Structure of the Replicating Complex of a Pol α Family DNA Polymerase

We describe the 2.6 Å resolution crystal structure of RB69 DNA polymerase with primer-template DNA and dTTP, capturing the step just before primer extension. This ternary complex structure in the human DNA polymerase α family shows a 60° rotation of the fingers domain relative to the apo-protein structure, similar to the fingers movement in pol I family polymerases. Minor groove interactions near the primer 3′ terminus suggest a common fidelity mechanism for pol I and pol α family polymerases. The duplex product DNA orientation differs by 40° between the polymerizing mode and editing mode structures. The role of the thumb in this DNA motion provides a model for editing in the pol α family.