, the structures posita mechanism for template selection wherein dNTP binding induces a conformational switch in template purines for productive Hoogsteen base pairing.